carbocations

Jul 25, 2022, 16:45 IST

About Carbocations

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 are of about 120°. There is a vacant unhybridized p orbital which (e.g. in the case of CH3+) lies perpendicular to the plane of C—H bonds.

Stability of Carbocations

Lower the charge in carbon higher will be the 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