Substitution Reaction (Displacement Reaction)

A substitution reaction (also called a displacement reaction) is one in which an atom or a group of atoms already attached to a carbon is replaced by a new atom or group. Based on the electronic nature of the attacking reagent, substitution reactions are classified into three types:

1. Nucleophilic substitution. Represented generally as

   $A\text{-}X + Y^- \longrightarrow A\text{-}Y + X^-$

   where $Y^-$ is the incoming nucleophile (the attacking species) and $X^-$ is the leaving group. The textbook example is the hydrolysis of alkyl halides: methyl bromide reacts with aqueous hydroxide,

   $CH_3Br + OH^-_{(aq)} \longrightarrow CH_3OH + Br^-$

   Aliphatic nucleophilic substitution reactions of this kind proceed by either an $S_N1$ or an $S_N2$ mechanism -- the detailed mechanistic distinction between the two is studied later in the syllabus.
2. Electrophilic substitution. Represented generally as

   $A\text{-}X + Y^+ \longrightarrow A\text{-}Y + X^+$

   where $Y^+$ is the incoming electrophile. The textbook example is the nitration of benzene, in which the electrophile $^+NO_2$ (nitronium ion) replaces a ring hydrogen to give nitrobenzene. The detailed mechanism of aromatic electrophilic substitution (EAS) is likewise studied in detail later.
3. Free-radical substitution. Represented generally as

   $A\text{-}X + Y^\bullet \longrightarrow A\text{-}Y + X^\bullet$

   The textbook example is the reaction of methane with a chlorine radical, …