organic synthesis bysir faheem

7/30/2019 Organic Synthesis Bysir Faheem

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Organic Synthesis (CH-205) Chemical Kinetics

Activation Energy and Temperature

Dependence of Rate Constants

The Collision Theory of Chemical KineticsCommon sense dictates that a reaction between two particles can occur only if

they collide (usually in a gas or liquid). Therefore it is expected that a reaction rate

is proportional to the rate of collision or the number of collisions per second.

The percentage of successful collisions in chemical systems varies greatly . Insome systems, almost every collision results in chemical change. (Instantaneous

reaction). In other systems, collisions rarely produce chemical change. (A slow

reaction)Therefore, the reaction rate is not the same as the collision rate but is often directlyproportional to it.

Chemical reactions occur when the energy of collision is enough to break

reactant bonds and form product bonds. If there is not enough kinetic energy

in colliding species, reactant bonds will not break and new, product bonds will

not form.

The kinetic energy of the colliding particles must equal or exceed a

certain minimum value in order for a reaction to proceed. Thisminimum energy is called the Activation Energy, EA.

The Arrhenius EquationThe Arrhenius Equation relates the rate constant k to the activation energy, R the

gas law constant and T the absolute temperature.

k=A eEA/RT

Where A = Collision frequency/frequency factor (dimensionless)

EA = Reaction activation energyR = Gas law constant

T = TemperatureGraphable equation becomes

ln k = (- Eact/R) (1/T) + lnA, corresponding to y = mx + b.

This form of the equation shows that ln k varies as minus T-1

.


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Temperature Dependence of Rate ConstantsFor using the Arrhenius Equation algebraically to compare rate constant value atdifferent temperatures, write two equations for different temperatures and therefore

different k values.

ln (k1/k2) = EA/R(T1-1

- T2-1

)

Q: The rate constant for a first-order reaction is 4.50 x 10 -4 s-1 at 25.0 C.

What is its rate constant at 50.0 C if the activation energy is 35.6kJ/mol?

ln (k1/k2) = EA/R(T1 - T2)(T1 T2) T1 = 298.2 K, T2 = 323.2 Kln k1 = EA/R(T1 - T2)(T1 T2) + ln k2ln (k2) = ln k 1 - EA/R(T1 - T2)(T1 T2)

= -7.71 (35.6 kJ . mol

-1

) / (8.314 J mol

-1

K

-1

) (1000 J . kJ

-1

) (-2.589 x 10-4 K-1)

= -7.71 + 1.11 = - 6.60

k = 1.36 x 10-3

s-1

Examples1. The rate constant of the decomposition of N2O5 increases from 1.5210

-51/s

at 25C to 3.8310-31/s at 45C. Calculate the activation energy.2. Calculate k 2 at 780 K for the reaction 2HIH2 + I2, if k1 is 3.510

-51/Ms at

550 K and EA is 183 kJ/mol.


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Chemical Process Kinetics

Chemical Process Kinetics is a study of the influence of the physical factor that

affects chemical reactions, e.g.

1. The type and shape of rector used,

2. The method of operation,

3. Temperature control,

4. Batch or continuous process

5. Backmixing,

6. Fixed or fluidized bed (in the case catalytic reactions).

Choice of Reactor

The type of (tube, tower or tank), the type of operation(batch, recycle,

continuous or once through), and the means of temperature control,(adiabatic or

isothermal), may depend upon the type of reaction involved in order to choose

the best reactor and method of operation, the specific type of system must be

considered.

Backmixing

Backmixing is the mixing of the reactant and product of a chemical reaction by

upstream diffusion and reaction while the main flow is in the downstream

direction.

Backmixng is an extremely important phenomenon which must be considered

carefully in all process-reactor design.


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Type and Shape of Reactor

1. Simple batch homogeneous reactor,

a. Closed tank,

b. Rocking autoclave (laboratory tool),

c. Stirred Kettle,

d. Kettle with outside recirculation but with no material added or

removed,

e. Coil with outside recirculation but with no material added or remove,

2. Semi Batch Rectors

a. Batch with continuous addition of one reactant: Gas-phase, Liquid or

Solid addition

b. Batch with continuous removal of one Product:i. Gas formation,

ii. Solid precipitation,

iii. Formation of immiscible liquids.

c. Batch with combined addition of one reactant and removal of one

Product

3. Continuous Homogenous reactors

a. Longitudinal tubular reactor(no backmixing)

b. Stirred-tank reactor(complete backmixing)c. Tubular reactor with some backmixing

d. Tower reactor (Packed tower, Empty tower, Baffled reactor)

e. Baffled tank reactor

f. Longitudinal reactor with multiple injection of one reactant

4. Continuous Hetrogenous reactors

a. Packed-tower countercurrent reactor

b. Fixed-bed catalytic reactor: (Longitudinal, Backmixing)

c. Moving and fluidized-bed catalytic reactor: (Longitudinal,

Backmixing)

d. Distillation column


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Organic Synthesis (CH-205) Nitration

Nitration:

The nitration reaction serves to introduce one or more nitro groups (-NO2) into a

reacting molecule. The nitro group may become attach to carbon to form a nitro-aromatic or nitro-paraffinic compounds. It may become attached to oxygen to

form oxygen to form nitrate ester, or attached to nitrogen to form nitramine.

e.g.

CH4 + HNO3 CH3NC2 + H2O

+ HNO3

H2SO4

NO2

+ H2O

RX + AgNO3RNO3 + AgX

OH

SO3H

SO3H

HNO3

O2N NO2

OH

SO3H

HNO3

O2N NO2

OH

NO2

Trinirtophenol

(Picric Acid)

Uses Dyestuff

Pharmaceuticals

Intermediates for the production

of several other compounds.

Explosives

Organic solvents


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Nitrating Agent

The agents or reagents which are used to carry out in nitration reaction are called

nitrating agents. Some common of these are as follows:

Aqueous HNO3 (nitric acid)

Conc. HNO3

Fuming Nitric Acid

Mixture of HNO3 & H2SO4 (Mixed

acid)

Mixture of HNO3 & Acetic

anhydride

Mixture of HNO3 & H3PO3

Mixture of HNO3 & Chloroform

Nitrogen pentaoxide (N2O5)and

Nitrogen tetraoxide (N2O4)

The most important nitrating medium is nitric acid and sulfuric acid. In this

mixture HNO3 exists in the form of Nitryl (Nitronium) ion. The ionization of HNO3

in H2SO4 can be represented by:

HNO3 + 2H2SO4 NO2+

+H3O+

+ 2HSO4-

Ionization of HNO3 in H2SO4 mixture:

In solution of HNO3 and H2SO4, weaker than the 86% H2SO4, the ionization of

HNO3 is very slight, but rapidly rises as the H2SO4 becomes more concentrated. In

about 94% H2SO4 the HNO3 completely ionized.


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Nitration of Aromatics

The nitration of aromatics can be represented by the equation:

ArH + HNO3 ArNO2 + H2O

The nitrating agent NO2+

is an electropositive radical. Hence NO2+

will attach itself

to that Carbon of aromatic compoundwhere the charge density is higher.

Some functional groups cause the electron density to be higher at Ortho and Para

position than Meta position like Cl2, I2, Fluorine and CH2Cl.

Some functional groups cause the electron density to be higher at Meta position

e.g. CCl4, CHCl2 and COOC2H5.

