Question

How do you predict whether a reaction will undergo sn1 or sn2?

Answer 1

How much shrubbery is there around the ipso carbon?

Explanation:

Now of course `S_N1` and `S_N2` reactions are DISSOCIATIVE (i.e. bond-breaking), and ASSOCIATIVE (bond-making) respectively to a first approximation. Associative mechanisms are largely restricted to methyl, primary, and more rarely, secondary, carbon centres.

On the other hand, if there is some degree of steric bulk around the ipso carbon, a dissociative, bond-breaking mechanism is more likely. Anyway, we need data and context.