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NOTES - Unit 10: Organic Chemistry

PART 1: Functional Groups & Naming of Organic CompoundsFormulas for organic compounds: empirical, molecular and structural

Empirical: simplest whole number ratio of the atoms it contains.Example: empirical formula of ethane, C2H6 is CH3

Molecular: actual number of atoms of each present. It can be deduced if both the empirical formula and relative molecular mass, Mr, are known.

Example: What is the molecular formula of a compound with relative molecular mass, M r, of 30 and an empirical formula of CH3?

Structural: representation of molecule showing how the atoms are bonded to each other. Full structural formula – graphic formula showing every bond and atom. Usually 90 and 180

angles are used to show the bonds because this is the clearest representation on a 2-dimensional page, although it is not the true geometry of the molecule. (MUST DRAW H’s!)

Condensed structural formula – often omits bonds where they can be assumed, and groups atoms together. It contains the minimum information needed to describe the molecule unambiguously.

Stereochemical formula – attempts to show the relative position of atoms and groups around carbon in three dimensions. The convention is that a bond coming out of the page is shown as a solid, enlarging wedge; a bond sticking behind the page is shown as a dotted line; a bond in the plane of the paper is a solid line.

o Examples : methanol, CH3OH ethene, C2H4

Example: different formulas applied to three compoundsformula ethane ethanoic acid glucoseempirical

molecular

full structural

condensed structural

IUPAC: International Union of Pure and Applied ChemistryInternational, non-governmental organization that is best known for its system of nomenclature, which is now recognized as the world authority in this field.

Rule #1: Identify the longest chain of carbon atomsa) The longest chain of carbon atoms gives the stem/root of the name as shown in the table below:

# of C-atoms in longest chain Stem in IUPAC name Example (C2H2n+2 for alkanes)1 meth- CH4, methane

2 eth- C2H6, ethane

3 prop- C3H8, propane

4 but- C4H10, butane

5 pent- C5H12, pentane

6 hex- C6H14, hexane

7 hept- C7H16, heptane

8 oct- C8H18, octane

9 non- C9H20, nonane

b) If two chains have equal lengths, pick the one with more branch points.

Rule #2: Number the carbons in the main chainNumber chain to minimize the position/number of the following in order of priority:

a) thing you’re naming the compound after (double bond if alkene; -OH group if alcohol, etc)note: for multiple double bonds -diene, -triene, -tetraene

b) first branch/substituent group c) If both ends have the same first branching number, then number chain to minimize position of second

branch (and then third and so on).d) if still in need of a tie breaker, minimize # of substituent group that comes first alphabetically

Note: in cyclic and aromatic (benzene derivatives) compounds, no number needed if only one substituent.

Rule #3: Identify the functional group and attach appropriate suffixNote: the name for the stem/root is derived from the longest carbon chain, which may include the carbon of the functional group.

Homologous series

Functional group Suffix in IUPAC name

Example of compound

alkane -ane

alkene -ene

alkyne -yne

alcohol -anol

nitrile -anenitrile

aldehyde -anal

ketone -anone

carboxylic acid -anoic acid

ester* -anoate

amine* -anamine

amide* -anamide

*tricky naming… described below:

Ester functional group*: organic salts where the alkyl group of the alcohol has replaced the hydrogen of the carboxylic acid. Their name puts the alkyl group first, followed by the name of the acid anion.

Example: C2H5COOCH3

Amine functional group*: primary amines have an –NH2 group which can undergo substitution by alkyl groups, giving rise to secondary and tertiary amines. These are named using N- to show the position of the substituents.

Example 1: (primary amine)

Example 2: (secondary amine)

Example 3: (tertiary amine)

Amide functional group*: acid derivatives where the –OH of the acid has been replaced by –N<Example 1: (primary amide)

Example 2: (secondary amide)

Example 3: (tertiary amide)

Indicating position of the functional group: shown by a number inserted before the functional group ending. The number refers to the carbon atom to which the functional group is attached when the chain is numbered starting at the end that will give the smallest number to the group.

Example 1: Example 2:

Sometimes a functional group can only be in one place, and in these cases we do not need to give a number to show its position.

