alkane and cycloalkanes

ALKANE AND

CYCLOALKANES

SITI HAJAR ANAZIAH MUHAMAD

COURSE OUTCOME

Ability to explain the relationship between the structure, physical and chemical properties of the different bonds and functional groups in organic compounds (CO2)

Course Learning OutcomeThe student should be able to: -

Name alkanes.

Explain aliphatic properties.

Predict, draw and name the products of functional group reactions.

Draw the mechanistic pathway.

Fossil Fuels:

Many alkanes occur in

nature, primarily in

natural gas and

petroleum.

Natural gas is com-posed

largely of methane, with

lesser amounts of ethane,

propane and butane.

Petroleum is a complex mixture of compounds, most of which are hydrocarbons

containing one to forty carbon atoms. Distilling crude petroleum (called refining),

separates it into usable fractions that differ in boiling point.

gasoline: C5H12C12H26kerosene: C12H26C16H34diesel fuel: C15H32C18H38

Sources of Alkanes: Petroleum and Natural Gas

.

Petroleum is the source of alkanes. It is a complex mixture of

mostly alkanes and aromatic hydrocarbons with smaller amounts

of oxygen-, nitrogen-, and sulfur-containing compounds

Natural gas is a gaseous mixture of hydrocarbons recovered from

natural sources. It is mostly methane (CH4, BP -162oC) with small

amounts of ethane (C2H6, BP -88oC) and propane (C3H8, BP -42

o).

Petroleum Refining

Liquid petroleum and natural gas are usually separated at the

wellhead and shipped independently to processing (refining) plants.

The liquid petroleum (crude) is separated by distillation according to

the volatility (BPs) of the different size hydrocarbons. The fractions

collected by refining are still mixtures of hydrocarbons that have

commercial value.

Refining crude petroleum into usable fuel and other petroleum products.

(a) An oil refinery. At an oil refinery, crude petroleum is separated into

fractions of similar boiling point by the process of distillation.

(b) Schematic of a refinery tower. As crude petroleum is heated, the lower-

boiling, more volatile components distill first, followed by fractions of

progressively higher boiling point.

Hydrocarbon Fractions from Petroleum

boiling range

of fraction (oC)

size range name and use

below 20 C1 to C4natural gas, bottledgas, petrochemicals

20 to 60 C5 to C6 petroleum "ether". solvents

60 to 100 C6 to C7 ligroin, solvents

40 to 200 C5 to C10 straight-run gasoline

175 to 325 C12 to C18 kerosene and jet fuel

250 to 400 C12 and highergas oil, fuel oil and diesel oil

nonvolatileliquids

C20 and higher mineral oil, lubricating oil

nonvolatile solids

C20 and higherparaffin wax, asphalt, tar

Petroleum Technologies

.

Technologies exist to interconvert the various hydrocarbons using

catalysts

Cracking is a process for breaking down larger alkanes into

smaller alkanes by heating. The mixture of larger alkanes is

heated in the absence of oxygen at high temperatures (~500oC) for

only a few minutes in the presence of catalysts. By this method,

alkanes of size C12 and larger can be turned into gasoline-size

alkanes (C5 to C10).

Isomerization

Since the 1920s, it has been known that highly branched alkanes

perform better in the internal combustion engine than straight-

chain alkanes. Catalytic isomerization changes straight-chain

alkanes into the more useful branched alkanes.

hexane

acid catalyst

+

branched alkanes

CH3CH2CH2CH2CH2CH3

CH3CH2CH2CHCH3CH3

CH3CH2CHCH2CH3CH3

Catalytic Reforming

Alkanes are transformed into cycloalkanes and aromatic

hydrocarbons by catalytic reforming.

heptane

silica-alumina

catalyst, 500oC

20 atm H2

+ 4H2CH3CH2CH2CH2CH2CH2CH3

CH3

The aromatic hydrocarbons produced by catalytic reforming

are used as additives in gasoline and as starting materials for

the petrochemical industry. Production of these aromatics is in

the billions of pounds per year in the United States.

Crude

PetroleumRefining

straight-

chain

alkanes

of

different

sizes

Cracking

Isomerization

Reforming

smaller alkanes

branched alkanes aromatics

Petroleum Products

Daily consumption of petroleum in the United States is over 17

million barrels which amounts to close to one billion tons per year.