Mechanism of Aromatic Nitration: (Assignment)

Nitration of Paraffins

Gas Phase Nitration:

Paraffins are quite inert to electrophilic nitro-groups. Nitration of paraffin is a free

radical reaction carried out in a vapor phase at higher temperature of 350450oC.

Nitric acid of 70% strength or less is used as reagent. Nitration of paraffin yields

variety of products e.g. the nitration of 2-methyl butane.

When the nitration of 2-methyl butane occurs, above mentioned products are

formed with different percentages and then each product is separated by

appropriate separation techniques.


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The nitration of paraffin in liquid phase is less importance at commercial scale,

because of low yield, lower conversion and occurrence of side reactions. In liquid

phase nitration of paraffin replacement of hydrogen atom take place in contrast

to gas phase nitration where the cleavage of alkyl bond occurs. The liquid phase

nitration is generally not adopted in industries.

Nitration of Olefins

Nitration of Ethylene:

The Nitration of Ethylene is also a free radical reaction carried out in liquid

medium at low temperature of -10oC25

o. Nitrogen tetra oxide is used as nitrating

agent with some air/oxygen to oxidize any nitrogen oxide to nitrogen dioxide.


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Trinitrotoluene

Trinitrotoluene (TNT) is an important chemical and is used for the preparation of

other useful products. It is also used as an explosive chemical, so special caremust be taken during the large scale production.

Production:

Toluene and mixed acid are delivered from their storage tanks to the first nitrator.

Temperature parameters of the first nitrator are set at 46-52oC. The process is

continued for 15 minuets and mononitrotoluene (MNT) is produced. Spent acid is

taken out from the bottom of the nitrator. MNT along with fresh acid is delivered

to the second nitrator where the temperature is maintained at 77-78oC. Reaction

takes 20 minuets to complete and dinitrotoluene (DNT) is produced. Again the

spent acid is withdrawn from the bottom of the nitrator. DNT and fresh mixed

acid is delivered to third nitrator where the temperature is set at 90-231oC. After

45 minutes, the reaction is completed and trinitrotoluene (TNT) is produced.

Now the TNT is purified by passing it through the filter press and then it is washed

by water in a washing chamber. TNT carries some contents of acid with it which is

then neutralized by the sodium carbonate in a separate chamber and againwashed with water. Finally the TNT is crystallized and sent to the packaging

system.

Application

TNT is one of the most commonly used explosives for military and industrial

applications. It is valued because of its insensitivity to shock and friction, which

reduces the risk of accidental detonation. TNT melts at 80 C (176 F), far below

the temperature at which it will spontaneously detonate, allowing it to be poured

as well as safely combined with other explosives. TNT neither absorbs nor

dissolves in water, which allows it to be used effectively in wet environments.

Additionally, it is relatively stable when compared to other high explosives.


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Organic Synthesis (CH-205) Alkylation

Alkylation

Alkylation may be defined as the introduction of an alkyl radical by substitution or addition into an

organic compound.

1. Substitution of hydrogen in carbon compounds:

This is a nuclear Alkylation when aromatic hydrogen is substituted. The carbon of alkyl bound is

attached to carbon of either alphatic or aromatic compound. This is termed as carbon to carbon

alkylation.

CH3I + CH3I C2H6 + I2

2. Substitution of hydrogen in the hydroxyl of an alcohol group or phenol:

In this type of alkylation the alkyl group replaces the hydrogen in the hydroxyl group of alcohol or

phenol. This type of alkylation is termed as Carbon-Oxygen alkylation.

C2H5OH + CH3Cl C2H5-O-CH3 + HCl

3. Substitution of hydrogen in Amino group:

In this type of alkylation the alkyl group replaces the hydrogen of he amino group and directly

attaches to the nitrogen atom. This type of alkylation is termed as Carbon-Nitrogen alkylation.

+ 2CH3Cl + 2HCl

4. Addition of alkyl halide to Tertiary nitrogen compounds:

In this type of alkylation, addition of alkyl halide or alkyl ester takes place. Here the binding of the

alkyl is to the nitrogen, and the trivalent nitrogen is often assumed to be converted to a pentavalent

linkage.

+ CH3Br

NH2 N(CH3)2

N

CH3

CH3

CH3N

CH3

CH3

CH3

CH3

+

Br-


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5. Alkyl Metallic Compounds:

In this type of compounds, the alkyl group is attached to metals. E.g. (C2H5)4 or tetra ethyl lead (TEL).

4PbNa + C2H5Cl (C2H5)4Pb + 3Pb + 4NaCl

6. Miscellaneous Alkylation:

In mercaptans, the alkyl group is attached to the sulfur.

n-C12H25SH

Products Derived by Alkylation

Anesthetics, alkaloids, antiseptics detergents, dyes explosives, flavors, lubricants, plastics, perfumes,

rubber, petrochemicals (accelerators, antioxidants, modifiers, resins, stabilizers), solvents, synthetic

gasoline.

Alkylating Agents:

1. Olefins:

These are used extensively for CC alkylation. Olefins of high higher molecular weight react more

rapidly than then the Olefins of lighter molecular weight.

e.g.

Olefins are also used in making of the other alkylating agents like ethylene chloride and isopropyl

hydrogen sulfide.

2. Alcohols

In alcohols methanol and ethanol are mostly used as alkylating agents. In almost all the reactions a

catalyst is necessary to promote the alkylation smoothly and generally mineral acids are used as

catalysts. Alcohols are used for the manufacturing of ethers. The lower molecule weight alcohols are

also used mostly for the catalytic vapor phase synthesis of alkyl ammines and for the alkylation of

phenols.

e.g.

3. Alkyl Halides:

The alkyl halides are the most commonly used alkylating agents used for several manufacturing

processes such as production of Tetra Ethyl Lead TEL. some of the lower alkyl halides are so volatile

that they must be used in autoclaves.

Alkyl halides are commonly used for alkylation of phenols and alcohols.


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e.g

4. AlkylAryl HalidesAlkylAryl Halides are used for the reduction of aromatic ring in the reacting molecule. For example

benzyl chloride is used for the introduction of the benzyl group.

e.g

5. Alkyl Quaternary Ammonium Salts:

These compounds are not very common alkylating agents and used in very specific reactions like the

preparation of tri-methyl ammonium iodide:

e.g.

6. Metallic Alkyl Derivatives:

Metallic Alkyl Derivatives are prepared by the action of alkyl halide on metal (zinc magnesium alloy).

Alkyl magnesium halide (Grignards reagent) are frequently used as alkylating agent for the

production of alkyl phenols, metal alkyls, ethers etc.

e.g.


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Mechanism of Reaction

Mechanism for the Liquid-phase alkylation:

In this type of reaction is either Friedel-Craft or protonic acid catalyst are often used when liquid iso-

paraffins or aromatic hydrocarbons are alkylated with olefins. These catalysts are generally considered

to be proton donors which forms carbonium ions. The reaction temperature is relatively close to room

temperature, about -40 to 30oC

A typical alkylation may be represented by the following series of reaction.

The other acids can be used as catalyst for alkylation like H2SO4, HF depending upon the composition offeed stock and process parameters. The role of HCl with AlCl3, is to increase the acidity of the catalyst

and hence make it more effective.

Side reactions such as polymerization, isomerization, hydrogen transfer & destructive alkylation occurs

during the catalytic alkylation of iso-paraffins. In polymerization the carbonium ions such as formed in

eq. 1 and eq. 3 react with olefins causing chain growth. Polymerization is minimized by using a relatively

excess quantity of iso-paraffins.

