Halogenoalkanes & Alcohols

Halogenoalkanes — nucleophilic substitution

A halogenoalkane has a polar C–X bond ($\text{C}^{\delta+}\!-\!\text{X}^{\delta-}$); the carbon is attacked by nucleophiles (species with a lone pair). Substitution reactions:

with aqueous $\text{OH}^{-}$ (warm) → alcohol;
with $\text{CN}^{-}$ (in ethanol) → nitrile (adds a carbon — useful in synthesis);
with excess $\text{NH}_3$ (in ethanol, heat, sealed tube) → amine.

Mechanisms: primary halogenoalkanes react by $S_N2$ (one step, nucleophile attacks as X leaves, rate $= k[\text{RX}][\text{Nu}]$); tertiary by $S_N1$ (two steps via a stable carbocation, rate $= k[\text{RX}]$).