Nucleophilic substitution mechanism

A nucleophile is a species that donates an electron pair to form a new covalent bond.

A nucleophile can react with a polar molecule like a halogenoalkane by 'kicking out' the halogen functional group and taking its place.

This is called a nucleophilic substitution reaction and follows the mechanism below:

The key steps are:

1. A nucleophile (Nuc) approaches the halogenoalkane (RCH₂X), which has a partially positive carbon atom (δ⁺).
2. The nucleophile donates its lone pair of electrons to the δ⁺ carbon, forming a new covalent bond.
3. The original bond between the δ⁺ carbon and the halogen breaks heterolytically as the halogen atom takes both the shared electrons.
4. The halogen departs as a halide ion (X^-), being replaced by the nucleophile.

Halogenoalkanes readily undergo nucleophilic substitution reactions via this mechanism. The nucleophiles that can react via this mechanism include hydroxide ions (OH^-), water (H₂O), cyanide ions (CN^-) and ammonia (NH₃). The nature of the product formed depends on which nucleophile is used.

Reaction with hydroxides to form alcohols

Halogenoalkanes readily undergo nucleophilic substitution with aqueous hydroxide ions (OH^-) from bases like sodium hydroxide or potassium hydroxide when the reaction mixture is warmed.

For example, bromoethane reacts with hydroxide to form ethanol:

CH₃CH₂Br + OH^- ➔ CH₃CH₂OH + Br^-

The reaction mechanism is:

This reaction, which replaces the halogenoalkane with an alcohol product, is a type of hydrolysis reaction. Water molecules can also act as the nucleophile in similar hydrolysis reactions with halogenoalkanes to generate alcohols. However, the reaction rate is much slower with neutral water molecules than with hydroxide ions, which are more nucleophilic.