Alkanes are the bedrock of organic chemistry, but they're notoriously unreactive. Their C-C and C-H bonds are strong and non-polar, making them poor candidates for the ionic reactions that drive many syntheses. To use them as starting materials, we first need to install a functional group, a process called functionalisation. The most direct route is free-radical halogenation, which swaps a hydrogen atom for a halogen (like chlorine or bromine).

This transformation doesn't happen in a single step. It proceeds through a free-radical chain reaction, a self-propagating sequence of steps involving highly reactive species with unpaired electrons.

This mechanism unfolds in three distinct stages: initiation, propagation, and termination.

Not all halogens are created equal. Chlorine is highly reactive and not very selective. When reacting with an alkane that has different types of hydrogens (e.g., primary, secondary), chlorination produces a mixture of all possible isomers. For example, the free-radical chlorination of propane yields both 1-chloropropane and 2-chloropropane.

Separating such a mixture can be difficult and costly, making this reaction less useful for targeted synthesis. Bromine, on the other hand, is much less reactive and far more selective. It will preferentially abstract the hydrogen that leads to the most stable alkyl radical.

This difference in selectivity is explained by the Hammond Postulate, which relates the structure of a transition state to the species it is closest to in energy. Hydrogen abstraction by a chlorine radical is highly exothermic, so the transition state is early and resembles the reactants. In contrast, hydrogen abstraction by a bromine radical is endothermic. Its transition state is late, product-like, and strongly reflects the stability of the resulting alkyl radical.

Because of this, bromination of propane almost exclusively yields 2-bromopropane. The reaction proceeds via the more stable secondary radical intermediate over the primary one. The order of radical stability is tertiary > secondary > primary. Therefore, bromine is the reagent of choice when you want to form the most substituted alkyl halide possible.

In practice, if a synthesis requires converting an alkane into a specific alkyl halide, free-radical bromination is often the best first step, provided there's a tertiary or secondary hydrogen available to be selectively replaced. For example, to synthesise 2-bromo-2-methylpropane, you would treat 2-methylpropane with Br₂ and UV light. The reaction would overwhelmingly favour abstraction of the lone tertiary hydrogen.

While chlorination can be useful in industrial settings where complex mixtures are either acceptable or easily separated, for targeted, lab-scale synthesis, the regioselectivity of bromine makes it a much more predictable and valuable tool. This initial functionalisation step opens the door to countless other transformations.

This conversion of a simple, unreactive alkane into a versatile alkyl halide is a foundational strategy in building more complex molecules.