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.