CH 2 HC lCH 2 + A lC l3+ CH 3 CH 2+

AlCl4-

+

CH 3 CH

CH 3

+

CH 3

CH 3 CH 2+

AlCl4-

+ CH 3 C+

CH 3

CH 3

AlCl4-

+ CH 3 CH 3+

CH 3 C+

CH 3

CH 3

AlCl4-

+ CH 2 CH 2+ CH 3 C

CH 3

CH 3

CH 2 CH 2+

AlCl4-

+

CH 3 C

CH 3

CH 3

CH 2 CH 2+

AlCl4-

+ CH 3 C

CH 3

CH 3

CH 3CH+

AlCl4-

+ CH 3 C+

CH 3

C H C H 3 AlCl4-

+

CH 3

CH 3 C+

CH 3

C H C H 3

CH 3

AlCl4-

+ CH 3 CH

CH 3

CH 3

+ CH 3 CH

CH 3

C H C H 3

CH 3

+ CH 3 C+

CH 3

CH 3

AlCl4-

+


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Mechanism for the Vapor-phase alkylation

Paraffins can be alkylated in the absence of catalyst at sufficiently high temperature about 500oC and at

high pressures of 25004000 psi, so that small amount of the paraffins will decompose into free radical.

A general free radical mechanism is as follows:

Materials of Construction

In many cases, steel is suitable for the construction of alkylating equipment, even in the presence of the

strong acid catalyst, as their corrosive effect is greatly decreased by the formation of esters as catalytic

intermediate products. Lined equipments are satisfactory where conditions are not an-hydrous, lead-

lined, Monel-lined, or enameled. In a few cases, copper or tinned copper is still used as in the

manufacture of pharmaceutical and photographic products, to decreased with metals.

Corrosion of Apparatus

Corrosion may be caused by the catalyst used in the alkylation or by the hydrogen halides formed by

hydrolyses of alkyl halide. In the preparation of the n-alkyl compounds the original amine or the alkyl

amines formed have an inhibiting effect against corrosion.

Recovery of Alkylate

Distillation is usually the most convenient procedure for product recovery, even in those instances in

which the boiling points are rather close together. Frequently such a distillation will furnish a finished

material of quality sufficient to meet the demands of the market. If not, other means of purification may

be necessary, such as crystallization or separation by means of solvents. The choice of a proper solvent

CH 3

CH 3CH 2CH 3

CH 3+

CH 3CH 2 C2H5+

+

CH 3+

+

H

CH 3 C+

CH 3

CH 4+

H

CH3C

+

CH 3

+C

2H

4

H

CH3 C

CH 3

CH2

CH2

+

H

CH 3 C

CH 3

CH 2 CH 2+

CH 3CH 2CH 3+

H

CH 3 C

CH 3

CH 2 CH 3

H

CH 3 C+

CH 3

+


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will, in many instances, lead to the crystallization of the alkylated product and to its convenient

recovery.

Production of Dodecyl benzene

Alkyl aryl is used to designate the predominantly important aliphatic aromatic organic compounds which

are sulfonated to manufacture detergents. The dodecene is normally propane tetramer containing 12

carbon atoms and with boiling range of 350-420oF.

The benzene is alkylated with dodecene catalyzed by AlCl3 continuously introduced. The temperature is

kept at 115oF as maximum this being controlled by cooling coils or by circulating a of the benzene.

AlCl3

C6H6 + (C3H6)4 C6H5C12H25

115oF Dodecyl benzene

The increasing temperature is being controlled by cooling coils or by circulating a part of the benzene

alkylate through an external cooler and back to the agitated alkylator. The alkylator is followed by a

continuous settler. The benzene is in excess to suppress the formation of isomers (heavy alkyl-aryl

HC). After separation of the AlCl3 sludge, the charge goes to a benzene fractionator where the excess

benzene is distilled overhead and recycled. The bottom intermediates from the benzene fractionator are

pass through the intermediate fractionator, furnishing as overhead a small quantity of a light alkyl aryl

hydrocarbon, then to the dodecylbenzene vacuum fractionator. The dodecyl benzene has a boiling

range of 530 600

o

F.


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Alkylation in Petroleum Industry

In oil refining contexts, alkylation refers to a particular alkylation of isobutane

with olefins. It is a major aspect of the upgrading of petroleum products.

Alkylation processes at refineries are exothermic and are fundamentally similar to

polymerization processes. As a result, the alkylate product contains no olefins and

has a higher octane rating. These methods are based on the reactivity of the

tertiary carbon of the iso-butane with olefins, such as propylene, butylenes, and

amylenes. The product alkylate is a mixture of saturated, stable isoparaffins

distilling in the gasoline range, which becomes a most desirable component of

many high-octane gasolines.

Alkylation is carried out at petroleum industry by two methods.

1. Thermal alkylation in the vapor phase

2. Catalytic alkylation in the liquid phase

a. With Hydrogen Fluoride

b. Sulfuric acid

c. Aluminum Chloride

d. Hydrocarbon complex

In both of these cases it is necessary to keep the olefins in a low concentration,

which may cause the formation and gum and varnishes by the polymerization ofthese hydrocarbons, and the paraffin are in a high concentration to favor the

paraffin-olefin junction which help in favorable alkylation.

Thermal Alkylation

Thermal alkylation has ceased to be a commercial process. Neohexane, the main

product of the thermal process, has an octane no. of 104.8 with 3 ml of TEL per

gallon, while the corresponding number of diisopropyl with 6-6.8 ml of TEL per

gallon has the octane rating of 118.7. For this reason and because the thermal

process operated under the extreme conditions of 500 psi pressure and 950oF

temperature it is very doubtful if operations will be resumed.

Sulfuric Acid Alkylation Process


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In cascade type sulfuric acid (H2SO4) alkylation units, the feedstock (propylene,

butylene, amylene, and fresh isobutane) enters the reactor and contacts the

concentrated sulfuric acid catalyst (in concentration of 85% to 95% for good

operation and to minimize corrosion). The reactor is divided into zones, with

olefins fed through distributors to each zone, and the sulfuric acid and isobutanes

flowing over baffles from zone to zone.

The reactor effluent is separated into hydrocarbon and acid phases in a settler,

and the acid is returned to the reactor. The hydrocarbon phase is hot-water

washed with caustic for pH control before being successively depropanized,

deisobutanized, and debutanized. The alkylate obtained from the deisobutanizer

can then go directly to motor-fuel blending or be rerun to produce aviation-grade

blending stock. The isobutane is recycled to the feed. The product alkylate is a

mixture of saturated, stable isoparaffins distilling in the gasoline range, which

becomes a most desirable component of many high-octane gasolines.


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Page 1 of6

Sulfonation and Sulfation

Sulfonation

Any chemical process by which the sulfonic acid group SO2OH, or the corresponding salt or sulfonyl

halides.

Sulfation

Any chemical process in which OSO2OH group is introduced into organic carbon. Reacting with one

carbon it form acid sulfates ROSO2OH which on further alkylation forms alkyl sulfate with two carbon

atoms ROSO2OR.

Types of Sulfonation

1. Sulfochlorination introduction of SO2Cl in alkanes.2. Halo Sulfonation reaction of halosulfonic acid ClSO3H and FSO3H with an aromatic hetrocyclic

compounds.

