The true statement is that they require ultraviolet light to react.
The reaction between methane and chloride is a free radical substitution.
Chlorine molecule (Cl2) reacts with light to produce 2 radicals Cl ·, which will start a chain reaction, because the Cl · radicals react with CH4, to produce CH3· radicals and one CH3· radical joind to a Cl· radical to produce CH3Cl.
This can also lead to a double or triple substitution obtaning CH2 Cl2 or CH Cl3. Or even more substitution to obtain C Cl4.