Of this total, approximately 43% goes into gasoline, another 25%

into fuel oil, and approximately 7.5% into jet fuel. Thus, about

75% of all the petroleum consumed is burned as a source of

energy. The remainder is used as "feedstock" for polymers (~4%)

and the chemical industry (~3%), and the many other products

used in our society such as oils, lubricants and asphalt.

An Overview of Petroleum Refining

Combustion

All hydrocarbons undergo combustion, the reaction with oxygen

that liberates energy. Thus, all hydrocarbons are potential "fuels",

materials that burn in oxygen releasing energy.

Heat of Combustion

The heat of combustion (Hcomb) is the amount of heat liberated

when one mole of material undergoes combustion at 1 atm pressure

to produce gaseous CO2 and liquid water.

CH4 + 2O2 CO2(g) + 2H2O(l)methane

Hcomb = -882 kJ/mol (or -55.1 kJ/g)

For a gasoline-size hydrocarbon::

2C8H18 + 25O2 16CO2(g) + 18H2O(l)

Hcomb = -5452 kJ/mol (or -47.8 kJ/g)

Note, the total amount of heat liberated increases with the size of

the hydrocarbon, but that doesn't make it a better fuel. On a per

weight basis, methane is a better fuel than the octane.

Gasoline Performance: The Octane Rating

The combustion of alkanes is a complicated reaction probably

involving free radicals. Much attention has been directed towards

the combustion of hydrocarbons in the internal combustion engine.

Since the 1920s, it has been known that some hydrocarbons tend to

give better performance during combustion. Some fuels cause

"knocking", the premature ignition of the fuel before the piston is

in the firing position for a power stroke. Knocking causes loss of

power.

Branched hydrocarbons were found to perform better than straight-

chain alkanes in the internal combustion engine. In 1927, an

arbitrary engineering performance standard was developed called

"the octane rating." The performance of the branched alkane

"isooctane" (actually 2,2,4-trimethylpentane) in a specific internal

combustion engine was given a rating of 100. Heptane, which

causes severe knocking, was given a rating of 0.

A fuel with a

performance equivalent

to a mixture of 75%

isooctane and 25%

heptane is given an

octane rating of 75.

"isooctane"

100

heptane

0

CH3CCH2CHCH3CH3

CH3 CH3CH3CH2CH2CH2CH2CH2CH3

Octane Boosters

.

It is common practice to add octane boosters to gasoline to

improve the performance of the fuel. Many years ago,

tetraethyllead, (C2H5)4Pb, was an important additive for this

purpose. It is now illegal to use "leaded" gasoline in an

automobile in the United States. Aromatics and "oxygenated

fuels" are blended into gasoline to raise the octane rating

Some Octane Ratings of

Hydrocarbons and Additives

Octane Rating

heptane 0

1-pentene 91

2,2,4-trimethylpentane 100

benzene 106

methanol 107

ethanol 108

methyl t-butyl ether 116

toluene 118

Methyl t-butyl ether (MTBE) is an

oxygenated fuel blended into

gasoline to improve performance

and reduce air pollution.

Production of MTBE increased over

the past 10 years to many billions of

pounds per year in the United

States. However, MTBE is being

phased out because of

environmental and health concerns.

PHYSICAL PROPERTIES OF

ALKANES

Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups)

Connecting carbons can lead to large or small molecules

The formula for an alkane with no rings in it must be CnH2n+2 where the number of Cs is n

Alkanes are saturated with hydrogen (no more can be added)

They are also called aliphatic compounds

ALKANES AND ALKANE

ISOMERS

CH4 = methane, C2H6 = ethane, C3H8= propane

The molecular formula of an alkane with more than three carbons can give more than one structure

C4 (butane) = butane and isobutane

C5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane

Alkanes with Cs connected to no more than 2 other Cs are straight-chain or normal alkanes

Alkanes with one or more Cs connected to 3 or 4 Cs are branched-chain alkanes

ALKANE ISOMERS

Isomers that differ in how their atoms are arranged in chains are called constitutional isomers

Compounds other than alkanes can be constitutional isomers of one another

They must have the same molecular formula to be isomers

CONSTITUTIONAL ISOMERS

We can represent an alkane in a brief form or in many types of extended form

A condensed structure does not show bonds but lists atoms, such as

CH3CH2CH2CH3 (butane)

