Substituent Effects Among constitutional isomers,more substituted double bonds are usually more stable. Wider separation between the groups means less steric interaction and increased stability. less substituted more substituted H H H3 109.5° H H2C separation 120° CH; separation CH3 closer groups wider separation Copyright2010 Pearson Prentice Hall,Inc
Substituent Effects Among constitutional isomers, more substituted double bonds are usually more stable. Wider separation between the groups means less steric interaction and increased stability
Disubstituted Isomers trans more stable than cis cis geminal trans isomer Less stable isomer is higher in energy,has a more exothermic heat of hydrogenation. Cis-2-butene CH3-C-C-CH3 -120kJ H 入H Isobutylene (CH3)2C=CH2 -117kJ Trans-2-butene H-C-C-CH3 -116kJ C -H steric repulsion
Disubstituted Isomers trans more stable than cis cis < geminal < trans isomer Less stable isomer is higher in energy, has a more exothermic heat of hydrogenation. Trans-2-butene -116 kJ Isobutylene (CH3 )2 C=CH 2 -117 kJ Cis-2-butene C H 3 -120 kJ C C C H 3 H H H C C C H 3 C H 3 H steric repulsion C C H H H H H H H H C C H H H H H H H H
Substituent Effects More substituted alkenes are more stable. H,C=CH2 R-CH=CH2 R-CH=CH-R R-CH=CR2 R2C=CRZ unsub.monosub.disub.< trisub.tetra sub. Alkyl group stabilizes the double bond. Alkene less sterically hindered
Substituent Effects More substituted alkenes are more stable. H2 C=CH2 < R-CH=CH2 < R-CH=CH-R < R-CH=CR2 < R2 C=CR2 unsub. < monosub. < disub. < trisub. < tetra sub. Alkyl group stabilizes the double bond. Alkene less sterically hindered
Relative Stabilities ethylene, unsubstituted 2.7 kcal (11k) monosubstituted R 4.2 kcal (18kJ) 4.8 kcal CIS (20kJ) 5.2 kcal (22k) 5.9 kcal disubstituted geminal (25kJ) 6.2 kcal H trans H (26kJ C=C trisubstituted 一R tetrasubstituted R R R
Relative Stabilities
Stability of Cycloalkene ring connects CH,H CH,-CH2 H behind the double bond. CH2 CH2 -CH CH, CH2 H CH, H CH, H CH2 CH,-CH, H trans cyclic system trans-cycloheptene trans-cyclooctene cis-cyclooctene marginally stable stable more stable Copyright 2010 Pearson Prentice Hall,Inc. Cis isomer more stable Small rings have additional ring strain. Must have at least eight carbons to form a stable trans double bond. For cyclodecene (and larger),the trans double bond is almost as stable as the cis
Stability of Cycloalkene Cis isomer more stable Small rings have additional ring strain. Must have at least eight carbons to form a stable trans double bond. For cyclodecene (and larger), the trans double bond is almost as stable as the cis