Sn1 vs Sn2 reactions

[jackdamack10]

Hello.

How do we know which reaction is favored?
I know that:
-a primary alkyl halide favors the Sn2
-weak bases promote sn2 because they can attack sp3 carbon more easily

Let's say we are given an electrophile and a nucleophile. Is there a foolproof trick which determines which reaction occurs?

[GCT]

No foolproof tricks, you'll need to study this in detail because there are quite a few factors which influence Sn2 and E2 reactions. You've got a carbocation formation in the sn1 usually with the aid of a catalyst and sn2 requires a stronger nucleophile; in most cases, sn2 reactions occur at the terminal motif of the molecule, while sn1 reactions are prefered at the secondary, tertiary carbons due to the stabilization of the carbocation.

[so-crates]

The rate-limiting step of the Sn1 reactions is the formation of the carbocation/carbanion - it is limited by the degree to which the molecule disassosciates in a solution. Sn1 is essentially a Lewis acid-base reaction. As GCT indicated, this can be influenced by a variety of factors, including solvents. The classic example is that is that of a polar aprotic solvent can increase the rate of a Sn1 reaction by a million fold simply because it solvates the anions well but does not solvate cations to the same degree.