The trifluoroacetic acid catalyzed chlorination of aromatic hydrocarbons in carbon tetrachloride. Inhibition by acetic acid

Trifluoroacetic acid, which has previously been shown to be a very effective catalyst for the dissociation of iodobenzene dichlorides in carbon tetrachloride, also promotes the chlorination of aromatic hydrocarbons in this solvent. At relatively low catalyst concentrations durene chlorination occurs by a process which is 3/2 order in trifluoroacetic acid. Toluene chlorination, which requires a higher catalyst concentration, is mixed order (3/2 and 5/2) with respect to trifluoroacetic acid. Apparently a minimum of three catalyst monomers (or one monomer and one dimer) are incorporated in the activated complexes for the aromatic chlorinations. The catalyst activity of trifluoroacetic acid in durene and toluene chlorination is grossly diminished when acetic acid is also in the reaction mixtures. However acetic acid does not markedly. inhibit the action of the catalyst in inducing the dissociation of iodobenzene dichlorides or their formation from the components. Presumably in carbon tetrachloride the two acids interact to form a mixed dimer which is ineffective as a-chlorination catalyst but which compares favorably in catalyst strength with trifluoroacetic acid in the iodobenzene dichloride reaction. Explanations for the differences in the effect of acetic acid on the rates of the two reactions have been suggested. The equilibrium constant for formation of the mixed dimer has been estimated using the rate constants for trifluoroacetic acid catalyzed chlorination of toluene both in the presence and absence of acetic acid.