Question

What is the difference between a sn1 and sn2 reaction?

Answer 1

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 referred to as nucleophilic substitution, because a nucleophile substitutes a substituent that is likely to leave (a leaving group).

The rate laws dictate what each is sensitive two regarding reaction conditions. SN1 needs an amazing leaving group, and a substrate that can be stable with a carbocation intermediate (via hyperconjugation or resonance), the nucleophile doesn't have to be great (hence the neutral nucleophiles).

SN2 needs an amazing nucleophile, that isn't bulky, to substitute the leaving group in one concerted step. Hence, polar aprotic solvents are nearly always needed (`H_2O` could be used with `NaOH`, which is an exception that I remember my professor getting me with).

Does all this sound very confusing? If it does, you probably haven't put in the work with problem sets. This is where organic chemistry loses a lot of students who can't work hard and practice the material religiously.