Example 1: Example 2:

Rule #4: Identify the side chains or substituent groupsAssign number of carbon at point of attachment.Side chain/ substituent

group

Prefix in IUPAC name

Example of CompoundCondensed

formulaStructural formula name

-CH3 methyl- CH3CH(CH3)CH3 2-methylpropane

-C2H5 ethyl CH(C2H5)3 3-ethylpentane

-C3H7 propyl- CH(C3H7)3 4-propylheptane

-F, -Cl, -Br, -I

fluoro-, chloro-, bromo-, iodo-

CCl4 tetrachloromethane

-NH2 amino- CH2(NH2)COOH 2-aminoethanioic acid

Rule #5: Assemble name as a single word #, substituent, root, suffixa) List substituents alphabetically (i.e. butyl- before methyl-)b) If multiples of one substituent are present: “di-,” “tri-,” “tetra,” etc.

Note: “di-,” “tri-,” “tetra,” etc. aren’t part of alphabetical name (i.e. triethyl- before dimethyl-)c) punctuation: commas between numbers; hyphens between numbers and letters; merged into one word (exception: acid = word #2 for carboxylic acids)

EXAMPLES:

PART 2: Physical Properties & Isomerism (Structural- v. Stereo-)Homologous seriesSame general formula with neighboring members of the series differing by –CH2.

Examples:Alkanes: CnH2n+2 Alcohols: CnH2n+1OH

Boiling pointsLength of carbon chain

Structural isomers

Solubility in waterDetermined by polarity

Example: ethanol v. 1-hexanol (or hexan-1-ol)

Classification of alcohols & halogenoalkanesAlcohols and halogenoalkanes may be classified according to how many R- groups and how many hydrogen atoms are bonded to the carbon atom containing the functional group.

Primary (one R-group bonded to C atom)

Secondary (two R-groups bonded to C atom)

Tertiary (three R-groups bonded to C atom)

Unit 10: Organic Chemistry IB Topics 10 & 20 AP Chapters (Zumdahl): 21.4, 22(all) & beyond! AP Chapters 1-5

Low reactivity of alkanesC-C and C-H bonds

relatively strong low polarity

Only readily undergo combustion rxns w/ O2 and substitution rxns with halogens in the presence of high energy UV light

Structural isomerism: same molecular formula, but different structural formulasEx: n-butane and 2-methylpropane

Ex: 1-butene, 2-butene and methylpropene

Structural isomers v. stereoisomersStructural isomers: same molecular formula, but different structural formulasStereoisomers: same structural formula, but differ in their spatial arrangement

Geometrical isomerism Optical isomerism

Stereoisomerism1) Geometrical isomerism: occurs when rotation about a bond is restricted or prevented

Example: asymmetric non-cyclic alkenescis-2-butene v. trans-2-butene

Example: disubstituted cycloalkanescis- v. trans- form of 1,2-dichlorocyclopropane

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Physical and chemical properties of geometrical isomers Different physical properties: (common)

cis-1,2-dichloroethene: b.p.=60.3Ctrans-1,2-dichloroethene: b.p.=47.5C

Difference in both physical and chemical properties: (sometimes)Ex: heating of cis- v. trans- forms of 2-butene-1,4-dioic acid

Stereoisomerism2) Optical isomerism: shown by all compounds containing at least one asymmetric or chiral carbon atom within a

molecule (one that contains for different atoms or groups bonded to it).o Two isomers are known as enantiomers and are mirror images of each other (like your right and left hand).

Example:

o Enantiomers are optically active with plane polarized light (both rotate plane-polarized light)o Normal light consists of electromagnetic radiation vibrating in all planes – when passed through a polarizing

filter the transmitted waves only vibrate in one plane and the light is said to be plane-polarized.

Enantiomerso Apart from behavior toward plane-polarized light, enantiomers have identical physical properties.o Chemical properties of enantiomers are also identical except when they interact with other optically active

substances.o Often the case in the body, where different enantiomers can have completely different physiological

effects.o Optical activity of enantiomers can be detected and measured by an instrument called a polarimeter.o A polarimeter consists of a light source, two polarizing lenses, and between the lenses a tube to hold the sample

of the enantiomer.o Diagram:

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o If the analyzer has to be rotated clockwise, the enantiomer is said to be dextrorotatory (from the Latin, dexter, meaning “right”).

o If it has to be rotated counterclockwise, the enantiomer is said to be laevorotatory (from the Latin, laevus, meaning “left”).

o One enantiomer is thus known as the d-form and the other is the l-form.o If both enantiomers are present in equal amounts, the two rotations cancel each other out, and the mixture

appears to be optically inactive. Such a mixture is known as a racemic mixture.

