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

You could write whole chapters in an organic text to answer this question. And of course what follows is merely a guide to further reading. We know that $S_(N)1$ reactions...$"substitution...

In a $S_n1$ reaction the reaction rate does only depend on the concentration of the substrate. Mening that you can add all the nucleophile you wan't but the reaction wont...

SN2 (substitution, nucleophilic, bimolecular) takes place in a single step without intermediates when a nucleophile reacts with the substrate(e.g. alkyl halide) from a direction opposite opposite to the leaving group....

.......whereas $S_N1$ processes are essentially bond-breaking reactions., and of course, are only dependent on the concentration of the electrophile. A bond making process at a tertiary carbon centre, for...

1-more stearic hindrance, more chance to give SN1 Rn. vice versa 2-more hyperconjugation capability of a substrate, more stable will be the carbocation ultimately more chance to give SN1 Rn....

SN1 will be faster if: 1.Reagent is weak base. 2.C connected to the Leaving Group is tertiary (sometimes...

Not only the solvent The type of nucleophilic substitution is decided by nucleophilicity and basicity of the nucleophyle, steric hindrance around the putative carbon, and type of solvent, protic solvents...

The clearest difference is in their names! SN1 reactions are unimolecular, stepwise mechanisms, where, $"R" = k["LG"^(-)]$ SN2 reactions are bimolecular, concerted mechanisms, where, $"R" = k["LG"^-]["Nu"]$ These reactions are...

The point is that YOU DON'T KNOW. And for this we apply $"a posteriori"$ reasoning, argument AFTER the fact, rather than $"a priori"$ reasoning, argument BEFORE the fact. Mix...

For nucleophilic substitutions, SN1 and SN2 are equally important, however, for the $beta$-elimination, E2 is much more common and useful than E1.