H2 - Y2 Reactions (Y = Cl, Br, I): A Comparative and Mechanistic Aspect

The reaction between hydrogen and chlorine shows resemblance to that between hydrogen and bromine but is considerably more complicated, owing to the larger number of elementary processes that play a significant part in it. The reaction scheme proposed for the hydrogen-chlorine reaction differs in several important respects from that for the hydrogen-bromine reaction. The main differences are seen to be (1) the inclusion of the reaction H + HBr → H2 + Br and the exclusion of H + HCl → H2 + Cl, and (2) the different chain-terminating steps, Br + Br → Br2 being assumed for bromine and the three processes H + O2, Cl + O2, and Cl + X in the chlorine reaction. The rather marked difference between the reactions of chlorine, bromine, and iodine with hydrogen is that the hydrogen-iodine reaction is elementary while the others are not.