Are 1 degree allylic carbocations more stable than 3 degree regular carbocations?

Answer 1

tertiary carbocations are more stable than allylic. I was perplexed by it when learning about it, but Mother Nature is never wrong....

Contraintuitive, the resonance should make primiary allyl more stable, but it does not. The steric repulsion between three alkyls in tertiary will win: the bonding angle of 109 degrees in tertiary alkyl is opened to 120 deg when cation is formed, bringing the crowded alkyls further apart.

Answer 2

To use the correct terminology, yes, primary allylic carbocations are more stable than tertiary regular carbocations. The reason is the conjugation with the adjacent alkene group, spreading the positive charge over three carbon atoms. To illustrate the importance, if one were to do nucleophilic substitutions under Sn1 conditions, allyl chloride would react faster than t-butyl chloride because of the difference in cationic stability.
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Edit:
chem_star, I went back to the literature and you are right. The Enthalpy of formation of 3° carbocations are less than allylic cation by about 70 Kcal/mol; they are even lower than trityl cation. Who'da thunk it??