Decomposition of N-(pivaloyloxy)-2-(acetylamino)fluorene (1 b) and N-(sulfonatooxy)-4-(acetylamino)- biphenyl (2a) in MeOH occurs predominately via N-0 bond cleavage to yield oxazoles (5, 6, 23), methoxyadducts (7,8,24,25,26),andrearrangement products (lob, 11b,28). Minor estermethanolysis paths lead to the N-arylhydroxamic acids (9,271. In the presence of 0.1 M aniline (31, lb yields a number of adducts (14-18) identical to those previously obtained from the reaction of 3 with N-(sulfonatooxy)-2-(acetylamino)fluorene (la). This occurs with no change in the rate constant for decomposition of lb. At 0.1 M 3 all solvolysis products of lb, except the rearrangement products 10b and llb, are reduced below detectable levels. Similar results were obtained for 2a, which yields the adducts 30-35 in the presence of 3 and 36-38 in the presence of NJV-dimethylaniline (4). These results are consistent with a mechanism (Scheme V) in which the N-0 bond heterolysis leads to a tight ion pair that can undergo internal return to yield the rearrangement products or diffusional separation to yield the free ion. The free nitrenium ion can be trapped by solvent or added nucleophiles. Both the N-acetyl-N- (4-biphenyly1)nitrenium ion (45) and the N-acetyl-N- (2-fluoreny1)nitrenium ion (48) react slowly enough with the solvent to undergo selective reaction with strong nucleophiles. Since la, lb, and 2a span the reactivityrange of the ester derivatives of the common N-arylhydroxamic acids which undergo N-O bond heterolysis in HzO, it appears that all of the carcinogenic esters will react with simple aromatic amines via an sN1 mechanism.