Amino acid asparagine: H2N-CH(CH2CONH2)-COOHOne enantiomer tastes bitterOther enantiomer tastes sweet

Enantiomers: Critical considerations: Thalidomide

One enantiomer combats “morning sickness” (labeled as sleep inducing above)

Other enantiomer responsible for fetal abnormalities (labeled as teratogenic above) Drug released in 1958 by a German pharmaceutical company (sold OTC in some countries) Reports of severe adverse side effects began to appear in 1959 Later is was found that clinical trials revealed these problems as early as 1956, yet the company released the drug anyway Withdrawn from market in 1961 Has since been re-released (prescription only) to treat leprosy and certain types of cancer

PART 3: Reactions of Alkanes & AlkenesReactions of AlkanesCombustion of alkanes

All hydrocarbons burn in excess oxygen to give carbon dioxide and water:o CxHy + (x + y/4) O2 x CO2 + y/2 H2O

Although the C-C and C-H bonds (reactants) are strong, the C=O and O-H bonds (products) are stronger, so the rxn is very exothermic.

o Ex: natural gas (methane)

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o Ex: gasoline

Incomplete Combustion: If there is an insufficient supply of oxygen, incomplete combustion occurs and carbon monoxide and carbon are also produced as products.

Substitution Reactions Alkanes can react with halogens in the presence of UV light to form an acid and a substituted alkane.

o Ex:

o Ex:

Mechanism of chlorination of methane The mechanism of an organic reaction describes the individual steps. When a chemical bond breaks, it breaks heterolytically or homolytically. Heterolytic fission: both of the shared electrons go to one atom, resulting in a negative and a positive ion. Homolytic fission: each atoms keeps of the shared electrons, resulting in the formation of two free radicals. The halogen-halogen bond is weaker than the C-H and C-C bond in methane and can break homolytically in the

presence of UV light. This stage of the mechanism is called initiation:

Free radicals contain an unpaired electron and are highly reactive. When a chlorine free radicals come into contact with a methane molecule they combine with a hydrogen atom

to produce hydrogen chloride and a methyl radical. Since a new radical is produced, this stage of the mechanism is called propagation:

The methyl free radical is also extrememly reactive and reacts with a chlorine molecule to form the product and regenerate another chlorine radical.

Another propagation step:

In theory a single chlorine radical may cause up to 10,000 molecules of chloromethane to be formed. When two radicals react together, termination occurs:

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Further substitution can occur when chlorine radicals react with the substituted products. o For example:

The substitution can continue even further to produce trichloromethane and then tetrachloromethane. The overall mechanism is called free radical substitution.

o Note that in this mechanism hydrogen radicals, H•, are not formed.

Reactions of AlkenesAddition reactions

Bond enthalpies:o C-C 348 kJ mol-1

o C=C 612 kJ mol-1 (less than twice C-C enthalpy) Thus the C=C bond makes the molecule reactive. The most important reactions of alkenes are addition reactions, where an unsaturated reactant becomes

saturated.

Addition reactions include the addition of hydrogen, bromine, hydrogen halides and water.

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Uses of addition reactionso Bromination

Pure bromine is a red liquid, but has a yellow/orange color in sol’n. When a sol’n of bromine is added to an alkene, the product is colorless. Test for unsaturation: this decoloriation of bromine solution provides a useful test to indicate

the presence of an alkene group. o Hydration

Ethene is an important product formed during the cracking of oil. Although ethanol can be made from the fermentation of starch and sugars, much industrial

ethanol is formed from the addition of steam to ethene.o Hydrogenation

The addition of hydrogen to unsaturated vegetable oils is used industrially to make margarine. Hydrogenation reduces the number of double bonds in the polyunsaturated vegetable oils

present in the margarine, which causes it to become a solid at room temp (and have a longer shelf life).

