200. The kinetics of aromatic halogen substitution. Part VI. Iodine-catalysed bromination

Aromatic Molecular Complexes of the Electron Donor-Acceptor Type.

“…There are probably numerous examples of reactions in which molecular complexes serve as participants in rate-controlling steps in organic reactions. The high-order rate dependence on halogen concentration of certain halogenation reactions of aromatic hydrocarbons has been explained on the assumption that a 1:1 aromatic-halogen complex is attacked by one or more halogen molecules in the slow step (262,263,296).…”

Kinetics of Aromatic Halogenation. IV. The Bromination of Naphthalene in 50% Aqueous Acetic Acid1

“…In order to identify the substituting agent further, the reaction was studied over a 0.1 to 0.5 M range of bromide ion concentration; sodium perchlorate was used to keep the ionic strength constant (Table V). The addition of bromide ion retards the rate considerably, because of the instantaneous complexing of free bromine, (Br2)f, to form the tribromide ion according to (3) Br,-(Br2), + Br" The total bromine concentration, (Br2)t, determined iodometrically, is rigorously expressed as the sum of the free and complexed bromine, as well as all other bromine containing species (4) (Br2)t = (Br2)f + Br3~+ HOBr + CHsCOOBr + H2OBr + + CH3COOHBr+ (4) but the concentrations of the latter are undoubtedly so small that they can be neglected,20 and the equation reduces to (5) (Br2)t = (Br2)f + Br,"…”

Orientation of Substitution in the Benzene Nucleus.

“…(ß) At low concentrations (ca. 0.001 M) halogenation is bimolecular, while at higher concentrations (0.02 M) bromination is trimolecular, being of second order with respect to bromine (69). (3) At low concentrations the rates are sensitive to the presence of water and hydrogen halide (69).…”

“…On the other hand, recent studies have revealed several significant facts to alter this viewpoint. (1) The most important fact is that it appears that ionization of the halogen-halogen bond occurs during, rather than before, association with the aromatic nucleus (69).…”

“…0.001 M) halogenation is bimolecular, while at higher concentrations (0.02 M) bromination is trimolecular, being of second order with respect to bromine (69). (3) At low concentrations the rates are sensitive to the presence of water and hydrogen halide (69). ( 4) Aromatic compounds form reversibly 1:1 molecular addition compounds with bromine, iodine, or iodine chloride (9,29,37,49).…”