Final answer:
Positive charge in allylic cations is delocalized due to resonance, making the terminal carbon a strong electrophile and stabilizing the carbocation. Tertiary carbocations are favored for their stability and have a planar structure that can lead to racemic mixtures in non-enzymatic reactions.
Step-by-step explanation:
The positive charge in allylic cations is delocalized over the carbons adjacent to the carbocation. This delocalization is a result of resonance, where the charge can be shared between multiple atoms through the overlap of p-orbitals. Therefore, the positive charge in an allylic cation is not localized on a single atom but is spread out, which stabilizes the cation and makes the terminal carbon a strong electrophile. In biological contexts, like the ß-oxidation pathway in fatty acid metabolism, similar electrophilic reactions are commonly seen. Delocalization is critical in reducing the charge density on any one carbon, increasing the stability of the species involved in the reactions.
When forming carbocations, tertiary carbons are more favorable due to stabilization through the inductive effect from adjacent carbons. The sp2 hybridized planar structure of carbocations allows nucleophiles to attack from either side, leading to potential racemic mixtures when chiral products are formed. However, in enzymes, chiral active sites direct the addition to one side, providing enantioselectivity.