26772337 Matriculation Chemistry Introduction to Organic Compound Part 3

8/4/2019 26772337 Matriculation Chemistry Introduction to Organic Compound Part 3

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Lecture 4:Lecture 4:

12.4 Isomerism12.4 Isomerism

Learning Outcomes:Learning

Outcomes:

At the end of the lesson the students shouldAt the end of the lesson the students shouldbe able to :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

ChainIsomerism

PositionalIsomerism

Functional GroupIsomerism

diastreomer enantiomer

cis-trans

isomerism

other

diastereomers



IsomerismIsomerism

is the existence of differentis the existence of different

compounds with the same molecularcompounds with the same molecularformula but different structuralformula but different structural

formulae.formulae.

Different structural formula that haveDifferent structural formula that have

the same molecular formula arethe same molecular formula are

calledcalled isomersisomers..



1) Constitutional isomers1) Constitutional isomers(Structural isomers)(Structural isomers)

are isomers with the same molecularare isomers with the same molecularformula 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 molecularare 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 differentIsomerism 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

CC55HH1212

a) Chain/skeletal isomerisma) Chain/skeletal isomerism

The isomers differ in the carbon skeletonThe 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

CHCH33CCHCCH

33

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

CC33HH

77ClCl

CHCH33CHCH

22CHCH

22ClCl

1-chloropropane1-chloropropane

2-chloropropane

CHCH33CHCHCHCH

33

ClCl



CC44HH

88

CC88HH

1010

CH3

CH3

CH3

CH3

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

1-butene1-butene 2-butene2-butene

CHCH22=CHCH=CHCH

22CHCH33 CHCH

33CH=CHCHCH=CHCH33

CH3

C H 3

1,4-dimethylbenzene1,4-dimethylbenzene



CC66HH

1313NN

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 differentgroups and belong to differenthomologous series with the same generalhomologous series with the same general

formula.formula.

Different classes of compounds thatDifferent classes of compounds that

exhibit functional group isomerism :-exhibit functional group isomerism :- General formula Classes of compounds

CnH2n+2 O ; n > 1 alcohol and ether

CnH2n O ; n ≥ 3 aldehyde and ketone

CnH2n O2 ; n ≥ 2 carboxylic acid andester

CnH2n ; n ≥ 3 alkene and cycloalkane



ExamplesExamples

CC22HH

66OO

CC33HH

66OO

CC33HH66OO22

CH3CH2CH

O

CH3CCH3

O

CH3COCH

3

O

CH3CH2COH

O

ethanol dimethyl ether

propanalpropanone

propanoic acid methyl ethanoate

CHCH33CHCH

22OHOH CHCH33OCHOCH

33



Exercise:

1.State how many are isomers with the followingmolecular formulae, identify the type of isomerism

and draw the structural formula of the isomers.

a) C5H10

b) C5H10 O2

c) CH3

CH=C(Cl)CH3

d) C4H6Cl2

e) CH3CH2CH(OH)CH(Br)CH2CH3



Lecture 5Lecture 5

12.4 Isomerism12.4 IsomerismLearning Outcomes:Learning Outcomes:

At the end of the lesson the studentsAt the end of the lesson the studentsshould be able to :should be able to :

Define stereoisomerism.Define stereoisomerism. DescribeDescribe cis-transcis-trans isomerism due toisomerism due to

restricted rotation about C=C bond andrestricted rotation about C=C bond andCC−−C bond in cyclic compoundsC bond in cyclic compounds

IdentifyIdentify cis-transcis-trans isomerism of a givenisomerism of a givenstructural formula.structural formula.



Stereoisomerism / optical isomerism :Stereoisomerism / optical isomerism :

Isomerism that resulting from differentIsomerism that resulting from differentspatial arrangement of atomsspatial arrangement of 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)



Diastereomer Diastereomer

Cis-TransCis-Trans IsomerismIsomerism

The requirements for geometric isomerism : The requirements for geometric isomerism :

i) restricted rotation about a C=C,doublei) restricted rotation about a C=C,double

bond in alkenes, or a C-C single bondbond in alkenes, or a C-C single bond inincyclic compounds.cyclic compounds.

ii) each carbon atom of a site of restrictedii) each carbon atom of a site of restrictedrotation has two different groupsrotation has two different groups attachedattached

to it.to it.



ExamplesExamples

C C

CH3

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 2If one of the doubly bonded carbons has 2

identical groups, geometric isomerism isidentical groups, geometric isomerism is

not possiblenot possible

.

.

ExampleExample

NoNo cis – transcis – trans isomerisomer

C C

CH3

H

H3C

H3C



Lecture 6Lecture 6

12.4 Isomerism12.4 IsomerismLearning Outcomes:Learning Outcomes:

At the end of the lesson the studentsAt the end of the lesson the studentsshould be able to :should be able to :

IdentifyIdentify cis-transcis-trans isomerism of a givenisomerism of a givenstructural 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 compoundsState the applications of chiral compoundsin daily life.in daily life.



Optical IsomerismOptical Isomerism Optically active compounds have theOptically active compounds have the

ability to rotate plane-polarized light toability to rotate plane-polarized light to

the right (dextrorotary) and to the leftthe 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 ormolecule contains a chiral carbon or

chirality centre or stereogenic centre (achirality centre or stereogenic centre (aspsp33-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 itsii) molecule is not superimposable with its

mirror image.mirror image.

P

CQ

R

S*

P≠ Q≠ R≠ S*designates chiral centre



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

Example:-Example:-

i)i) 2-butanol ,2-butanol ,

C*

CH2CH3

H3COH

H

C

CH2CH3

CH3HOH

CH3CHCH2CH3

OH

enantiomers



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

enantiomersenantiomers

CH3CHCOOH

OH

COOH

HO H

CH3

COOH

H OH

CH3



12.4.9 Racemate12.4.9 Racemate

AA racemic mixtureracemic mixture oror racemate isracemate is an equimolar mixture of enantiomersan equimolar mixture of enantiomers

which is optically inactive because thewhich is optically inactive because thetwo components rotate plane-polarizedtwo components rotate plane-polarized

light equally (same degree of rotation)light equally (same degree of rotation)

but in opposite directions.but in opposite directions.

Hence it does not give a net rotation of Hence it does not give a net rotation of

plane-polarized light.plane-polarized light.



Applications of chiral compounds inApplications of chiral compounds in

daily life.daily life. e.g:e.g:

((−−) Dopa is used for treatment of Parkinson’s) Dopa is used for treatment of Parkinson’s

disease but (+) dopa is toxic to human.disease but (+) dopa is toxic to human.

(S)(S)-Ibuprofen the popular analgesic(the active-Ibuprofen the popular analgesic(the active

ingredient in motrin, advil, and many otheringredient in motrin, advil, and many other

nonaspirin analgesics)nonaspirin analgesics)

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26772337 Matriculation Chemistry Introduction to Organic Compound Part 3