3. Sulfoxidation use of SO2 and O2 to sulfonate an alkane

4. Sulfoalkylation and Sulfoarylation introduction of Sulfoalkyl and Sulfoaryl groups.

Types of Sulfation

1. Formation of sulfateded alkenes

2. Alcohol sulfation

3. Cyclic sulfates

4. Sulfated carbohydrates

5. Sulfated nitrogenous poly saccharides.

Procedure employing for Sulfonation

1. Treatment of organic compound with SO3

2. Treatment with a compound of SO2

3. Condensation and polymerization method

4. Oxidation of organic compound already containing sulfur.

Uses & Application of Sulfation and SulfonationSulfonation and sulfation is employed for production different salts and acid which consumes in the

manufacturing of the verity of products.

A few sulfonates are both marketed and used in the acid form, methane- and toulenesulfonic acid as

catalyst and phenolsulfonic acid as an electroplating additive. The major quantity of sulfonates and

sulfates is both marketed and used in salt form this category includes detergents, emulsifying agents,


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Page 2 of6

demulsifying agents, penetrating, wetting and solubilizing agents, lubricant additives and rust inhibitors.

Polymeric sulfonates includes dispersing agents, elastomers, water-soluble synthetic gumsand

thickening agents, ion-exchange resins which function as strong acids with complete water insolubility.

Sulfamates includes a herbicide, water sweetening agent, and a blood anticoagulant. Sulfonates and

sulfates find uses as intermediates for preparing organic compound not containing sulfur, notably

phenols and alcohols.

Sulfonating and Sulfating agents

With special reference to their properties and major application, theses are categorized in three groups:

1. Sulfur trioxide and compound

a) SO3 and oleum, conc. H2SO4

b) Chlorosulfonic acid (SO3 plus HCl)

c) Sulfamic acid (NH2SO3H)

d) SO3 adducts with organic compounds.

2. The sulfur dioxide group

a) Sulfurous acid, metallic sulfites.

b) SO2 with chlorine.

c) SO2 with oxygen.

3. Sulfoalkylating agents

a) Sulfomethylating agents

The most efficient sulfonating and sulfating is SO3 and its compounds.

Chemical and Physical Factors in Sulfonation and Sulfation

When employing SO3 or its compounds for sulfonation and sulfation, important variables determining

the rate and course of the reaction are:

a) Conc. of SO3,

b) Chemical structure of the organic compound,

c) Time in relation to temperature and reagent strength,

d) Catalyst and

e) Solvent.


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Page 4 of6

Material of Construction

Cast iron is resistant to the action of sulfuric acid in the range of 75-100 % in the strength over a

fairly wide temperature range has been a standard material of construction for Sulfonation kettles

for many years.

However, it has a poor tensile strength and is corroded by oleum or SO3. Improved corrosion

resistance results from the addition of silicon to cast iron (e.g. Duriron), but such alloys are also of

poor strength. Enameled cast iron is often employed because it is more resistant to corrosion.

The use of lined steel vessels combines low cost and high strength with good corrosion resistance.

Commonly used linings are glass, enameled, and various metals like lead, nickel and numerous type

of stainless steels. Glass-lined are commonly used for the batch Sulfonation of detergent alkylate.

Type 316 is one of the most widely used stainless steels.

For each substance being sulfonated, there is a critical concentration of acid below which

sulfonation ceases. The removal of the water formed in the reaction is therefore essential. The use

of a very large excess of acid, while expensive, can maintain an essentially constant concentration

as the reaction progresses. It is not easy to volatilize water from concentrated solutions of sulfuric

acid, but azeotropic distillation can sometimes help.

The sulfonation reaction is exothermic, but not highly corrosive, so sulfonation can be conducted in

steel, stainless-steel, or cast-iron sulfonators. A jacket heated with hot oil or steam can serve to

heat the contents sufficiently to get the reaction started, and then carry away the heat of reaction.

A good agitator, a condenser, and a fume control system are usually also provided.

Sulfonation reactions may be carried out in batch reactors or in continuous reactors. Continuous

operations are feasible and practical (1) where the organic compound (benzene or naphthalene)

can be volatilized, (2) when reaction rates are high (as in the chlorosulfonation of paraffins and the

sulfonation of alcohols), and (3) where production is large (as in the manufacture of detergents,

such as alkylaryl sulfonates).


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Page 5 of6

Production of Detergent by Dodecyl Sulfonation

In the production of detergents from the Sulfonation of dodecyl benzene, the docyl benzene and

sulfuric acid is introduced in the sulfonator. From where the dodecyl benzene sulfonates sulurry is

introduced in the neutralizer, in which caustic is used as the neutralizing media. The Sodium slurry

is the introduced in the crutcher with continuous agitating arrangement. In the crutcher buildersand additives are added which stabilize the physical and chemical properties of detergents. The

slurry is heated and then pumped to the top of a tower where it is sprayed through nozzles under

high pressure to produce small droplets and hot flue gases is used for the drying media. The

droplets fall through a current of hot air, forming hollow granules as they dry. The dried granules

are collected from the bottom of the spray tower where they are screened to achieve a relatively

uniform size.

After the granules have been cooled, heat sensitive ingredients that are not compatible with the

spray drying temperatures such as bleach, enzymes and fragrance are added. Traditional spray

drying produces relatively low density powders.


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Organic Synthesis (CH-205) Sulfonation & Sulfation

Page 6 of6

Sulfuric Acid

Dodecyl

Benzene

Spent Acid

Spent AcidsettlerSulfonator

Fuel Gas Furnace

FlueGases

Neutralizer Crutcher

Builders

Sodiumdodecyl

benzenesulfonate

slurry

NaOH

Solution

Dodecyl

Benzene

Sulfonic

Acid

Water

Productionof Detergent fromDodecyl BenzeneSulfonation


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Organic Synthesis (CH-205) Amination

Amination

Amination is the process of introducing the amino group (NH2) into an organic compound. Amination

by reduction involves the synthesis of amines by reductive methods. The production of aniline (C6H5NH2)

by the reduction of nitrobenzene (C6H5NO2) in the liquid phase is the example of reductive amination.

Amines may be derivatives of ammonia, where one or more of the hydrogen is replaced by alkyl, aryl,

cycloalkyl or heterocyclic groups. Amines are divided into three classes, primary, secondary and tertiary

depending upon the number of replaced hydrogen in the parent substance ammonia.

N

H

H

H

N

R

H

H

N

R

R'

H

N

R

R'

R"

Ammonia Primary Amines Secondray Amines Tertiary Amines

Primary Amines

CH3 NH2

NH2

Methlamine

Aniline

Secondary Amines

(CH2)2NH

NH

Dimethlamines

Diphenylamine


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Tertiary Amines

(CH3)3N

Trimethyl Amines

N

Triphenylamine

Amination is also achieved by the use of ammonia (NH 3), in a process referred to as ammonolysis. An

example is the production of aniline (C6H5NH2) from chlorobenzene (C6H5Cl) with ammonia (NH3). The

reaction proceeds only under high pressure.

C6H5OH + NH3 -------> C6H5NH2 + H2O

Methods of Reduction

A great variety of reduction methods have been used for the preparation of amines. Among these are:

1. Metal and acid

2. Catalytic

3. Sulfide

4. Electrolytic

5. Metal and alkali

6. Sodium Hydrosulfide

7. Metal Hydrides

8. Sodium and Sodium alcoholate

By proper selection of reducing agent and careful regulation of the process, reduction may often be

stopped at intermediate stages and valuable products other than amines obtained. Metal and acid

reductions are most vigorous and usually yield amines as end products. When nitrobenzene is treated

with zinc a mineral acid, the resultant product is aniline. When alkaline solution is employed,

hydrazobenzene is generally obtained, but very vigorous conditions sometimes results in the formation

of aniline. When zinc dust & water are used, the reaction product is Phenylhydroxylamine.


