Lecture 4:Lecture 4:12.4 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 isomerism.Define isomerism. Explain constitutional isomerism. Explain constitutional isomerism.

chain isomerschain isomers positional 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 have Different structural formula that have the same molecular formula are the same molecular formula are called called isomersisomers..

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

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

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

formula but different arrangement of formula but different arrangement of atoms in spaceatoms in space

Constitutional isomerismConstitutional isomerism

Isomerism 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) Chain/skeletal isomerisma) Chain/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

2)Positional isomerism2)Positional isomerism These isomers have a substituent group/ These isomers have a substituent group/

functional group in different positions.functional group in different 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=CHCH=CHCH22CHCH33 CHCH33CH=CHCHCH=CHCH33

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

ExamplesExamples

CC22HH66OO

CC33HH66OO

CC33HH66OO22

CH3CH2CH

O

CH3CCH3

O

CH3COCH3

O

CH3CH2COH

O

ethanol dimethyl ether

propanal propanone

propanoic acid methyl ethanoate

CHCH33CHCH22OHOH CHCH33OCHOCH33

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 isomerism :Stereoisomerism / optical isomerism :

Isomerism that resulting from Isomerism that resulting from different spatial arrangement of different spatial arrangement of atoms atoms

in molecules.in molecules.

Two subdivisions of stereoisomers:Two subdivisions of stereoisomers:

i)i) Enantiomers (mirror image)Enantiomers (mirror image)

ii)ii) Diastereomers (non-mirror image)Diastereomers (non-mirror image)

DiastereomerDiastereomer

Cis-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 C=C,double bond in alkenes, or a bond in alkenes, or a C-C single bond C-C single bond in cyclic compounds.in cyclic compounds.

ii) each carbon atom of a site of ii) each carbon atom of a site of restricted restricted rotation has two different rotation has two different groups groups 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 carbons has 2 If one of the doubly bonded carbons has 2 identical groups, geometric isomerism is identical groups, geometric isomerism is not possiblenot possible..

ExampleExample

No No cis – transcis – trans isomer isomer

C CCH3

H

H3C

H3C

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 compounds have the Optically active compounds have the

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

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

Enantiomer

Polarimeter

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

i)i) molecule contains a chiral carbon or molecule contains a chiral carbon or chirality centre or stereogenic centre (a chirality centre or stereogenic centre (a spsp33-hybridized carbon atom with 4 -hybridized carbon atom with 4 different groups attached to it)different groups attached to it)

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

P

CQR

S*

PQRS*designates chiral centre

EnantiomersEnantiomers 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

enantiomers

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

enantiomersenantiomers

COOH

HO H

CH3

COOH

H OH

CH3

12.4.9 Racemate12.4.9 Racemate

A A racemic mixtureracemic mixture or or racemate isracemate is an equimolar mixture of an equimolar mixture of enantiomers which is optically enantiomers which is optically inactive because the two inactive 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 compounds in Applications of chiral compounds in daily life.daily life.

e.g: e.g:

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

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