Nucleophilic vs Electrophilic Substitution: Key Differences Explained
Nucleophilic substitution swaps an atom or group with a nucleophile—an electron-rich species that “attacks” a positive carbon. Electrophilic substitution replaces a hydrogen on an aromatic ring with an electrophile—an electron-seeking species that targets electron-rich π clouds.
People confuse these because both involve substitution reactions on carbon skeletons. In a busy lab or exam hall, the terms sound alike, so they assume “nucleo” and “electro” describe the carbon, not the attacking reagent.
Key Differences
Nucleophilic substitution: SN1/SN2 on sp³ carbons, driven by nucleophile attack. Electrophilic substitution: EAS on aromatic rings, driven by electrophile attack forming a carbocation intermediate. Solvent, leaving group, and π electrons decide which path wins.
Which One Should You Choose?
Synthesize alcohols or alkyl halides? Pick nucleophilic substitution. Want nitrobenzene or bromobenzene? Choose electrophilic substitution. Match your substrate: saturated alkyl vs aromatic ring.
Examples and Daily Life
Nucleophilic: CH₃Br + OH⁻ → CH₃OH + Br⁻. Electrophilic: benzene + NO₂⁺ → nitrobenzene for dyes. Baking soda is a nucleophile; FeBr₃ catalyst in sunscreen synthesis acts via electrophilic substitution.
Can SN2 happen on benzene?
No. Benzene’s delocalized π electrons favor electrophilic substitution, not SN2.
Why does FeCl₃ speed up bromination?
FeCl₃ forms Br⁺, the electrophile needed for aromatic substitution.