CH3(CH2)2CH3 (butane)

Structural formulas

CONDENSED STRUCTURES OF

ALKANES

NAMING STRAIGHT CHAIN

ALKANES

Alkyl group remove one H from an alkane (a part of a structure) General abbreviation R (for Radical, an incomplete species or the rest

of the molecule)

Name: replace -ane ending of alkane with yl ending -CH3 is methyl (from methane)

-CH2CH3 is ethyl from ethane

ALKYL GROUPS

ALKYL GROUPS

(CONTINUED)

Classified by the connection site

TYPES OF ALKYL GROUPS

* There is no 4 hydrogenWhy or why not? Lets talk about this

ALKYL GROUPS

(CONTINUED)

Hydrogen atoms are classified as primary (10), secondary (20), or tertiary (30) depending on the type of carbon atom to which they are bonded

CYCLOALKANES

Cycloalkanes have molecular formula CnH2n and contain carbon atoms

arranged in a ring. Simple cycloalkanes are named by adding the prefix

cyclo- to the name of the acyclic alkane having the same number of

carbons.

Compounds are given systematic names by a process that uses

Follows specific rules

Find parent hydrocarbon chain

NAMING ALKANES

1. Find the parent carbon chain and add the suffix.

Note that it does not matter if the chain is straight or it bends.

IUPAC systematic Nomenclature -

Alkanes

Also note that if there are two chains of equal length, pick the

chain with more substituents. In the following example, two

different chains in the same alkane have seven C atoms. We

circle the longest continuous chain as shown in the diagram

on the left, since this results in the greater number of

substituents.

2. Number the atoms in the carbon chain to give the first substituent the

lowest number.

NAMING ALKANES

(CONT..)

If the first substituent is the same distance from both ends, number the

chain to give the second substituent the lower number.

NAMING ALKANES

(CONTINUED)

When numbering a carbon chain results in the same numbers from either

end of the chain, assign the lower number alphabetically to the first

substituent.

NAMING ALKANES

(CONTINUED)

3. Name and number the substituents.

Name the substituents as alkyl groups.

Every carbon belongs to either the longest chain or a substituent, not both.

Each substituent needs its own number

If two or more identical substituents are bonded to the longest chain, use prefixes to indicate how many: di- for two groups, tri- for three groups,

tetra- for four groups, and so forth.

NAMING ALKANES

(CONTINUED)

4. Combine substituent names and numbers + parent and

suffix.

Precede the name of the parent by the names of the substituents.

Alphabetize the names of the substituents, ignoring all prefixes except iso, as in isopropyl and isobutyl.

Precede the name of each substituent by the number that indicates its location.

Separate numbers by commas and separate numbers from letters by hyphens. The name of an alkane is a single word, with no spaces after

hyphens and commas.

NAMING ALKANES

(CONTINUED)

NAMING ALKANES

(CONTINUED)

Substituents are identified and numbered

A)

B)

NAMING ALKANES

(CONTINUED)

CYCLOALKANES

NAMING

Cycloalkanes are named by using similar rules, but the prefix cyclo-

immediately precedes the name of the parent.

1. Find the parent cycloalkane.

2. Name and number the substituents. No number is needed to

indicate the location of a single substituent.

For rings with more than one substituent, begin numbering at one

substituent and proceed around the ring to give the second substituent the

lowest number.

CYCLOALKANES NAMING (CONT..)

With two different substituents, number the ring to assign the

lower number to the substituents alphabetically.

Note the special case of an alkane composed of both a ring and a long

chain. If the number of carbons in the ring is greater than or equal to the

number of carbons in the longest chain, the compound is named as a

cycloalkane.


Some organic compounds are identified using common names that do not

follow the IUPAC system of nomenclature. Many of these names were

given long ago before the IUPAC system was adopted, and are still widely

used. Additionally, some names are descriptive of shape and structure, like

those below:

NomenclatureCommon Names


REACTION OF ALKANES

Combustion of Alkanes

Alkanes undergo combustionthat is, they burn in the presence of oxygen to form carbon dioxide and water.

This is an example of oxidation. Every CH and CC bond in the starting material is converted to a CO bond in the product.

