radical halogenation of alkanes

Alkanes are famously unreactive, so it takes something fierce to make them react. Radical halogenation is one of their two real reactions: shine ultraviolet light on (or heat) a mixture of an alkane and a halogen such as chlorine or bromine, and the halogen replaces a hydrogen, turning the alkane into a haloalkane. Methane plus chlorine and light, for instance, gives chloromethane plus hydrogen chloride.

It works by a chain of steps driven by free radicals — atoms or fragments with a lone unpaired electron, hungry to pair it up. In plain terms: light snaps a Cl-Cl bond into two chlorine radicals (initiation); a chlorine radical yanks a hydrogen off the alkane, making HCl and a carbon radical, which then grabs a chlorine from another Cl2, making the product and regenerating a chlorine radical to continue (propagation); and occasionally two radicals collide and quench each other (termination). One photon can thus trigger thousands of product-forming cycles. (The detailed mechanism, energetics, and selectivity live in the radical-chemistry field; here we only place the reaction.)

Two honest caveats. First, halogenation is hard to control: it tends to over-react, replacing a second and third hydrogen, so you get a mixture (mono-, di-, tri-, and tetra-chloromethane), which limits its use in fine synthesis. Second, chlorine is fast but unselective (it attacks most C-H bonds at similar rates), while bromine is slower but far more selective, preferring the weakest C-H bonds; fluorine is violently uncontrollable and iodine essentially won't react. So radical halogenation is the alkane's signature reaction, but a blunt instrument compared with the precise chemistry of functional groups.