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 Structrual Chain Functional Positional Ring Chain Metamerism Tautomerism

Isomerism Isomerism Stereoisomerism Previous slide Conformational Configurational Optical Geometrical

Structural Isomerism

Structural Isomerism Functional Isomerism Same molecular formulae but differ in the functional groups. CH3 CH2 OH CH3OCH3 Positional Isomerism same molecular formulae but differ in the position of the same functional group.

Structural Isomerism Disubstituted benzene has three positional isomers. e.g.

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 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 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 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 Isomerism Shows keto-enol tautomerism Does not show keto enol tautomerism

Structural Isomerism Nitro-aci tautomerism nitrite-nitro tautomerism

Difference between resonance and keto-enol tautomerism 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. 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.

Stereoisomerism Have the same molecular formula and structure but differ in the arrangement of atoms in space. Tautomerism Conformational Geometrical Optical

Stereoisomerism Conformational CH3 H3C H3C CH3 H H Conformations of Butane (Sawhorse model)

Newmann projection 1800

Stereoisomerism Conformations of Cycloalkanes

Interconversions Fischer Sawhorse H3C CO2H OH H H CO2H OH

Interconversions Br H CH3 CH3 H Br

Interconversions H CHO Br Cl CH2OH B F CH2OH Cl CHO Br H

Interconversions H CO2H OH CH3

Interconversions Ph OH Br CH3

Interconversions H CO2H OH CH3

Geometrical isomerism

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 isomerism For two identical terminal groups in alkene For even no. of double bonds For odd no. of double bonds 3 geometrical isomers

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 isomerism 2. No plane or centre of symmetry CH3 CO2H OH H H Br Plane of symmetry (Meso) No plane of symmetry (Enantiomer)

Optical isomerism Centre of symmetry results optical inactivity CH3 H NH C CH3 H Centre of symmetry results optical inactivity

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 isomerism 3. For symmetrical molecule and odd number of asymmetric centres

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