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Iron and acid (Bechamp) Reduction

In the metal-acid reduction, using iron as metal, the product is aniline. Initially the acetic acid was used

for such reduction, but the technical progress in the application of this reaction was first made by

substituting hydrochloric acid for the acetic acid originally employed. Subsequently, it was discovered

that by using ferrous salt in this reduction process, the amount of acid consumed is far less than the

theoretical quantity of acid calcualted.

C6H5NO2 + 2Fe + 6HCl ----------- C6H6NH2 + 2H2O + 2FeCl3

In the industrial practice, it has been shown that 3.0 lb of HCl is sufficient to bring about a satisfactory

reduction of 100 lb of nitrobenzene to aniline.

Production of AnilineAniline can be produced by the use of ammonia (NH3), in a process referred to as ammonolysis. On a

small scale, cracking ammonia can produce hydrogen for reduction. Transport and storage of hydrogen

as ammonia is compact, and the cracking procedure involves only a hot pipe packed with catalyst and

immersed in a molten salt bath. The nitrogen that accompanies the generated hydrogen is inert.

C6H5OH + NH3 --> C6H5NH2 + H2O


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Ammonia is a comparatively low cost reagent, and the process can be balanced to produce the desired

amine. The other routes to amines through reduction use expensive reagents (iron, zinc or hydrogen)

that make ammonolysis costs quite attractive. Substituted amines can be produced by using substituted

ammonia (amines) in place of simple ammonia.

Thus, amination, or reaction with ammonia, is used to form both aliphatic and aromatic amines.

Reduction of nitro compounds is the traditional process for producing amines, but ammonia or

substituted ammonias (amines) react directly to form amines. The production of aniline by amination

now exceeds that produced by reduction (of nitrobenzene).

Material of Construction

Amination by reduction is usually carried out in cast-iron vessels (1600 gallons capacity or higher) and

alkali reductions in carbon steel vessels of desired sizes. The vessel is usually equipped with a nozzle atthe base so that the iron oxide sludge or entire charge may be run out upon completion of the reaction.


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Production of Ethanolamines

An equilibrium mixture of the three ethanolamines is produced when ethyleneoxide is bubbled through

28% aqueous ammonia at 30 to 40oC. By recirculating the products of the reaction, altering the

temperatures, pressures, and the ratio of ammonia to ethylene oxide, but always having an excess of

ammonia, it is possible to make the desired amine predominate. Diluent gas also alters the productratio.

CH2CH2O +NH3 --> HOCH2CH2NH2 + H2O

monoethanolamine

2CH2CH2O + NH3 --> (HOCH2CH2)2NH + 2H2O

diethanolamine

3CH2CH2O + NH3 --- (HOCH2CH2)3N + 3H2O

triethanolamine

After the strongly exothermic reaction, the reaction products are recovered and separated by flashing

off and recycling the ammonia, and then fractionating the amine products. Monomethylamine is used in

explosives, insecticides, and surfactants. Dimethylamine is used for the manufacture of

dimethylformamide and acetamide, pesticides, and water treatment; these can also used for the

sweetening agent in the gas processing plants. Trimethylamine is used to form choline chloride and to

make biocides and slimicides.


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Organic Synthesis (CH-205) Halogenation

Halogenation

Halogenation is a chemical reaction that incorporates a halogen atom into a molecule.

The preparation of organic compounds containing halogens such as Chlorine, fluorine, Iodine, and

bromine, can be done by a variety of manners. The conditions and procedures not only differ for each

member of halogen family but also with the type and structure and of reacting compound.

The chlorine derivatives because of the greater economy in effecting their preparation are most

important of the other halogens and for the reasons are given by primary consideration.

The bromine derivatives, however sometimes has certain advantages because of the greater ease in

effecting the replacement of this halogen in subsequent reaction or because it possesses certain

pharmaceutical or dying properties.

The fluorine derivatives are as well established in the field of refrigerants and aerosol (a cloud of liquid

or solid carried under high pressure and released as a spray) propellants because of their stability and

low boiling points.

All the halogenation reactions are fairly exothermic reactions.

Halogenation may be involved the addition, substitution and replacement reaction. E.g.

Halgenating Agents

1. Chlorine and derivatives

2. Fluorine and derivatives

3. Iodine and derivatives

4. Bromine and derivatives

C2H2 + 2Cl2

FeCl3Cl2HC CHCl2

CH3COOH + Cl2 PCl3 CH2ClCOOH + HCl

C2H5OH + HCl C2H5Cl + H2OZnCl2


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Catalyst:

From experiments it becomes clear that each type of reaction may involve not only a specific

halogenating agent but also a suitable catalyst or activator. Iron antimony and phosphorus, which are

able to exist in low valencies, are less stable at higher valence and give up part of their halogen during

process. Iodine, bromine and chlorine which are capable of forming mixed halogens are also frequentlyused as catalyst in halogenation. Active carbon, clay and other compounds also serves to catalyze

halogenation process. Where the halogen is energized to an activated state by means of light, heat,

nuclear energy, or free radical, it may then proceed to react by addition or substitution reaction without

catalyst.

Chlorination:

Halogenation is almost always chlorination, for the difference in cost between chlorine and the other

halogens, particularly on a molar basis, is quite substantial. In some cases, the presence of bromine (Br),

iodine (I), or fluorine (F) confers additional properties to warrant manufacture.

Chlorination proceeds

1. By addition to an unsaturated bond,

2. By substitution for hydrogen, or

3. By replacement of another group such as hydroxyl (OH) or sulfonic (SO3H). Light catalyzes

some chlorination reactions, temperature has a profound effect, and polychlorination almost

always occurs to some degree.

The most important methods of chlorination are as follows:

Direct Action of Chlorine GasC2H4 + Cl2 ClH2CCH2Cl

+ Cl2

FeCl3

30-100oC

Cl

+ HCl

Hydrochloric Acid

HC CH + HClHgCl2

H2C CHCl

2C6H6 + 2HCl + O2CuCl2 on Al2O3

2C6H5Cl + 2H2O


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Sodium Hypochlorite

OH

NaOCL

alkaline solution

OH

Cl

Chlorination with Phosgene (COCl2)

CHO

+ COCl2

CHCl3

+ CO2

Chlorination with sulfuryl chloride

+ 3SO2Cl2

C6H6

Solvent

NH3Cl

Cl

Cl

Cl

NH3Cl

+ 3HCl + 3SO2

Chlorination with Phosphorus Chlorides:

3RCOOH + PCl3 3RCOCl + H3PO3

Design and Construction of Equipment for Halogenation

From the several experiment on pilot scale as well on large scale, it is obvious that no general rules can

be formulated for the design and construction of the plant.

For non-aqueous media, apparatus constructed of iron and lined plastics, such as Teflon, PVC,

polyesters, epoxy resins, or with stoneware, enamels, porcelain, glass, nickel, inconel, stainless steel,

hestelloy, can be used.

For aqueous media like HCl or hypochloric acid the above mentioned materials are severely limited.

Tantalum, zirconium and titanium are usually resistant but expensive. The plastics of variable ranges arelimited due to temperature and solvent attacks. In dilute solutions wood is satisfactory. For HCl acid

rubber lined steel is excellent at low temperatures even in the absence of organic solvents.

It is usually desirable in the pilot laboratory stage to make measurements of potential across propose

materials under operating conditions.


