Objectives 1.Structural isomerism: chain, positional, functional, ring-chain, isomerism, metamerism, tautomerism Homologous series 2.Stereoisomerism: conformational, optical and geometrical isomerism

Isomerism Structrual Isomerism Chain Functional Positional Ring Chain Metamerism Tautomerism Next slide

Isomerism Configurational Geometrical Optical Isomerism Previous slide Stereoisomerism Conformational

Structural Isomerism

Same molecular formulae but differ in the functional groups. CH 3 CH 2 OH CH 3 OCH 3 Positional Isomerism Functional 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. C 3 H 6

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 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. keto-enol isomerism

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 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. 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 Conformations of Butane (Sawhorse model) H3CH3C CH 3 H H H H H H H3CH3C H H

Newmann projection 180 0

Stereoisomerism Conformations of Cycloalkanes

Interconversions Fischer Sawhorse H H CO 2 H OH CO 2 H H3CH3C OH CO 2 H OH H

Interconversions CH 3 H Br H H CH 3 Br H CH 3

Interconversions H H CHO Br Cl CH 2 OH F B Cl CHO Br H H

Interconversions H CO 2 H OH CH 3

Interconversions Ph OH Br CH 3

Interconversions H CO 2 H OH CH 3 HOH

Geometrical isomerism

Number of geometrical isomers = 2 x x = No. of double bonds For compounds with two different terminal groups Example No. of geometrical isomers =2 3 =8

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

Optical isomerism Criterion for optical activity 1.Presence of chiral C–atom (for single asymmetric centre) For more than one asymmetric centres – 1.Non-superimposable mirror images Enantiomers (d,l–pair) Note: Superimposable mirror images Meso compounds

Optical isomerism 2. No plane or centre of symmetryH H CO 2 H OH CO 2 H Plane of symmetry (Meso) Br H CH 3 Br H CH 3 No plane of symmetry (Enantiomer)

Optical isomerism NH C C CH 3 H H Centre of symmetry results optical inactivity

Optical isomerism 1.When the molecule is asymmetrical: Number of enantiomers = 2 m Number of meso isomers = 0 2.For symmetrical molecule with even number of asymmetric centres : Number of enantiomers = 2 m–1

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