Which type of halides cannot participate in SN2 reactions?

In the context of organic chemistry, the ability of a substrate to undergo an SN2 (bimolecular nucleophilic substitution) reaction is critically dependent on its steric hindrance. SN2 reactions involve a backside attack by the nucleophile on the electrophilic carbon, leading to the inversion of configuration at that carbon center.

Tertiary (3°) halides are characterized by having three other carbon groups attached to the carbon bearing the halide. This steric bulk around the electrophilic carbon significantly impedes the approach of the nucleophile towards the carbon-halide bond. As a result, tertiary halides are generally unable to undergo SN2 reactions; instead, they tend to undergo SN1 reactions, where the leaving group departs first, forming a carbocation that can then be attacked by the nucleophile.

In contrast, primary (1°) halides, secondary (2°) halides, and methyl halides possess less steric hindrance. Primary halides have one carbon group, secondary halides have two, and methyl halides have none, which would allow efficient backside attack by the nucleophile. Therefore, these substrates can participate in SN2 reactions.

This understanding of steric hindrance and reaction mechanisms is essential for