Question

in goc chapter explain the carbo cation stability

Answer 1

Carbocations are the key intermediates in several reactions and particularly in nucleophilic substitution reactions.

Structure of Carbocations : Generally, in the carbocations the positively charged carbon atom is bonded to three other atoms and has no nonbonding electrons. It is sp2 hybridized with a planar structure and bond angles of about 120°. There is a vacant unhybridized p orbital which in the case of CH3+ lies perpendicular to the plane of C—H bonds.

Stability of Carbocations: There is an increase in carbocation stability with additional alkyl substitution. Thus one finds that addition of HX to three typical olefins decreases in the order (CH3)2C=CH2>CH3—CH = CH2 > CH2 = CH2. This is due to the relative stabilities of the carbocations formed in the rate determining step which in turn follows from the fact that the stability is increased by the electron releasing methyl group (+I), three such groups being more effective than two, and two more effective than one.



Stability of carbocations 3°> 2° > 1° > CH3+

Electron release : Disperses charge, stabilizes ion.

Further, any structural feature which tends to reduce the electron deficiency at the tricoordinate carbon stabilizes the carbocation. Thus when the positive carbon is in conjugation with a double bond, the stability is more. This is so, because due to resonance the positive charge is spread over two atoms instead of being  concentrated on only one. This explains the stability associated with the allylic cation. The benzylic cations are stable, since one can draw canonical forms as for allylic cations.



The benzyl cation stability is affected by the presence of substituents  on the ring. Electron donating p-methoxy and p-amino groups  stabilize the carbocation by 14 and 26 kcal/mole, respectively. The electron withdrawing groups like p-nitro destabilize by 20 kcal/mol.

A hetero atom with an unshared pair of electrons when present adjacent to the cationic centre strongly stabilizes the carbocation. The methoxymethyl cation has been obtained as a stable solid CH3O+CH2SbF6-. Cyclopropylmethyl cations  are even more stable than the benzyl cations. This special stability is a result ofconjugation between the bent orbitals of the cyclopropyl ring and the vacant  p orbital of the cationic carbon. That the carbocations  are planar is shown by the fact that these are difficult or impossible to form at bridgeheads,  where they cannot be planar.

The stability order of carbocation is explained by hyperconjugation. In vinyl cations

(CH2 = C+H), resonance stability lacks completely and these therefore are very much less stable.