Chemistry Session Stereochemistry – 1 Session Objectives 1. Structural isomerism: chain, positional, functional, ring-chain, isomerism, metamerism , tautomerism Homologous series 2. Stereoisomerism: conformational, optical and geometrical isomerism Stereoisomerism Isomers same molecular formula Constitutional Isomers Different nature/sequence of bonds Conformational Isomers Differ by rotation about a single bond Enantiomers Non-superposable mirror images Stereoisomers Different arrangement of groups in space Configurational Isomers Interconversion requires breaking bonds Diastereoisomers Not mirror images Structural isomerism Chain Isomerism: When two or more compounds have similar molecular formula but different carbon skelton. Functional Isomerism: Having the same molecular formula but different functional groups. For example, CH3 CH2 OH & CH3OCH3 Positional Isomerism: When two or more compounds differ in the position of substituent atom and group on the carbon skelton. Metamerism: Arises due to unequal distribution of alkyl groups on either side of the functional group in the molecule. CH3 CH2 O CH2 CH3 Ethoxyethane CH3 O CH2 CH2 CH3 1 - Methoxypropane Conformational Carbon atoms connected through bonds in alkanes can undergo rotation leading to conformational isomers. Conformations of ethane H H H H H Staggered H H H H H H H Eclipsed Newmann and Sawhorse projection CH3 HH HH H H Newman projection H H3C CH3 H H H CH3 CH3 3 H H CH3 Eclipsed H H H CH H CH3 Staggered Sawhorse projection Interconversions Fischer Sawhorse H CO2H H OH H OH CO2H H 3C OH CO2H OH CO2H Interconversions H Br H CH3 CH3 Br CH3 H Br Br H CH3 Interconversions CHO B H H H H Cl Br Cl F CH2OH H H Cl CH2OH Br CHO CHO CH2OH Interconversions COH 2 CO2H H OH CH3 H HO CH3 Interconversions OH OH Br Ph CH3 Ph Br CH3 Conformational Isomers Results from free rotation about C-C single bond. Conformers differ in energy through i. Torsional strain: through repulsive interaction between bonding electrons in adjacent -bonds. ii. 'Steric strain’ or ‘van der Waals’ strain – through space interaction between filled orbitals on groups attached to adjacent atoms. Ethane H H H H H H H H H H ‘sawhorse’ ‘Newman’ ‘staggered’ H H H H H H H H H HH H ‘sawhorse’ HH ‘Newman’ ‘eclipsed’ Conformations of Cycloalkanes H H H H H H Chair form H H H H H H H H H H H H H H H H Boat form H H Geometrical isomerism H H C C C H 25 C H 2 5 Cis-isomer C H 3 H C C H 3 C C H 25 Trans-isomer E-Z configuration When all the groups attached to the carbon carbon double bond are different, cis trans nomenclature is not possible. We give the priority to the groups on each carbon atom on the bases of atomic number. Now, we see whether the high priority groups are at same side or not. For example: 1 1 Cl Br C H 2 C 1 2 CH3 Br C CH3 2 Higher priority substituents on same side. Z-isomer. C H Cl 2 1 Higher priority substituents on opposite side. E-isomer. Geometrical isomerism Number of geometrical isomers = 2x x = No. of double bonds For compounds with two different terminal groups Example H3C –CH= CH–CH= CH–CH= CH–C2H5 No. of geometrical isomers =23=8 Geometrical isomerism Number of geometrical isomers =2x 1 2y 1 For two identical terminal groups in alkene y= x For even no. of double bonds 2 y= x +1 For odd no. of double bonds 2 2,4-hexadiene 3 geometrical isomers Optical isomerism Required asymmetric carbon atom called ‘achiral’. Optical isomerism Same molecular formula but different behaviour with the plane polarised light. 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 symmetry CH3 CO2H H OH H Br H OH Br H CO2H Plane of symmetry (Meso) CH3 No plane of symmetry (Enantiomer) Optical isomerism CH3 O H || NH C C C C || H O NH CH3 Centre of symmetry results optical inactivity Optical isomerism 1. 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 Number of m 1 meso isomers=2 2 Optical isomerism 3. For symmetrical molecule and odd number of asymmetric centres Number of m1 2 m1 enantiomers = 2 2 m1 Number of meso isomers = 2 2 Thank you
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