Halogenation of Alkanes

C H + X2

250-400o

or hC X + HX

Reactivity:- X2 : F2 > Cl2 > Br2 (> I2)

H : 3o > 2

o > 1

o > H3C-H

Chlorination - a substitution reaction

CH4 + Cl2 h

or

CH3Cl + HCl

REACTION OF ALKANES (CONT..)

Polychlorination

CH3Cl + Cl2 CH2Cl2 + HCl

CH2Cl2 + Cl2 CHCl3 + HCl

CHCl3 + Cl2 CCl4 + HCl

dichloromethane

methylene chloride

trichloromethane

chloroform

tetrachloromethane

carbon tetrachloride

Iodination and fluorination

iodine does not react while, fluorine reacts very readily

order of halogen reactivity: F2 > Cl2 > Br 2 (> I2)

REACTION OF ALKANES (CONT..)

E.G: CHLORINATION OF METHANE

Requires heat or light for initiation.

The most effective wavelength is blue, which is absorbed by chlorine gas.

Many molecules of product are formed from absorption of only one photon of light (chain reaction).

CHAPTER 4 45

THE FREE-RADICAL CHAIN REACTION

Initiation: Generates a radical intermediate.

Propagation: The intermediate reacts with a stable molecule to produce another reactive intermediate (and a product molecule).

Termination: Side reactions that destroy the reactive intermediate.

INITIATION STEP: FORMATION OFCHLORINE ATOM

A chlorine molecule splits homolytically into chlorine atoms (free radicals).

LEWIS STRUCTURES OF FREE RADICALS

Free radicals are reactive species with odd numbers of electrons.

PROPAGATION STEP: CARBON RADICAL

The chlorine atom collides with a methane molecule and abstracts (removes) an H, forming another free radical and one of the products (HCl).

PROPAGATION STEP: PRODUCTFORMATION

The methyl free radical collides with another chlorine molecule, producing the organic product (methyl chloride) and regenerating the chlorine radical.

TERMINATION STEPS

OVERALL REACTION

OVERALL REACTION

(CONT..)

54

The intermediate alkyl radical

The nature of the intermediate free radical determines the product:

CH3CH2CH3X

propane

CH3CH2CH2n-propyl radical

CH3CHCH3

isopropyl radical

CH3CH3X

CH3CH2

ethane ethyl radical

X2CH3CH2X

haloethane

X2CH3CH2CH2X

1-halopropane

X2CH3CHXCH32-halopropane

X2CH3X

halomethane

CH4X CH3

methane methyl radical

SYNTHESIS OF ALKANES AND

CYCLOALKANES

Hydrogenation of alkenes and alkynes

CnH2n CnH2n+2

H2

Pt, Pd or Ni

alkene alkane

H2/Ni

C2H5OH

25o, 50 atm

(CH3)3CH

Pt+ 2 H2

+ H2Pd

Reduction of alkyl halides

i. Hydrolysis with Grignard reagent

R-X + Mg R Mg X

RMgX + HOH R-H + Mg(OH)X

CH3CH2CH2MgBr + H2O CH3CH2CH3 + Mg(OH)Br

ii. Reduction of alkyl halide with metal and acid

(Zn in CH3COOH or HBr)

R-X R-H

CH3CHBrCH2CH3 CH3CH2CH2CH3

iii. Reaction with LiAlH4

C9H19CH2-Br C9H19CH3

Alkylation of terminal alkynes

An acetylenic hydrogen is weakly acidic:

C C HRNa

NH3C CR

-Na

+ + 1/2H2

a sodiumacetylide

(CH3)2CHC C HNaNH2

ether(CH3)2CHC C

-Na

+

+ NH3

Alkylation of terminal alkynes (cont..)

The anion formed will react with a primary halide:

C C- Na

+R + CH3X C CCH3 + NaXR

1. NaNH2

2. CH3Br

H2/Pt

Corey Posner Whitesides - House Synthesis

R-X + 2Lidiethyl ether

RLi + LiX

alkyllithium1o, 2

o,

or 3o

2RLi + CuI R2CuLi + LiI

lithium dialkylcupratea Gilman reagent

R2CuLi + R'X R-R' + RCu +LiX

1o alkyl or 2

o

cycloalkyl halide

alkane and cycloalkanes

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