Unformatted text preview: 4.10 Structure, Bonding, and Stability of Carbocations
\Cg R Most carbocations are too unstable to be isolated, but occur as reactive intermediates in
£3 a number of reactions. When R is an alkyl group, the carbocation is stabilized compared
C toR =H.
’d ‘ “J Ethyl cation (a primary carbocation) CHgCH2" is more stable than CH3+ Methyl carbocation CH3+ least stable. § lsopropyl cation (a secondary carbocation) (CH3)ZCH“ is more stable than CH30H2+
$ ' tert-Butyl cation (a tertiary carbocation) (CH3)3C+ is more stable than (CH3)2CH W\ Exxwp WWW W3: mm H H H CH H CH HC CH
\c,( \cg 3 \0g 3 3x454 3 I I 4‘
H .” CH3 0mm Lao};
(3mm 6mm MOW N
sta PfL 0 car . LemonsS I least stable most stable Stabilization of carbocations is due to two factors: - inductive effects - hyperconjugation Stabilization of carbocations via the inductive effect - positively charged carbon pulls electrons in 6 bonds closer to itself so that positive
We?“ charge is "dispersed “, i.e., shared by carbon and the three atoms attached to it. Electrons
W ' in 0—0 bonds are more polarizable than those in C—H bonds; therefore, alkyl groups
(_______,___. stabilize carbocations better than H. Electronic effects transmitted through obonds are called "inductive effects." ...
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- Summer '08
- Organic chemistry