Final answer:
The primary electrophile depends on the structure of the substrate in SN2 vs. E2 reactions. Methyl and primary substrates are primary electrophiles in SN2 reactions, while tertiary substrates are primary electrophiles in E2 reactions.
Step-by-step explanation:
The primary electrophile refers to the carbon atom that is attacked by a nucleophile in a reaction. In the context of SN2 vs. E2 reactions, the structure of the substrate determines the primary electrophile.
For SN2 reactions, the primary electrophile is a methyl or primary substrate. These substrates have no or minimal steric hindrance and allow the nucleophile to approach the carbon atom easily.
On the other hand, for E2 reactions, the primary electrophile is a tertiary substrate. Tertiary substrates have steric hindrance that prevents the nucleophile from approaching the carbon atom directly. Instead, the base attacks a hydrogen adjacent to the carbon atom, leading to elimination of the leaving group.