Chemistry · Ch 11 — Organic Chemistry: Some Basic Principles
Carbocations and Carbanions: Structure and Relative Stability
Carbocations and Carbanions: Structure and Relative Stability
Carbocations are carbon species bearing a positive charge, formed when heterolytic fission leaves a carbon atom with only three bonds and six electrons around it (an 'electron sextet') rather than the usual eight. The positively charged carbon is hybridised and trigonal-planar, with the three groups attached to it in one plane and an empty, unhybridised p orbital lying perpendicular to that plane. The stability of a carbocation rises sharply with the number of alkyl groups attached to the charged carbon, giving the order tertiary (3 degree) > secondary (2 degree) > primary (1 degree) > methyl: every alkyl group donates electron density towards the electron-deficient carbon by two separate mechanisms simultaneously -- the inductive effect (Section 11.5) pushing density through the sigma-bond framework, and hyperconjugation (Section 11.7) donating electron density directly from adjacent C-H sigma bonds into the empty p orbital -- and more alkyl substitution means more of both kinds of stabilisation acting at once.
Carbanions are the opposite case: carbon species bearing a negative charge, formed when heterolytic fission leaves the carbon retaining the bonding electron pair. A carbanion carbon is hybridised, with three bonding pairs and one lone pair of electrons (a full octet), giving it a pyramidal shape closely resembling ammonia, , with the lone pair occupying the fourth position. Crucially, the stability order for carbanions is the exact reverse of the order for carbocations: methyl > primary (1 degree) > secondary (2 degree) > tertiary (3 degree). This is because alkyl groups are electron-donating by the inductive effect; attaching more of them to a carbon that is already electron-rich and negatively charged only concentrates even more electron density onto that one carbon, which is destabilising rather than stabilising, and the increasing steric crowding around a more heavily substituted carbanion adds a further destabilising factor. …
What this figure shows. A diagram in two parts. Part (a) shows four carbocations placed left to right in increasing order of stability -- methyl cation CH3(+), a primary cation CH3CH2(+), a secondary cation (CH3)2CH(+), and a tertiary cation (CH3)3C(+) -- each drawn with the positively charged carbon as a trigonal-planar centre with an empty p orbital shown as two lobes above and below the plane, with the label 'stability increases' under an arrow running from the methyl cation to the tertiary cation. Part (b) zooms in on the tertiary (tert-butyl) cation and shows its three attached methyl groups, with dashed lines drawn from three of the adjacent C-H bonds toward the empty p orbital on the central carbon, each dashed line labelled 'hyperconjugative (no-bond) overlap', with a caption noting that the tert-butyl cation has nine such alpha C-H bonds available …