D. A. Evans. B Breit Carbocations: Stability Chem 206 Carbocation subclasses Hydride ion affinities(Hi) Carbon- substituted Heteroatom-stabilized Me2-CH_-18 Me3c 231 R3 R2 R3 R2 +21 81 H3C—CH2 -CH R-R3= alkyl or aryl R-R3=alkyl or aryl R-R3= alkyl or aryl The following discussion will focus on carbocations unsubstituted with heteroatoms 287 Classical ys nonclassical carbonium ions PhcH2→ Me-CH, -20 hyperconjugation unsymmetrical symmetrical The effect of beta substituents: Rationalize trivalent increasing nonclassical character -CH 2- Me-CH2-CH2 classical nonclassical 276 270 Stability: Stabilization via alkyl substituents(hyperconjugation) Order of carbocation stability: 3>2>1 Hydride ion affinities versus Rates of Solvolysis >R-C④>Hc>Hce asing number of substituents PhCH2-Br CH=CH-CHr-Br Me2 CH-Br le of hyperconjugation rel rate 100 0.7 The relative stabilities of various carbocations can be measured in the gas phase by their Hydride ion Hl239 256 affinity for hydride ion. +17 R⊕+H R-H+ (CH3)2CH249 ative Solvolysis rates in 80% EtOH, 80C Hydride Affinity =-AG AHI increases-C(+) stability decreases Note:As S-character increases, cation stabilityPhCH2' decreases due to more electronegative carbon Conclusion: Gas phase stabilities do not always correlate with rates of solvolysis J. Beauchamp, J Am. Chem. Soc. 1984, 106, 3917. arey& Sundberg -A, pp 276-C C C C C C C C C C C C Å Å Å Å R3 R2 R1 R3 R2 O R1 R3 R2 N R R H3C CH2 Me CH2 H2C CH Me2 CH R R R H R R H H R H H H C C + H Ph CH2 Me CH2 PhCH2–Br CH=CH–CH2–Br HI D-HI rel rate Me CH2 Me–CH2 CH2 Hydride Affinity = –DG° C C C C CH3 + CH3CH2 + (CH3)2CH+ (CH3)3C + H2C=CH+ PhCH2 + R R–H Me3 C Me2CH–Br HC C H2C CH CH2 D. A. Evans. B. Breit Chem 206 Carbocation Subclasses Å R–R3 = alkyl or aryl Å R–R3 = alkyl or aryl Å R–R3 = alkyl or aryl Carbon-substituted Heteroatom–stabilized The following discussion will focus on carbocations unsubstitutred with heteroatoms Stability: Stabilization via alkyl substituents (hyperconjugation) Order of carbocation stability: 3˚>2˚>1˚ > > > Due to increasing number of substituents capable of hyperconjugation 314 276 249 231 287 386 239 Hydride ion affinities The relative stabilities of various carbocations can be measured in the gas phase by their affinity for hydride ion. J. Beauchamp, J. Am. Chem. Soc. 1984, 106, 3917. 276 287 +21 386 +81 276 249 –27 231 –18 239 276 –37 –20 256 Hydride ion affinities (HI) Hydride ion affinities versus Rates of Solvolysis Relative Solvolysis rates in 80% EtOH, 80 °C 100 52 0 +17 239 256 A. Streitwieser, Solvolytic Displacement Reactions, p75 Conclusion: Gas phase stabilities do not always correlate with rates of solvolysis 0.7 +10 249 + H 276 270 –7 open The effect of beta substituents: Rationalize trivalent hyperconjugation no bridging unsymmetrical bridging symmetrical bridging classical nonclassical increasing nonclassical character Classical vs nonclassical carbonium ions Note: As S-character increases, cation stability decreases due to more electronegative carbon. + HI HI increases C(+) stability decreases Carey & Sundberg–A, pp 276- Carbocations: Stability