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Rates of nitrosation are reported for benzene, toluene, diphenyl ether, anisole, and phenol in 1 0 . 4 ~-H C J 0 , a t 52.9 "C, for benzene in 1 ~. ~M -H , S O , , and for [aH6]benzene in 1 2M-D2S0,. The effective reagent under these conditions is NO+ and substitution occurs predominantly a t the para-position of monosubstituted substrates. Benzene reacts about 50 times more rapidly in HzS04 than in HCIO, of the same Ho acidity, and about 8.5 times more rapidly than [2H6]benzene in D2S04, which shows that H+-loss from the Wheland intermediate is ratelimiting. Rates of nitrosation are moderately increased by electron-donating substituents and log (partial rate factors) for the para-position correlate with Q+ parameters to give p = -6.9. This relatively low p value is consistent with the rate-limiting step suggested. The reactivity of NO+ is discussed and it is shown to be at least 1 014 times less reactive than NOz+. The significance of the findings to recent explanations of aromatic reactivity in nitrations is also discussed.London S.W.7 THE reaction of nitrous acid with aromatic substrates is usually difficult in the absence of powerfully activating substituents such as OH, OR, and NR,. Often the nitroso-product formed initially undergoes further transformation and nitrous acid is unstable in moderately acidic solutions at high temperature^.^ This combination of adverse factors has discouraged detailed investigation of these reactions and relatively little is

Kinetic studies of the para-nitrosation of phenol, anisole, and their [ P -~H ~] -analogues by sodium nitrite in aqueous H CIO, are reported. With anisole, ether cleavage of the initial product occurs rapidly to give p-nitrosophenol. The rate for phenol is virtually constant below 1 M-HCIO, but thereafter increases rapidly with acidity to reach a maximum in ~' . ~M -H C I O , . A similar rate-acidity profile is observed with anisole, apart from the absence of an acidindependent pathway a t low acidities. Both phenol and anisole reactions show substantial primary deuterium isotope effects. The data suggest that aromatic nitrosation proceeds via an SE2 mechanism in which slow expulsion of the aromatic proton is a characteristic feature, and that dienone intermediates lie on the reaction coordinate for phenol. This limits deduction as to the nature of the nitrosating agent, but there is good evidence for reaction via NO+ above 5M-HCIO,.AROMATIC nitrosation has not been widely examined, formation is available from kinetic studies with phenols,2 presumably because the reaction is difficult with all but which do react under mild conditions to form mainly the highly activated compounds. Some mechanistic in-(a) T.

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