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Chlorination of Methane

Chlorine and methane (fresh and recycled) are charged in the ratio 0.6/1.0 to a reactor in which the

temperature is maintained at 340 to 370oC. The reaction product contains chlorinated hydrocarbons

with unreacted methane, hydrogen chloride, chlorine, and heavier chlorinated products. Secondary

chlorination reactions take place at ambient temperature in a light-catalyzed reactor that convertsmethylene chloride to chloroform, and in a reactor that converts chloroform to carbon tetrachloride. By

changing reagent ratios, temperatures, and recycling ratio, it is possible to vary the product mix

somewhat to satisfy market demands. Ignition is avoided by using narrow channels and high velocities in

the reactor. The chlorine conversion is total, and the methane conversion around 65 percent.


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Organic Synthesis (CH-205) Hydrogenation

Hydrogenation

In its simplest interpretation, hydrogenation is the addition of hydrogen to a chemical compound.

Generally, the process involves elevated temperature and relatively high pressure in the presence of

a catalyst.

Uses

Hydrogenation yields many useful chemicals, and its use has increased phenomenally, particularly in

the petroleum refining industry. Besides saturating double bonds, hydrogenation can be used to

eliminate other elements from a molecule. These elements include oxygen, nitrogen, halogens, and

particularly sulfur. Cracking (thermal decomposition) in the presence of hydrogen is particularly

effective in desulfurizing high-boiling petroleum fractions, thereby producing lower-boiling and

higher-quality products.

Health concerns associated with the hydrogenation of unsaturated fats to produce saturated fats

and trans-fats are important aspect of current consumer awareness. Hydrogenation is widely

applied to the processing of vegetable oils and fats. Complete hydrogenation converts unsaturated

fatty acids to saturated ones. Hydrogenation results in the conversion of liquid vegetable oils to

solid or semi-solid fats, such as those present in margarine.

Conditions

Hydrogenation is generally carried out in the presence of a catalyst and under elevated

temperature and pressure. Noble metals, nickel, copper, and various metal oxide combinations are

the common catalysts.

Catalyst:

Nickel, prepared in finely divided form by reduction of nickel oxide used in a stream of hydrogen gas

to reduce the temperature about 300C to 175C, for the reaction of hydrogen with unsaturated

organic substances. Platinum black, palladium black, copper metal, copper oxide, aluminum, and

other materials have subsequently been developed as hydrogenation catalysts.

Temperatures:

The reaction is carried out at different temperatures and pressures depending upon the substrate.

Hydrogenation is a strongly exothermic reaction. In the hydrogenation of vegetable oils and fatty

acids, for example, the heat released is about 25 kcal per mole (105 kJ/mol), sufficient to raise the

temperature of the oil by 1.6-1.7 C.


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Production of Methyl Alcohol:

Methyl alcohol (methanol) is manufactured from a mixture of carbon monoxide and hydrogen

(synthesis gas), using a copper-based catalyst.

CO + 2H2 CH3OH

In the process, the reactor temperature is 250 to 260oC at a pressure of 725 to 1150 psi (5 to 8

MPa). High- and low-boiling impurities are removed in two columns and the unreacted gas is

recirculated.

New catalysts have helped increase the conversion and yields. The older, high-pressure processes

used zinc-chromium catalysts, but the low pressure units use highly active copper catalysts. Liquid

entrained micrometer-sized catalysts have been developed that can convert as much as 25 % perpass. Contact of the synthesis gases with hot iron catalyzes competing reactions and also forms

volatile iron carbonyl that fouls the copper catalyst. Some reactors are lined with copper.

Because the catalyst is sensitive to sulfur, the gases are purified by one of several sulfur-removing

processes, then are fed through heat exchangers into one of two types of reactors. With bed-in-

place reactors, steam at around 4.5 kPa, in quantity sufficient to drive the gas compressors, can be

generated.

Reaction vessels are usually of two types: one in which the contents are agitated or stirred in some


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such as solids or liquids that need to be brought into intimate contact with the catalyst and the

hydrogen. The second type is used where the substance may have sufficient vapor pressure at the

temperature of operation so that a gas-phase as well as a liquid-phase reaction is possible. It is also

most frequently used in continuous operation where larger quantities of material need to be

processed than can be done conveniently with batch methods.

In hydrogenation processes, heating of the ingoing materials is best accomplished by heat exchange

with the outgoing materials and adding additional heat by means of high-pressure pipe coils. A pipe

coil is the only convenient and efficient method of heating, for the reactor is usually so large that

heating it is very difficult. It is usually better practice to add all the heat needed to the materials

before they enter the reactor and then simply have the reactor properly insulated thermally.

Hydrogenation reactions are usually exothermic, so that once the process is started; the problem

may be one of heat removal. This is accomplished by allowing the heat of reaction to flow into the

ingoing materials by heat exchange in the reactor, or, if it is still in excess, by recycling and cooling

in heat exchangers the proper portion of the material to maintain the desired temperature.


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Dehydrogenation

Dehydrogenation is a reaction that results in the removal of hydrogen from an organic compound

or compounds, as in the dehydrogenation of ethane to ethylene:

CH3CH3 CH2=CH2 + H2

This process is brought about in several ways. The most common method is to heat hydrocarbons

to high temperature, as in thermal cracking, that causes some dehydrogenation, indicated by the

presence of unsaturated compounds and free hydrogen.

In the chemical process industries, nickel, cobalt, platinum, palladium, and mixtures containing

potassium, chromium, copper, aluminum, and other metals are used in very large-scale

dehydrogenation processes.

Styrene is produced from ethylbenzene by dehydrogenation. Many lower molecular weight

aliphatic ketones are made by dehydration of alcohols. Acetone, methyl ethyl ketone, and

cyclohexanone can be made in this fashion.

Acetone is the ketone used in largest quantity and is produced as a by-product of the manufacture

of phenol via cumene. Manufacture from iso-propanol is by the reaction:

(CH3)2CHOH (CH3)2C=O H2

This reaction takes place at 350oC and 200 kPa with copper or zinc acetate as the catalyst;

conversion is 85 to 90 percent. Purification by distillation follows.

The dehydrogenation of n-paraffins yields detergent alkylates and n-Olefins. The catalytic use of

rhenium for selective dehydrogenation has increased in recent years since dehydrogenation is one

of the most commonly practiced of the chemical unit processes.

Dehydrogenation of Ethyl Benzene

Styrene is most commonly produced by the catalytic dehydrogenation of ethyl benzene. Ethybenzene is mixed in the gas phase with 1015 times its volume in high-temperature steam, and

passed over a solid catalyst bed. Most ethyl benzene dehydrogenation catalysts are based on iron

(III) oxide (Ferric oxide), promoted by several percent potassium oxide or potassium carbonate. On

this catalyst, an endothermic, reversible chemical reaction takes place.

C6H5CH2CH3 C6H5CH=CH2 + H2


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+ H2

Steam serves several roles in this reaction. It is the source of heat for powering the endothermic

reaction, and it removes coke that tends to form on the iron oxide catalyst. The potassium

promoter enhances this decoking reaction. The steam also dilutes the reactant and products,

shifting the position of chemical equilibrium towards products. A typical styrene plant consists of

two or three reactors in series, which operate under vacuum to enhance the conversion and

selectivity. Typical per-pass conversions are 65%. The main byproducts are benzene and toluene.

Because styrene and ethyl benzene have similar boiling points (145 and 136 C, respectively), their

separation requires tall distillation towers and high return/reflux ratios.


