Halogenoalkanes — AQA A-Level Study Guide

Overview

Halogenoalkanes are organic compounds where one or more hydrogen atoms in an alkane have been replaced by a halogen atom (fluorine, chlorine, bromine, or iodine). They are incredibly important in organic synthesis because they are much more reactive than alkanes.

This reactivity stems from the polar carbon-halogen bond. Because halogens are more electronegative than carbon, the carbon atom becomes electron-deficient (\delta+), making it a prime target for nucleophiles. Understanding these mechanisms is crucial, as they form the foundation for many synthetic pathways in chemistry.

In exams, you will frequently be asked to draw mechanisms, explain reactivity trends based on bond enthalpies, and compare substitution versus elimination reactions. You'll also need to know the environmental impact of chlorofluorocarbons (CFCs).

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Key Concepts

Concept 1: Nucleophilic Substitution ($S_N2$)

Because the carbon attached to the halogen is \delta+, it is susceptible to attack by nucleophiles (electron pair donors). The nucleophile donates a lone pair to the carbon, forming a new bond, while the carbon-halogen bond breaks heterolytically, releasing the halide ion (the leaving group).