Bromination is more selective than chlorination because of differences in reactivity and energy:
- Reactivity of the halogen radicals
- Chlorine radicals are very reactive. They will react with almost any hydrogen, whether it’s on a primary, secondary, or tertiary carbon.
- Bromine radicals are less reactive. They “carefully” pick which hydrogen to react with, preferring the ones that lead to the most stable intermediate.
- Energy of hydrogen abstraction
- Chlorination releases energy when it removes a hydrogen, so it doesn’t matter much which hydrogen it takes. This means it can react at multiple positions, leading to a mixture of products.
- Bromination requires energy to remove a hydrogen, so it only occurs where it’s easiest—usually at the most substituted carbon, which gives the most stable radical. This makes bromination highly selective.
- Stability of the intermediate radical
- Bromination favors forming the most stable radical intermediate, such as tertiary over secondary or primary radicals.
- Chlorination doesn’t discriminate as much; even less stable radicals can form, producing less selective outcomes.
In short: Bromination is slower and “picky,” so it goes mostly to the most stable site. Chlorination is faster and “aggressive,” so it attacks multiple sites.