Addition polymerization Under certain conditions, ethene can also undergo addition reactions with itself to form a long chain polymer

containing many thousands (typically 40,000 to 800,000) of carbon atoms.

These addition reaction scan be extended to other substituted alkenes to give a wide variety of different addition polymers.

o Example 1: formation of PVC

o Example 2: formation of PTFE (Teflon)

PART 4: Reactions of Alcohols; Substitution Reactions

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Reactions of AlcoholsCombustion

General eq’n for an alcohol combusting completely in oxygen:o CnH(2n+1)OH(l) + (2n+1)O2(g) nCO2(g) + (n+1)H2O(l)

Ethanol is used as both a solvent and as a fuel. It combusts completely in excess oxygen to produce carbon dioxide and water.

C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l) H=-1371 kJmol-1

Ethanol is already partially oxidized, so it releases less energy than burning and alkane of comparable mass. However, it can be obtained by the fermentation of biomass; thus, in some countries it is mixed with gasoline to

produce “gasohol” which decreases dependence on crude oil.Oxidation of alcohols

Ethanol can be readily oxidized by warming with an acidified sol’n of potassium dichromate (VI). During the process, the orange dichromate(VI) ion Cr2O7

2- is reduced from an oxidation state of +6 to the green Cr3+ ion. Breathalyzer test: blow into bag through tube of acidified KCr2O7 crystals. If orange crystals turn green, this

indicates presence of a lot of ethanol (high BAC). Ethanol is initially oxidized to ethanal. The ethanal is then oxidized further to ethanoic acid.

To stop rxn at ethanal stage, distill ethanal from the rxn mixture as soon as it is formed. If complete oxidation to ethanoic acid is desired, heat the mixture under reflux so that none of the ethanal can

escape.

Ethanol is a primary alcohol. The oxidation reactions of alcohols can be used to distinguish between primary, secondary and tertiary alcohols.

All primary (1) alcohols are oxidized by acidified KCr2O7, first to aldehydes then to carboxyllic acids.

All secondary (2) alcohols are oxidized to ketones, which cannot undergo further oxidation.

Tertiarary (3) alcohols cannot be oxidized by acidified K2Cr2O7 as they have no hydrogen atoms attached directly to the carbon atom containing the –OH group.

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It is not true to say that tertiary alcohols can never be oxidized, as they burn readily. However, when this happens, the carbon chain is destroyed.

Substitution reactions and reaction pathwaysSubstitution reactions of halogenoalkanes

Because of the greater electronegativity of the halogen atom compared with the carbon atom, halogenoalkanes have a polar bond.

Reagents that have a non-bonding pair of electrons are attracted to the carbon atom in halogenoalkanes and a substitution rxn occurs.

Such reagents are called nucleophiles.

Mechanism of nucleophilic substitution Primary halogenoalkanes (one alkyl group attached to the carbon atom bonded to the halogen)

o Example: CH3CH2Br + OH- CH3CH2Br + Br-

Determined experimentally: rate=k[C2H5Br][OH-] The proposed mechanism involves the formation of a transition state which involves both of the

reactants.

Because the molecularity of this single-step mechanism is two it is known as an SN2 mechanism (bimolecular nucleophilic substitution).

o Primary halogenoalkanes tend to react by an SN2 mechanism. (bimolecular nucleophilic substitution)

Tertiary halogenoalkanes (three alkyl groups attached to the carbon atom bonded to the halogen).o Example: C(CH3)3Br + OH- C(CH3)3OH + Br-

Determined experimentally: rate=k[C(CH3)3Br] A two-step mechanism is proposed that is consistent with this rate expression.

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The 1st step is rate-determining. Because the molecularity is one the mechanism is known as SN1 (unimolecular nucleophilic substitution).

o Tertiary halogenoalkanes tend to react by an SN1 mechanism. (unimolecular nucleophilic substitution)

What about secondary halogenoalkanes?o Proceed by a mixture of SN1 and SN2 mechanisms

You need to be able to deduce rxn pathways given the starting material and the product. Conversions with more than two stages will not be assessed. Reagents, conditions and equations should be included.

