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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
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Isomerism
Structural/
Constitutional Isomerism Stereoisomerism
ChainIsomerism
PositionalIsomerism
Functional GroupIsomerism
diastreomer enantiomer
cis-trans
isomerism
other
diastereomers
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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..
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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
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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
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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
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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
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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
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CC66HH
1313NN
CH3
H2N
CH3
NH2
CH3
NH2
CH2NH2
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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
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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
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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
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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.
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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)
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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.
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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
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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
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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.
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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
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Polarimeter
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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
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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
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ii) 2-hydroxypropanoic acidii) 2-hydroxypropanoic acid,,
enantiomersenantiomers
CH3CHCOOH
OH
COOH
HO H
CH3
COOH
H OH
CH3
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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.
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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)