Chemistry · Reactivity 3: What are the mechanisms of chemical change?. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A radical is a species with an unpaired electron, shown by a dot, as in Cl•. Radicals form by homolytic fission: a covalent bond breaks and each atom keeps one of the two shared electrons. The unpaired electron makes a radical very reactive.
Alkanes are unreactive because their bonds are strong and non-polar. They do react with chlorine or bromine in ultraviolet light, by radical substitution: a hydrogen atom is replaced by a halogen atom.
The mechanism is a chain reaction with three stages. Initiation makes radicals. Propagation uses a radical and makes a new one, so the chain continues. Termination removes radicals when two of them join.
Students write H• as a product of the first propagation step. The chlorine radical takes the hydrogen atom, so the products are an alkyl radical and HCl, never CH3Cl and H•.
Answer: The main products are bromoethane and HBr; butane forms when two ethyl radicals combine in a termination step.
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