Why can't tert-bromide react with NaCN/DMSO through SN1?

Question

Why can't tert-bromide react with NaCN/DMSO through SN1? I know that it can't through SN2 because tert-bromide is teritary, but why not SN1? Is it because DMSO is aprotic and SN1 prefers protic solvents? But it shouldn't be impossible to occur, right?

Answer 1

SN1 has the following requirements:

1. Sterics to make a tertiary carbonation. We have this because the tert-butyl bromide has three carbons around its electrophilic site (where it would get attacked by a nucleophile).
2. A good leaving group. Br fulfills this.
3. A weak nucleophile. CN is a strong nucleophile, so we do NOT have this.
4. Protic solvents, like you mentioned (these are molecules like H2O, MeOH, or carboxylic acids because they can stabilize the carbocation through hydrogen bonding)

Since we are missing the weak nucleophile requirement, what other type of reaction could happen? E2 or E1. Let's see what their requirements are.

E1 requires a weak base and sterically hindered electrophile. E2 requires a strong base, good leaving group, and that the hydrogen getting deprotonated and the leaving group are anti to each other (180 degrees in a Newman projection).

Since we have a strong base (CN), a good leaving group (Br), and a hydrogen anti to the leaving group that can get deprotonated, E2 happens.

Feel free to message me if you have any other questions!