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

Jan 10, 2015

Yes, if the substrate is tertiary, can we rule out an $\text{S"_"N} 2$ mechanism.

An $\text{S"_"N} 2$ 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.