Nucleophilic substitution is a class of reactions where an electron-rich nucleophile attacks a positively charged (or partially positive) electrophilic center, replacing a leaving group.

1. Alkyl Halides: The carbon-halogen bond is polar ($C^{\delta+}-X^{\delta-}$). A nucleophile (like $OH^-$, $CN^-$, or $NH_3$) attacks the carbon, displacing the halogen atom. This is the characteristic reaction of alkyl halides (e.g., $R-X + KOH(aq) \rightarrow R-OH + KX$) [1], [2].
2. Comparison:

• Nucleophilic Addition: Characteristic of Aldehydes and Ketones (carbonyl group).
• Electrophilic Substitution: Characteristic of Benzene and its derivatives (Haloarenes) [3].
• Free Radical Substitution: Characteristic of Alkanes (e.g., chlorination in sunlight) [4].