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When alkanes are mixed with chlorine or bromine under normal conditions, nothing happens. Alkanes are unreactive towards halogens in the dark. However, expose the mixture to ultraviolet (UV) light and a vigorous reaction occurs. A hydrogen atom on the alkane is replaced by a halogen atom — this is free radical substitution.
Understanding this mechanism is essential for Edexcel A-Level Chemistry. It is one of the first reaction mechanisms you will learn, and it introduces key concepts — radicals, homolytic fission, and chain reactions — that appear throughout organic chemistry.
A free radical is a species (atom or group of atoms) with an unpaired electron. Free radicals are represented with a dot to show the unpaired electron, for example Cl• or CH₃•.
Free radicals are:
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