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)