How can sodium vapour and an alkyl halide be used to make free radicals?

In the study of radical chemistry, the reaction between sodium vapour and an alkyl halide is a recognized method for producing unstable free radicals, such as ethyl ($·C_2{H}_5$) or propyl ($·C_3{H}_7$) species.1

This process involves the interaction of the metal vapour with the organic halide to generate these highly reactive, short-lived intermediates. While sodium is also famously used in the Wurtz reaction to synthesize hydrocarbons by coupling two alkyl groups ($2RCl+2Na\to R-R+2NaCl$), the specific use of sodium vapour with alkyl halides serves as a technique to isolate transient free radicals for experimental study. These radicals are typically identified by their ability to react with metal mirrors (such as zinc or antimony) or through spectroscopic techniques like flash photolysis.12