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Chemistry. GOC–3. Session Objectives. Session Objectives. Structural isomerism: chain, positional, functional, ring-chain, isomerism, metamerism , tautomerism Homologous series Stereoisomerism: conformational, optical and geometrical isomerism. Isomerism. Isomerism. Next slide.

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Session objectives
Session Objectives

  • Structural isomerism: chain, positional, functional, ring-chain, isomerism, metamerism , tautomerism Homologous series

  • Stereoisomerism: conformational, optical and geometrical isomerism


Isomerism
Isomerism

Isomerism

Next slide

Structrual

Chain

Functional

Positional

Ring Chain

Metamerism

Tautomerism


Isomerism1
Isomerism

Isomerism

Stereoisomerism

Previous slide

Conformational

Configurational

Optical

Geometrical



Structural isomerism1
Structural Isomerism

Functional Isomerism

Same molecular formulae but differ in the functional groups.

CH3 CH2 OHCH3OCH3

Positional Isomerism

same molecular formulae but differ in the position of the same functional group.


Structural isomerism2
Structural Isomerism

Disubstituted benzene has three positional isomers. e.g.


Structural isomerism3
Structural Isomerism

Metamerism

same molecular formula but the distribution of alkyl groups on either side of the functional group is dissimilar. Occurs in amines, ketones, ethers and esters.


Structural isomerism4
Structural Isomerism

Ring chain isomerism

Due to the difference in linkage of carbon atoms in the form of ring or open chain structure, i.e. C3H6


Structural isomerism5
Structural Isomerism

Tautomerism

Existing of single compound in two readily interconvertible structures called as tautomers which appears in acid catalysed or base catalysed conditions.

Tautomerism

keto-enol

nitro-aci

nitrite-nitro


Structural isomerism6
Structural Isomerism

keto-enol isomerism

Contains a keto and an enol group. For example, in the presence of an acidic or basic catalyst a rapid equilibrium is established between an aldehyde or ketone and its isomeric (tautomeric) forms.


Structural isomerism7
Structural Isomerism

Shows keto-enol tautomerism

Does not show keto enol tautomerism


Structural isomerism8
Structural Isomerism

Nitro-aci tautomerism

nitrite-nitro tautomerism


Difference between resonance and keto enol tautomerism
Difference between resonance and keto-enol tautomerism

  • Keto-enol tautomerism

  • Change in the position of an atom, generally a H–atom.

  • Exist in solution as they are different compounds.

  • Exist in dynamic equilibrium.

  • Possess different functional groups.

  • Have no stabilization effect on the molecule.

  • Resonance

  • Shift in the position of electrons only.

  • Structures are arbitrary and do not exist.

  • Do not exist in equilibrium.

  • The functional group does not change.

  • Lower potential energy stabilize the molecule.


Stereoisomerism
Stereoisomerism

Have the same molecular formula and structure but differ in the arrangement of atoms in space.

Tautomerism

Conformational

Geometrical

Optical


Stereoisomerism1

CH3

H

H

H3C

H3C

H

H

CH3

H

H

H

H

Stereoisomerism

Conformational

Conformations of Butane (Sawhorse model)


Newmann projection

1800

Newmann projection


Stereoisomerism2
Stereoisomerism

Conformations of Cycloalkanes


Interconversions

H

CO2H

OH

H

OH

H3C

CO2H

H

OH

OH

CO2H

CO2H

Interconversions

Fischer

Sawhorse


Interconversions1

CH3

H

Br

H

Br

Br

H

Br

CH3

H

CH3

CH3

Interconversions


Interconversions2

CHO

B

H

Br

H

Cl

F

CH2OH

H

Br

H

Cl

CHO

CH2OH

Interconversions


Interconversions3

CO2H

H

OH

CH3

Interconversions


Interconversions4

OH

Ph

Br

CH3

Interconversions


Interconversions5

CO2H

H

OH

H

OH

CH3

Interconversions



Geometrical isomerism1
Geometrical isomerism

Number of geometrical isomers =2x x = No. of double bonds

For compounds with two different terminal groups

Example

No. of geometrical isomers =23=8


Geometrical isomerism2

For even no. of double bonds

For odd no. of double bonds

Geometrical isomerism

For two identical terminal groups in alkene

3 geometrical isomers


Optical isomerism
Optical isomerism

Criterion for optical activity

  • Presence of chiral C–atom(for single asymmetric centre)

For more than one asymmetric centres –

  • Non-superimposable mirror images

Enantiomers (d,l–pair)

Note: Superimposable mirror images

Meso compounds


Optical isomerism1

CH3

CO2H

H

OH

H

Br

H

OH

Br

H

CO2H

CH3

Optical isomerism

2. No plane or centre of symmetry

Plane of symmetry (Meso)

No plane of symmetry (Enantiomer)


Optical isomerism2

CH3

H

NH

C

C

NH

H

CH3

Optical isomerism

Centre of symmetry results optical inactivity


Optical isomerism3
Optical isomerism

  • When the molecule is asymmetrical:

  • Number of enantiomers = 2m

  • Number of meso isomers = 0

2. For symmetrical molecule with even number of asymmetric centres :

Number of enantiomers = 2m–1


Optical isomerism4
Optical isomerism

3. For symmetrical molecule and odd number of asymmetric centres



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