lecture 4: 12.4 isomerism - the great chemistrythegreatchemistry.yolasite.com/resources/lecture 4,5...
TRANSCRIPT
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Lecture 4:
12.4 Isomerism
Learning Outcomes:
At the end of the lesson the students should be able to :
Define isomerism.
Explain constitutional isomerism.
chain isomers
positional isomers
functional group isomer
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Isomerism
Structural/
Constitutional Isomerism Stereoisomerism
Chain
Isomerism
Positional
Isomerism
Functional Group
Isomerism
diastreomer enantiomer
cis-trans
isomerism
other
diastereomers
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Isomerism
is the existence of different compounds with the same molecular formula but different structural formulae.
Different structural formula that have the same molecular formula are called isomers.
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1) Constitutional isomers (Structural isomers)
are isomers with the same molecular formula but differ in the order of attachment of atoms.
2) Stereoisomers
are isomers with the same molecular formula but different arrangement of atoms in space
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Constitutional isomerism
Isomerism resulting from different order of attachment of atoms.
Three types
a) Chain/skeletal isomerism
b) Positional isomerism
c) Functional group isomerism
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CH3CH2CH2CH2CH3
C5H12
a) Chain/skeletal isomerism
The isomers differ in the carbon skeleton
(different carbon chain).
They possess the same functional group and
belong to the same homologous series.
Example:
CH3CHCH2CH3
CH3
CH3CCH3
CH3
CH3
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2)Positional isomerism
These isomers have a substituent group/ functional group in different positions.
Examples
C3H7Cl
CH3CH2CH2Cl 1-chloropropane
2-chloropropane
CH3CHCH3
Cl
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C4H8
C8H10
CH3
CH3
CH3
CH3
1,2-dimethylbenzene 1,3-dimethylbenzene
1-butene 2-butene
CH2=CHCH2CH3 CH3CH=CHCH3
C H 3
C H 3
1,4-dimethylbenzene
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C6H13N
CH3
H2N
CH3
NH2
CH3
NH2
CH2NH2
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3)Functional group isomerism
These isomers have different functional groups and belong to different homologous series with the same general formula.
Different classes of compounds that exhibit functional group isomerism :-
General formula Classes of compounds
CnH2n+2O ; n > 1 alcohol and ether
CnH2nO ; n ≥ 3 aldehyde and ketone
CnH2nO2 ; n ≥ 2 carboxylic acid and ester
CnH2n ; n ≥ 3 alkene and cycloalkane
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Examples
C2H6O
C3H6O
C3H6O2
CH3CH2CH
O
CH3CCH3
O
CH3COCH3
O
CH3CH2COH
O
ethanol dimethyl ether
propanal propanone
propanoic acid methyl ethanoate
CH3CH2OH CH3OCH3
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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
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Lecture 5
12.4 Isomerism
Learning Outcomes: At the end of the lesson the students
should be able to :
Define stereoisomerism.
Describe cis-trans isomerism due to restricted rotation about C=C bond and CC bond in cyclic compounds
Identify cis-trans isomerism of a given structural formula.
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Stereoisomerism / optical isomerism :
Isomerism that resulting from different spatial arrangement of atoms
in molecules.
Two subdivisions of stereoisomers:
i) Enantiomers (mirror image)
ii) Diastereomers (non-mirror image)
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Diastereomer
Cis-Trans Isomerism
The requirements for geometric isomerism :
i) restricted rotation about a C=C,double bond in alkenes, or a C-C single bond in cyclic compounds.
ii) each carbon atom of a site of restricted rotation has two different groups attached to it.
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Examples
C C
CH3
H
H
H3CC C
CH3
H
H3C
H
H
CH3
H
CH3
trans-2-butene cis-2-butene
cis-1,2-dimethylcyclohexane trans-1,2-dimethylcyclohexane
CH3
CH3
H
H
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If one of the doubly bonded carbons has 2 identical groups, geometric isomerism is not possible.
Example
No cis – trans isomer
C C
CH3
H
H3C
H3C
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Lecture 6
12.4 Isomerism Learning Outcomes: At the end of the lesson the students should be
able to : Identify cis-trans isomerism of a given structural
formula. Define chirality centre and enantiomers. Identify chirality centre in a molecule. Explain optical activity of a compound. Draw a pair of enantiomers using 3-dimensional
formula. Define racemate. State the applications of chiral compounds in
daily life.
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Optical Isomerism
Optically active compounds have the ability to rotate plane-polarized light to the right (dextrorotary) and to the left (levorotary)
The angle of rotation can be measured with an instrument called polarimeter.
Enantiomer
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Polarimeter
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The requirements for optical isomerism :-
i) molecule contains a chiral carbon or chirality centre or stereogenic centre (a sp3-hybridized carbon atom with 4 different groups attached to it)
ii) molecule is not superimposable with its mirror image.
P
CQ
R
S*
PQRS
*designates chiral centre
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Enantiomers
a pair of mirror-image that are not superimposable.
Example:-
i) 2-butanol ,
C*
CH2CH3
H3C
OHH
C
CH2CH3
CH3HOH
CH3CHCH2CH3
OH
enantiomers
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12.4.9 Racemate
A racemic mixture or racemate is an equimolar mixture of enantiomers which is optically inactive because the two components rotate plane-polarized light equally (same degree of rotation) but in opposite directions.
Hence it does not give a net rotation of plane-polarized light.
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Applications of chiral compounds in
daily life.
e.g:
() Dopa is used for treatment of Parkinson’s disease but (+) dopa is toxic to human.
(S)-Ibuprofen the popular analgesic(the active ingredient in motrin, advil, and many other nonaspirin analgesics)