Lecture 4:Lecture 4:12.4 Isomerism12.4 Isomerism

Learning Outcomes:Learning Outcomes:At the end of the lesson the At the end of the lesson the students should be able to :students should be able to :

Define isomerism.Define isomerism. Explain constitutional isomerism. Explain constitutional isomerism.

chain isomerschain isomerspositional isomerspositional isomers functional group isomerfunctional group isomer

Isomerism

Structural/Constitutional Isomerism

Stereoisomerism

Chain Isomerism

Positional Isomerism

Functional Group Isomerism

Diastreomer Enantiomer

cis-transisomerism

otherdiastereomers

IsomerismIsomerism is the existence of different is the existence of different

compounds with the same molecular compounds with the same molecular formula but different structural formula but different structural formulae.formulae.

Different structural formula that Different structural formula that have the same molecular formula are have the same molecular formula are called called isomerisomer

1) Constitutional isomers 1) Constitutional isomers (Structural isomers) (Structural isomers)

are isomers with the same are isomers with the same molecular formula but differ in molecular formula but differ in the order of attachment of the order of attachment of atoms.atoms.

2) Stereoisomers2) Stereoisomers are isomers with the same are isomers with the same

molecular formula but different molecular formula but different arrangement of atoms in spacearrangement of atoms in space

Constitutional isomerismConstitutional isomerismIsomerism resulting from different Isomerism resulting from different order of attachment of atoms.order of attachment of atoms.

Three types Three types

a)a) Chain/skeletal isomerismChain/skeletal isomerism

b)b) Positional isomerismPositional isomerism

c)c) Functional group isomerismFunctional group isomerism

CHCH33CHCH22CHCH22CHCH22CHCH33

CC55HH12 12

a) a) Chain/skeletal isomerismChain/skeletal isomerism The isomers differ in the carbon skeleton The isomers differ in the carbon skeleton (different carbon chain).(different carbon chain). They possess the same functional group and belong to the same homologous series.

Example:

CHCH33CHCHCHCH22CHCH33

CHCH33

CHCH33CCHCCH33

CHCH33

CHCH33

PentanePentane

2-methylbutane2-methylbutane

2,2-dimethylpropane2,2-dimethylpropane

2)Positional isomerism2)Positional isomerism

These isomers have a substituent These isomers have a substituent group/ functional group in different group/ functional group in different positions.positions.

ExamplesExamples CC33HH77ClCl

CHCH33CHCH22CHCH22ClCl 1-chloropropane1-chloropropane

2-chloropropane

CHCH33CHCHCHCH33

ClCl

CC44HH88

CC88HH1010

CH3

CH3

CH3

CH3

1,2-dimethylbenzene1,2-dimethylbenzene 1,3-dimethylbenzene1,3-dimethylbenzene

1-butene1-butene 2-butene2-butene

CHCH22==CHCHCHCH22CHCH33 CHCH33CHCH==CHCHCHCH33

CH3

C H 3

1,4-dimethylbenzene1,4-dimethylbenzene

CC66HH1313NN

CH3

H2N

CH3

NH2

CH3

NH2

CH2NH2

3)Functional group isomerism3)Functional group isomerism These isomers have different functional These isomers have different functional

groups and belong to different groups and belong to different homologous series with the same general homologous series with the same general formula.formula.

Different classes of compounds that Different classes of compounds that exhibit functional group isomerism :-exhibit functional group isomerism :-

General formulaGeneral formula Classes of Classes of compoundscompounds

CCnnHH2n+22n+2OO ; n > 1 ; n > 1 alcohol and etheralcohol and ether

CCnnHH2n2nOO ; n ≥ 3 ; n ≥ 3 aldehyde and ketonealdehyde and ketone

CCnnHH2n2nOO22 ; n ≥ 2 ; n ≥ 2 carboxylic acid and carboxylic acid and esterester

CCnnHH2n2n ; n ≥ 3 ; n ≥ 3 alkene and alkene and cycloalkanecycloalkane

Examples:Examples:CC22HH66OO

CC33HH66OO

CC33HH66OO22

CH3CH2CH

O

CH3CCH3

O

CH3COCH3

O

CH3CH2COH

O

ethanol dimethyl ether

propanal propanone

propanoic acid methyl ethanoate

CHCH33CHCH22OHOH CHCH33OCHOCH33

Exercise 1:Identify the relationship between the following pairs of compounds.

Identical pairs

Chain

Position

*

Position

Functional group

*

Exercise:

1.State how many are isomers with the following molecular formulae, identify the type of isomerism and draw the structural formula of the isomers.

a) C5H10

b) C5H10O2

c) CH3CH=C(Cl)CH3

d) C4H6Cl2

e) CH3CH2CH(OH)CH(Br)CH2CH3

Lecture 5Lecture 512.4 Isomerism12.4 Isomerism

Learning Outcomes:Learning Outcomes:At the end of the lesson the students At the end of the lesson the students should be able to :should be able to :

Define stereoisomerism.Define stereoisomerism. Describe Describe cis-transcis-trans isomerism due to isomerism due to

restricted rotation about C=C bond restricted rotation about C=C bond and Cand CC bond in cyclic compoundsC bond in cyclic compounds

Identify Identify cis-transcis-trans isomerism of a isomerism of a given structural formula.given structural formula.

Stereoisomerism / Optical Stereoisomerism / Optical IsomerismIsomerism

Isomerism that resulting from Isomerism that resulting from different spatial arrangement of different spatial arrangement of atoms in molecules.atoms in molecules.