Example: deduce a reaction pathway for the conversion of 2-butene to butanone can be done in two stages: Step 1: 2-butene can be heated with steam and a catalyst to form 2-butanol.

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Step 2: 2-butanol can then be oxidized by heating with acidified KCr2O7 to form butanone.

PART 5: SN1&SN2; Elimination & Condensation RxnsNucleophilic Substitution

Common nucleophiles include OH- (from dilute NaOH sol’n), CN-, NH3 and H2O. The nucleophiles are attracted to the δ+ carbon atom and substitute the halogen atom in halogenoalkanes. Primary halogenoalkanes react by an SN2 mechanism. Tertiary halogenoalkanes react by an SN1 mechanism.

Factors affecting the rate of nucleophilic substitution 1) The nature of the nucleophile

o Effectiveness of nucleophile depends on its electron density. Anions tend to be more reactive than the corresponding neutral species.

Ex: the rate of substitution w/ OH- is faster than with H2O. Among species with same charge, a less electronegative atom carrying a non-bonded pair of

electrons is a better nucleophile than a more electronegative one. NH3 is a better nucleophile than H2O This is because the less electronegative atom can more easily donate its pair of

electrons as they are held less strongly. Example: Put the following nucleophiles on order of increasing reactivity: NH3, H2O, CN-, OH-

2) The nature of the halogeno For both SN1 an SN2 rxns, iodoalkanes react faster than bromoalkanes, which react faster than

chloroalkanes. C-I bond is weaker than the C-Cl bond and breaks more readily.

3) The nature of the halogenoalkaneo Tertiary react faster than secondary, which are faster than primary halogenoalkanes.o The SN1 route is faster than the SN2 route

SN1 involves formation of intermediate carbocation SN2 involves a transition state w/ relatively high activation energy

Substitution with ammonia and potassium cyanide Halogenoalkanes can react with NH3 to form amines and with CN- to form nitriles.

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With 1 halogenoalkanes the mechanism is SN2.o Example: bromoethane + CN-

o Example: bromoethane + NH3

The rxn with KCN provides a means for extending the C-chain length by one C. The nitrile can then be converted either into amines by reduction using H2 with a Ni catalyst or into carboxyllic

acid by acid hydrolysis.o Example:

Elimination and Condensation ReactionsElimination rxns of halogenoalkanes

Altering rxn conditions can cause same reactants to make completely different products.o Example:

With dilute NaOH sol’n, the OH- ion acts as a nucleophile and substitution occurs to produce an alcohol.

With hot alcoholic NaOH sol’n (i.e. NaOH dissolved in ethanol), elimination occurs and an alkene is formed.

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In this second rxn, OH- acts as a base. Elimination of HBr can proceed by either a carbocation or as a concerted process…

In the presence of ethanol there will also be some ethoxide ions present. Ethoxide is a stronger base than OH-, so the equilibrium lies to the left but some

ethoxide ions will be present and these may be the actual species acting as the base.

Condensation rxns Rxns between 2 molecules to form a larger molecule with the elimination of a small molecule such as H2O or

HCl.o alcohol + carboxylic acid ester + water

Example:

Esters Most have distinctive, pleasant, fruity smell Many uses:

o Used as natural and artificial flavoring agents in foodo Used as solvents in perfumeso used as plasticizers, substances used to modify the properties of polymers (i.e. PVC) by making them

more flexible. Phthalate esters (esters of phthalic acid) - Many health concerns!

More condensation rxnso Example: Show the general rxn for a carboxylic acid with an amine.

This rxn is important in biological rxns --- amino acids contain an amine group and a carboxylic acid group , thus amino acids can condense together in the presence of enzymes to form poly(amides).

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Condensation polymerization If each of the reactants in a condensation rxn contains two functional groups that can undergo condensation,

then the condensation can continue to form a polymer.o Example: benzene-1,4-dicarboxylic acid + ethane-1,2-diol

o Example: hexane-1,6-dioic acid + 1,6-diaminohexane

Example: Propose a process for the conversion of 1-bromopropane to 1-aminobutane (a.k.a. 1-butylamine) o Step 1:

o Step 2:

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Example: Propose a process for the conversion ethane into ethylamineo Step 1:

o Step 2:

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