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Oxidation

Oxidation is the interaction between oxygen molecules and all the different substances they may

contact, from metal to living tissue. Sometimes oxidation is not such a bad thing, as in the formationof super-durable anodized aluminum. Other times oxidation can be destructive, such as the rusting

of an automobile or the spoiling of fresh fruit.

Oxidation is the addition of oxygen to an organic compound or, conversely, the removal of

hydrogen.

Reaction control is the major issue during oxidation reactions. Only partial oxidation is required for

conversion of one organic compound into another or complete oxidation to carbon dioxide and

water will ensue.

Oxidizing agent

The most common oxidation agent is air, but oxygen is frequently used. Chemical oxidizing agents

(nitric acid, dichromates, permanganates, chromic anhydride, chlorates, and hydrogen peroxide) are

also often used.

An oxidizing agent (also called an oxidant or oxidizer) can be defined as either:

A chemical compound that readily transfers oxygen atoms, or

A substance that gains electrons in a Redox chemical reaction

Common Oxidizing Agents are:

1. Hypochlorite (ClO-)and other hypohalite compounds such as Bleach

2. Iodine (I, I3

)and other halogens

3. Chlorite, chlorate, perchlorate, and other analogous halogen compounds

4. Permanganate salts (MnO4)

5. Ammonium cerium(IV) nitrate and probably related Cerium(IV) compounds [(NH4)2Ce(NO3)6]

6. Peroxide compounds (HOO)

7. Sulfoxides R-S(=O)-R',

oxidation state 1 +1 +3 +5 +7

anion name chloride hypochlorite chlorite chlorate perchlorate

formula Cl

ClO

ClO2

ClO3

ClO4


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8. Persulfuric acid H2SO5

9. Ozone

10. Nitric acid

11. Nitrous oxide (N2O)

As examples of oxidation processes, two processes are available for the manufacture of phenol, and

both involve oxidation. The major process involves oxidation of cumene (C9H12 isopropyl benzene) to

cumene hydroperoxide, followed by decomposition to phenol and acetone. A small amount of

phenol is also made by the oxidation of toluene to benzoic acid, followed by decomposition of the

benzoic acid to phenol.

Liquid Phase Oxidation

Liquid-phase reactions in which oxidation is secured by the use of oxidizing compounds need nospecial apparatus in the sense of elaborate means for temperature control and heat removal. There

is usually provided a kettle form of apparatus, closed to prevent the loss of volatile materials and

fitted with a reflux condenser to return vaporized materials to the reaction zone, with suitable

means for adding reactants rapidly or slowly as may be required and for removing the product, and

provided with adequate jackets or coils through which heating or cooling means may be circulated

as required.

In the case of liquid-phase reactions in which oxidation is secured by means of atmospheric

oxygenfor example, the oxidation of liquid hydrocarbons to fatty acids special means must be

provided to secure adequate mixing and contact of the two immiscible phases of gaseous oxidizingagent and the liquid being oxidized.

Mixing may be obtained by the use of special distributor inlets for the air, designed to spread the air

throughout the liquid and constructed of materials capable of withstanding temperatures that may

be considerably higher at these inlet ports than in the main body of the liquid. With materials that

are sensitive to overoxidation mixing may be provided by the use of mechanical stirring or frothing

of the liquid.

Vapor Phase Oxidation

By the very nature, the vapor-phase oxidation processes result in the concentration of reaction heat

in the catalyst zone, from which it must be removed in large quantities at high-temperature levels.

Removal of heat is essential to prevent destruction of apparatus, catalyst, or raw material, and

maintenance of temperature at the proper level is necessary to ensure the correct rate and degree

of oxidation. With plant-scale operation and with reactions involving deep-seated oxidation, removal

of heat constitutes a major problem. With limited oxidation, however, it may become necessary to

supply heat even to oxidations conducted on a plant scale.


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Stereochemistry

Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of

atoms within molecules. An important branch of stereochemistry is the study of chiral molecules.

Stereochemistry is a hugely important facet of chemistry and the study of stereochemical problemsspans the entire range of organic, inorganic, biological, and physical chemistries.

Stereochemistry includes methods for determining and describing these relationships; the effect on the

physical or biological properties these relationships impart upon the molecules in question, and the

manner in which these relationships influence the reactivity of the molecules in question (dynamic

stereochemistry).


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In chemistry, cis-trans isomerism or geometric isomerism is a form of stereoisomerism describing the

orientation of functional groups within a molecule. In general, such isomers contain double bonds,

which cannot rotate. There are two forms of a cis-trans isomer, the cis and trans versions. When the

substituent groups are oriented in the same direction, the diastereomer is referred to as cis, whereas,

when the substituents are oriented in opposing directions, the diastereomer is referred to as trans.


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A stereocenter is defined as an atom bearing groups of such nature that an interchange of any two

groups will produce a stereoisomer.

A tetrahedral atom with four different groups attached to it is a stereocenter (chiral center, stereogenic

center)


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A demonstration of chirality of a generalized molecule containing one tetrahedral stereocenter. (a) The

four different groups around the carbon atom in III and IV are arbitrary. (b) III is rotated and placed in

front of a mirror. III and IV are found to be related as an object and its mirror image. (c) III and IV are not

superposable; therefore, the molecules that they represent are chiral and are enantiomers.

2-Propanol (V) and its mirror image (VI), (b) When either one is rotated, the two structures aresuperposable and so do not represent enantiomers. They represent two molecules of the same

compound. 2-Propanol does not have a stereocenter.


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THE BIOLOGICAL IMPORTANCE OF CHIRALITY

The human body is structurally chiral.

Helical seashells are chiral, and most spiral like a right-handed screw.

Many plants show chirality in the way they wind around supporting structures.

All but one of the 20 amino acids that make up naturally occurring proteins are chiral, and all of

them are classified as being left handed (S configuration).

The molecules of natural sugars are almost all classified as being right handed (R configuration),

including the sugar that occurs in DNA.

DNA has a helical structure, and all naturally occurring DNA turns to the right.

Limonene: S-limonene is responsible for the odor of lemon, and the R-limonene for the odor of orange.

Asparagine: A crystalline amino acid found in proteins and in many plants.


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TESTS FOR CHIRALITY: PLANES OF SYMMETRY1. Superposibility of the models of a molecule:

1) If the models are superposable, the molecule that they represent is achiral.

2) If the models are nonsuperposable, the molecules that they represent are chiral.

2. The presence of a single tetrahedral stereocenter chiral molecule.

3. The presence of a plane of symmetry achiral molecule

1) A plane of symmetry (also called a mirror plane) is an imaginary plane that bisects a molecule in such

a way that the two halves of the molecule are mirror images of each other.


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DESIGNATION OF STEREOCENTER

2-Butanol (sec-Butyl alcohol):

R. S. Cahn (England), C. K. Ingold (England), and V. Prelog (Switzerland)

devised the (RS) system (Sequence rule) for designating the configuration of

chiral carbon atoms.

(R) and (S) are from the Latin words rectus and sinister:

i) R configuration: clockwise (rectus, right)

ii) S configuration: counterclockwise (sinister, left)

THE (R-S) SYSTEM: (CAHN-INGOLD-PRELOG SYSTEM)1. Each of the four groups attached to the stereocenter is assigned a priority.

1) Priority is first assigned on the basis of the atomic number of the atom that is directly attached to the

stereocenter.

2) The group with the lowest atomic number is given the lowest priority, 4; the

group with next higher atomic number is given the next higher priority, 3; and so on.

3) In the case of isotopes, the isotope of greatest atomic mass has highest priority.

