Why are tertiary carbocations the most reactive, if they're already stable?

Question

This may seem silly, but doesn't it seem weird for a compound that's stable (in this context, the tertiary carbocation) to be the most reactive? I mean, wouldn't it be the least, given that it's already stable and wouldn't want to leave that stable configuration. On the same lines, the methyl carbocation sounds like the most reactive due to its high instability, right? wouldn't it want to react in order to form a stable compound?

Answer 1

A tertiary carbocation is not really the most reactive. A molecule that is able to form a tertiary carbocation during a chemical process is the most reactive in that process. For example, tertiary halides do the fastest S_N1 reactions.

Carbocation formation is usually the rate limiting step. This means that the reactivity of the carbocation doesn't usually matter for the reaction rate. It just matters how quickly the carbocation can form. A tertiary carbocation forms the most quickly because it is the most stable. All carbocations are very reactive, so their relative reactivity doesn't matter much for the rate of a reaction.