Facile Halogen Exchange Reactions: Chloroform with Bromoform and Carbon Tetrachloride with Carbon Tetrabromide

Both of the title systems undergo rapid halogen exchange (half-life ca. 1−2 min) in N -methylpyrolidinone with catalytic sodium hydroxide at room temperature. Yet they differ markedly in response to added p -dinitrobenzene. The rate of the haloform exchange is unaffected, whereas the rate of the carbon tetrahalide exchange is severely retarded. The known base-induced halogen exchange reaction between chloroform and bromoform is shown not to proceed through a reversible carbene intermediate as claimed in the literature. It appears to be best described in terms of the so-called RARP mechanism (radical anion−radical pair). The mechanism proposed for the rapid exchange between carbon tetrachloride and carbon tetrabromide is initial electron transfer, halide ion loss, and ensuing radical chain scrambling of halogen atoms. The acronym RARC, standing for radical anion−radical chain, is proposed.