Halogenoalkanes undergo substitution reactions with the – – nucleophiles…

The idea

A nucleophile is an electron-pair donor attracted to the δ⁺ carbon. The mechanism is the same for all three: a curly arrow runs from the lone pair on the nucleophile to the δ⁺ carbon, and a second arrow runs from the C–X bond to the halogen, which leaves as X⁻. Warm aqueous sodium hydroxide gives an alcohol; potassium or sodium cyanide in aqueous ethanol, heated under reflux, gives a nitrile and lengthens the chain by one carbon; excess ammonia in ethanol in a sealed tube gives an amine, with a second ammonia molecule removing the H⁺. Rate is set by the carbon–halogen bond enthalpy: C–I is the weakest bond so iodoalkanes hydrolyse fastest, and C–F the strongest so fluoroalkanes are slowest.

What earns the marks

• The curly arrow from the lone pair on the nucleophile to the δ⁺ carbon.
• The curly arrow from the C–X bond to the halogen, with X⁻ shown as a product.
• The correct organic product, such as the nitrile from cyanide.
• Naming the reagent as a bottle chemical with its conditions, for example KCN in aqueous ethanol, heated under reflux.
• Explaining the rate order by carbon–halogen bond enthalpy rather than by polarity.

Common mistakes and how to put them right

• Giving the reagent as CN⁻ or OH⁻ when AQA requires a reagent that could be taken from a bottle, such as KCN or NaOH(aq).

• Starting the curly arrow at the negative charge rather than at a lone pair, which gains no credit.

• Explaining the rate trend by the polarity of the carbon–halogen bond instead of its enthalpy.

• Leaving the lone pair off NH₃, or forgetting the second ammonia molecule that removes H⁺ from the salt.

Exam tips

• Name reagent and conditions together; cyanide needs aqueous ethanol and heating under reflux, ammonia needs excess in a sealed tube.
• Check every arrow starts on a lone pair or a bond and ends where the new bond forms.
• Make the comparison explicitly: the C–I bond has the lowest bond enthalpy so breaks most easily and the iodoalkane hydrolyses fastest; use bond enthalpy values only where the question supplies them.