Question

How can you determine carbocation stability?

Answer 1

Carbocation stability is mainly due to hyperconjugation of the carbocation by inductive sharing of electron density from adjacent alkyl groups filled with `C-H` bonds,

Consider a methyl and tertiary carbocation under this perspective,

The methyl carbocation doesn't have nearly any hyper conjugation so it's going to be very unstable, whereas the tertiary carbocation will be relatively stable due to the large amount of potential electron density to inductively participate in its `"p"`-orbital.

For the preceding reasoning, SN1 reactions will favor tertiary `alpha` carbons, whereas SN2 reactions will favor methyl `alpha` carbons.

SN1 reactions have stepwise mechanisms that include the carbocation (and thus value its stability), whereas SN2 reactions have concerted mechanisms that don't really include the carbocation for more than an instant.