Two subdivisions of stereoisomers:Two subdivisions of stereoisomers:

i)i) Diastereomers (non-mirror Diastereomers (non-mirror image) image)

ii)ii) Enantiomers (mirror image)Enantiomers (mirror image)

DiastereomerDiastereomerCis-TransCis-Trans Isomerism Isomerism The requirements for geometric The requirements for geometric

isomerism :isomerism :

i) Restricted rotation about a i) Restricted rotation about a C=C,double bondC=C,double bond in in alkenesalkenes, or a , or a C-C single bondC-C single bond in in cycliccyclic compounds.compounds.

ii) Each carbon atom of a site of ii) Each carbon atom of a site of restricted rotation has restricted rotation has two different two different groupsgroups attached to it.attached to it.

ExamplesExamples

C CCH3

H

H

H3CC C

CH3

H

H3C

H

H

CH3

H

CH3

trans-2-trans-2-butenebutene ciscis-2-butene-2-butene

ciscis-1,2-dimethylcyclohexane-1,2-dimethylcyclohexanetranstrans-1,2-dimethylcyclohexane-1,2-dimethylcyclohexane

CH3

CH3

H

H

If one of the doubly bonded If one of the doubly bonded carbons has carbons has 2 identical groups2 identical groups, , geometric isomerism is not geometric isomerism is not possiblepossible..

ExampleExample

No No cis – transcis – trans isomer isomer

C CCH3

H

H3C

H3C

CH3CH2 H H CH3

C=C C=C

H3C H Cl CH3

2-methyl-2-butene 1-chloro-2-methylpropene

No No cis – transcis – trans isomer isomer

Lecture 6Lecture 612.4 Isomerism12.4 Isomerism

Learning Outcomes:Learning Outcomes:At the end of the lesson the students At the end of the lesson the students should be able to :should be able to :

Identify Identify cis-transcis-trans isomerism of a given isomerism of a given structural formula.structural formula.

Define chirality centre and enantiomers. Define chirality centre and enantiomers. Identify chirality centre in a molecule.Identify chirality centre in a molecule. Explain optical activity of a compound.Explain optical activity of a compound. Draw a pair of enantiomers using 3-Draw a pair of enantiomers using 3-

dimensional formula.dimensional formula. Define racemate.Define racemate. State the applications of chiral compounds State the applications of chiral compounds

in daily life.in daily life.

Optical IsomerismOptical Isomerism Optically active compoundsOptically active compounds

have the ability to rotate have the ability to rotate plane-polarized light to the plane-polarized light to the right (dextrorotary) and to the right (dextrorotary) and to the left (levorotary)left (levorotary)

The angle of rotation can be The angle of rotation can be measured with an instrument measured with an instrument called called polarimeter.polarimeter.

EnantiomEnantiomerer

Unpolarized light Polarized light

Dextrorotatory (+) Levorotatory (-)

The requirements for optical The requirements for optical isomerism :-isomerism :-

i)i) Molecule contains a Molecule contains a chiral carbonchiral carbon or or chirality centre or stereogenic centre chirality centre or stereogenic centre (a sp(a sp33-hybridized -hybridized carbon atom with 4 carbon atom with 4 different groups attached to it)different groups attached to it)

ii) Molecule is not superimposable with ii) Molecule is not superimposable with its mirror image.its mirror image.

P

CQR

S*

PQRS*designates chiral centre PQRS*designates chiral centre

a pair of mirror-image that are not a pair of mirror-image that are not superimposable.superimposable.

Example:-Example:- i)i) 2-butanol , 2-butanol ,

C*

CH2CH3

H3C

OHH

C

CH2CH3

CH3HOH

CH3CHCH2CH3

OH

EnantiomersEnantiomers

EnantiomeEnantiomersrs

*

ii) 2-hydroxypropanoic acidii) 2-hydroxypropanoic acid,,

EnantiomersEnantiomers

*

C *

COOH

H

CH3

OHC

COOH

HHOCH3

- A compound with n chiral centers can have a maximum of 2n stereoisomers.

- If a molecule contains two or more chiral centers, diastereomers may exist.

- A compound with n chiral centers can have a maximum of 2n stereoisomers.

- If a molecule contains two or more chiral centers, diastereomers may exist.

Diastereomers are stereoisomers that are not mirror images of each other.Diastereomers are stereoisomers that are not mirror images of each other.

Racemate MixtureRacemate MixtureRacemate MixtureRacemate Mixture

A A racemic mixtureracemic mixture or or racemate isracemate is an an equimolar mixtureequimolar mixture of of enantiomers which is enantiomers which is optically optically inactiveinactive because the two because the two components rotate plane-polarized components rotate plane-polarized light equally (same degree of light equally (same degree of rotation) but in opposite directions. rotation) but in opposite directions.

Hence it does not give a net rotation Hence it does not give a net rotation of plane-polarized light.of plane-polarized light.

Applications of chiral Applications of chiral compounds in daily life.compounds in daily life.

(() Dopa is used for treatment ) Dopa is used for treatment of Parkinson’s disease but of Parkinson’s disease but (+) dopa is toxic to human.(+) dopa is toxic to human.

(S)(S)-Ibuprofen the popular -Ibuprofen the popular analgesic(the active analgesic(the active ingredient in motrin, advil, ingredient in motrin, advil, and many other nonaspirin and many other nonaspirin analgesics)analgesics)