AS & A-Level Chemistry 08 — Halogenoalkanes and alcohols

The polar $\text{C-X}$ bond renders halogenoalkanes susceptible to attack by nucleophiles—electron-pair donors. When a nucleophile attacks, the halogen is displaced as a halide ion. Heating a halogenoalkane with aqueous sodium hydroxide, $\text{NaOH(aq)}$, yields an alcohol via hydrolysis. Heating with potassium cyanide ($\text{KCN}$) dissolved in ethanol produces a nitrile, synthetically extending the carbon chain by one atom. Heating with excess ammonia ($\text{NH}_3$) in ethanol under pressure favours a primary amine; excess ammonia reduces further alkylation of the amine product.

Solvent choice can dramatically alter the reaction outcome. While $\text{NaOH(aq)}$ favours nucleophilic substitution, heating a halogenoalkane with $\text{NaOH}$ dissolved in ethanol promotes an elimination reaction. Here, the hydroxide acts as a base rather than a nucleophile, removing a proton from an adjacent carbon as the halide leaves to form an alkene and water.

Nucleophilic substitution proceeds via two dominant mechanisms. The $\text{S}_\text{N}2$ mechanism is a single-step, concerted process where the nucleophile attacks the back of the $\text{C-X}$ bond simultaneously as the halogen leaves, passing through a transition state with partial incoming and outgoing bonds, rather than a stable carbon with five full bonds. The $\text{S}_\text{N}1$ mechanism involves two steps: first, the $\text{C-X}$ bond breaks heterolytically in a slow rate-determining step to form a planar carbocation intermediate. Alkyl groups attached to the carbocation exert an electron-donating inductive effect, significantly stabilising this positive charge. In the fast second step, $\text{OH}^-$ attacks the carbocation to form the alcohol. If the nucleophile is neutral water, attack instead forms a protonated alcohol, followed by proton loss.

An analytical test for halogenoalkanes involves warming them with aqueous silver nitrate ($\text{AgNO}_3$) in ethanol. The water acts as a nucleophile to displace the halide ion, which then reacts with $\text{Ag}^+\text{(aq)}$ to form a distinct precipitate, identifying the specific halogen present.