The reactivity patterns in the SNAr reactions of the ambident nitrite ion with nitrohalobenzenes depend on the solvent, the leaving group, and the position of substituents. The rate-determining step can be either the formation or decomposition of the intermediate complex, depending on the leaving group and mode of attack by the nitrite ion. The activating effect of a para relative to the same ortho substituent depends on the leaving group and on whether nitrite is attacking via its nitrogen or oxygen atom. The nitrite ion is of comparable nucleophilicity to azide ion, but is a much weaker carbon base than azide when attacking through its nitrogen atom, although it is a strong carbon base when attacking through oxygen. The reactivity patterns of nitrite ion in SNAr reactions are compared with those of other nucleophiles, such as N3-, SCN-, the halides, RS-and RO-.In Part 11' we gave further evidence for a mechanism (Scheme I) first proposed by Rosenblatt, Dennis, and Goodin2 for the reactions of nitrite ion with aromatic compounds, ArX, which are suitably activated for aromatic nucleophilic substitution. A number of rate constants were reported and used to support the mechanism. This paper takes those rate constants and some new ones and discusses reactivity patterns,3 i.e., nucleophilicities, leaving group tendencies, solvent effects, and substituent effects in these reactions and in reactions involving related nucleophiles, such as azide, thiocyanate, and the halides.The ambident nitrite i 0 n~3~ bonds to aromatic carbon via its nitrogen or its oxygen atom (N-attack or 0-attack), but the end product of reaction with ArX is always the phenoxide, ArO-, owing to the reactivity of the nitro intermediate ArN02, which can be isolated in certainThe reactions of nitrite ion have similarities to the SNAr reactions of the ambident thiocyanate ion, which were discussed in Part I.5 Thus the compounds ArSCN and ArN02 are reactive intermediates, and SCN-and NO2-are weak carbon bases. The nitrite ion has a soft6 nitrogen atom and two harder6 oxygen atoms, whereas SCN-has soft sulfur and harder nitrogen. These properties cause interesting variations in leaving group tendencies? in the activating effect of substituents,3 and even in the effect of solvents on rates and mechanism, as discussed below.
Results and DiscussionRate constants were measured and processed as described in Parts I5 and 11.' Leaving Group Tendencies? Table I compares the rates of reaction of three sets of fluoro-, chloro-, and iodonitro-substituted benzenes, ArX, with nitrite ion, via Nattack as log NkX, and O-attack as log Ohx. Three comparisons are made, NkX/OkX (the ratio of N-to 0-attack), log NkF/NkC1, and log OkF/OkC1. The latter two comparisons show leaving group tendencies of fluorine relative to chlorine for N-and 0-attack, respectively. In some cases, only upper or lower limits to these ratios have been recorded because the other rate constant was too slow to measure in competition with other reactions. However, even these ratios give useful inform...
