Question

If the substrate is tertiary, can we rule out SN2?

Answer 1

Yes, if the substrate is tertiary, can we rule out an `"S"_"N"2` mechanism.

An `"S"_"N"2` mechanism involves backside attack on the α carbon atom.

The three H atoms on a methyl carbon are tiny, so the nucleophile has a clear path for backside attack.

In a 3° substrate like t-butyl chloride, however, the bulky methyl groups block backside attack on the tertiary carbon.