2. Assign a priority at the first point of difference.

1) When a priority cannot be assigned on the basis of the atomic number of theatoms that are diredtly

attached to the stereocenter, then the next set of atoms in the unassigned groups are examined.

3. View the molecule with the group of lowest priority pointing away from us.

1) If the direction from highest priority (4) to the next highest (3) to the next (2) is clockwise, the

enantiomer is designated R.

2) If the direction is counterclockwise, the enantiomer is designated S.


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PROPERTIES OF ENANTIOMERS: OPTICAL ACTIVITY1. Enantiomers have identical physical properties such as boiling points, melting points, refractive

indices, and solubilities in common solvents except optical rotations.

2) Enantiomers have identical infrared spectra, ultraviolet spectra, and NMR (nuclear magnetic

resonance) spectra if they are measured in achiral solvents.

3) Enantiomers have identical reaction rates with achiral reagents.

4) Enantiomers show different rates of reaction toward other chiral molecules.

5) Enantiomers show different solubilities in chiral solvents that consist of a single enantiomer or an

excess of a single enantiomer.


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ENANTIOSELECTIVE SYNTHESESEnantioselective:

Enantioselective organocatalysis has emerged as a powerful synthetic paradigm that is complementary

to metal-catalysed transformations and has accelerated the development of new methods to make

diverse chiral molecules. The operational simplicity, ready availability of catalysts and low toxicity

associated with organocatalysis makes it an attractive method to synthesise complex structures.

1) In an enantioselective reaction, one enantiomer is produced predominantly over its mirror image.

2) In an enantioselective reaction, a chiral reagent, catalyst, or solvent must assert an influence on the

course of the reaction.

2. Enzymes:

1) In nature, where most reactions are enantioselective, the chiral influences come from protein

molecules called enzymes.

2) Enzymes are biological catalysts of extraordinary efficiency.

i) Enzymes not only have the ability to cause reactions to take place much more rapidly than they would

otherwise, they also have the ability to assert a chiral influence on a reaction.

ii) Enzymes possess an active site where the reactant molecules are bound, momentarily, while the

reaction take place.

iii) This active site is chiral, and only one enantiomer of a chiral reactant fits it properly and is able to

undergo reaction.

Enzyme-catalyzed organic reactions: (Example)Hydrolysis of esters:

Hydrolysis, which means literally cleavage (lysis) by water, can be carried out in a variety of ways that do

not involve the use of enzyme. Lipase catalyzes hydrolysis of esters:

1) Use of lipase allows the hydrolysis to be used to prepare almost pure enantiomers.

2) The (R) enantiomer of the ester does not fit the active site of the enzyme and is, therefore,

unaffected.


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3) Only the (S) enantiomer of the ester fits the active site and undergoes hydrolysis.

SEPARATION OF ENANTIOMERS: RESOLUTIONHow are enantiomers separated?

1) Enantiomers have identical solubilities in ordinary solvents, and they have identical boiling

points.

2) Conventional methods for separating organic compounds, such as crystallization and distillation,

fail to separate racemic mixtures.

Resolution via Different Methods:

1) Diastereomers, because they have different melting points, different boiling points, and

different solubilities, can be separated by conventional methods.

2) Resolution via Molecular Complexes, Metal Complexes.

3) Chromatographic Resolution:

4) Kinetic Resolution:


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Chemical Kinetics - Reaction Rates

Chemical kinetics is the branch of chemistry which addresses the question: "how fast do reactions go?"

Chemistry can be thought of, at the simplest level, as the science that concerns itself with making new

substances from other substances.

If Chemistry is making new substances out of old substances (i.e., chemical reactions), then there are two

basic questions that must be answered:

1. Does the reaction want to go? This is the subject of chemical thermodynamics.

2. If the reaction wants to go, how fast will it go? This is the subject of chemical kinetics.

Thermodynamics is not the whole story in chemistry. Not only do we have to know whether a reaction is

thermodynamically favored, we also have to know whether the reaction can or will proceed at a finite

rate. The study of the rate of reactions is called chemical kinetics.

Reaction Rates

Consider the reaction,

2 NO(g) + O2(g) 2 NO2(g)

We can specify the rate of this reaction by telling the rate of change of the partial pressures of one the

gases. However, it is convenient to convert these pressures into concentrations, so we will write our rates

and rate equations in terms of concentrations, where square brackets, [ ], mean concentration in mol/L.

We might try to write the rate variously as,

or as

For a general reaction,

aA + bB cC + dD

the reaction velocity can be written in a number of different but equivalent ways,

As in our previous example, the negative signs account for material that is being consumed in the

reaction and the positive signs account for material that is being formed in the reaction.


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The Rate Law

For the general expression of a chemical reaction, aA + bB cC + dD, the rate law is expressedas the rate law constant, k, times the reactant concentrations raised to the power of the order of

each reactant.Rate = k [A]

x[B]

y

It is tempting to assign x = a, y = b, etc. for the reactant orders, but this is generally not a valid

assumption. Reaction order often does not correlate to reactant coefficient. Rate measurements

are needed to determine the values of x and y. The reactions is described as being first, second,

etc. order in A and first, second etc. order in B. The allover reaction order is the sum of x + y+ Reactant orders are typically 0, 1, 2, 3, or sometimes 0.5.

A zero order reactant means that the rate of the reaction is not influenced by the concentration of

that particular reactant.

First-Order Reactions

The rate of a first-reaction depends only on the concentration of one reactant, A. A productsBy definition, rate = - [B]/ t and rate = k[B]Therefore, - [B]/ t = k[B] d[B]/dt = -kt (Instantaneous rate of change described by calculus.)

ln([B]/[ Bo]) = - kt (Calculus solution to the equation by integration)

ln[B] - ln[Bo] = - kt orln[B] = - kt + ln[Bo]

This is a useful equation for graphing since it is in the form y = mx + b

Half-life

The half-life, usually symbolized by t1/2, is the time required for [B] to drop from its initial value

[B]o to [B]o/2.

Using the integrated form of the first order rate law we find that


56/57


Taking the logarithm of both sides gives,

Example: For a reaction with k = 5.50 x 10-3

s-1

at 45.0 C, what will be the concentration of A

remaining after 12.0 min if the initial concentration of A is 0.200 M?

ln[A] = - kt + ln[Ao] = - 5.50 x 10-3

s-1

(12.0 min x 60 s/1 min) + ln (0.200)ln[A] = - 3.96 + - 1.61 = - 5.57

[A] = 3.81 x 10-3

M.

The half-life of a reaction is a useful parameter. The half-life of a first-order reaction is

characteristic of the reaction and is independent of the starting concentration of A.t = (1/k) ln [Ao]/[A]

t = 1/k ln (1/0.5) = (1/k) ln 2 = (1/k) 0.693 = 0.693/kt = 0.693/k or k = 0.693/t

Second Order Reactions

Second order reactions in A can be easily described mathematically.

For second order reactions in B:

2B products

rate = - [B]/ t, but now rate = k[B]2 or

=>

This equation can be integrated to give,

The graphable equation becomes:


57/57


1/[B] = 1/[ Bo] + kt; again, y = mx + b

The half-life equation can be obtained by substituting [ Bo] = [B]

The half-life, t then becomes,

t = 1/k[Bo]

This makes sense because a large rate constant leads to a faster reaction and short half -life. In second

order reactions, half-life does depend on the initial [B] because a higher [Bo] means more frequent

collisions between A molecules.

organic synthesis bysir faheem

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