Question

Explain why carbocations are higher in energy than oxonium ions, despite oxygen being more electronegative than carbon. Second, state four factors that account for the relative stabilities of carbocations.

Answer 1

The least a species in stability is, higher would be its energy. That is we need greater amount of energy to stabilize a less stable state. A carbocation with carbon with a +ve charge is having incomplete octet and is thus less stable than oxygen having a -ve charge as oxygen can stabilize the -ve charge due to its greater electronegativity.

Factors that stabilize a carbocation,

a. branching at the carbon having the +ve charge (carbocation center) increases, the stability of carbocation increases. That is as the number of alkyl groups on carbon goes up, the carbocation becomes more stable. So a primary is less stable than secondary which is less stable than tertiary center.

b. hyperconjugation : As the branching increases more possibility for the +ve charge stabilization by hyperconjugation and this enhances carbocation stability.

c. Resonance : a double bond or triple bond next to a carbocation as in benzyl or allyl system makes the carbocation more stable as charge is stabilized by resonance.

d. Electron donating groups increase stability of carbocation. So as more such group are connected to a carbocation carbon, the stability of the system increases.