THE NITROSATION OF N-ALKYLUREAS - EVIDENCE FOR A PROTON-TRANSFER MECHANISM

The kinetics of the nitrosation of methyl, ethyl, propyl, butyl, and a llyl urea were studied by conventional and stopped-flow spectrophotome try in the presence or absence of acetate or mono-, di-, or trichloroa cetate anions. In the presence of a large excess of urea, the observed rate equation was v = [urea][nitrite][H+](2)/K-a + [H+](theta + xi K- R[carboxylic acid]/K-R + [H+]) where K-a is the acidity constant of ni trous acid and K-R that of the carboxylic acid. The ureas exhibited th e reactivity order methylurea much greater than (ethylurea approximate to propylurea approximate to butylurea) much greater than allylurea. Experiments in D2O afforded values of k(H2O)/kD(2)O in general agreeme nt with the values 4.1-5.5 predicted by a semiclassical transition sta te theory of kinetic isotope effects [i.e., kH(2)O/kD(2)O = exp(0.130h <(nu)over bar>/kT)], where <(nu)over bar> is the frequency of [GRAPHIC S] stretching (2700-2250 cm(-1)) in the protonated urea. This result, the observed catalysis by carboxylate ions and the value of the Bronst ed parameter beta(0.45) show the rate-controlling step of these reacti ons to be the transfer of a proton from the protonated N-alkyl-N-nitro sourea to the solvent or to the organic anion, if present. The observe d order of substrate reactivities is explicable in terms of the capaci ty of the protonated N-alkyl-N-nitrosourea for forming a hydrogen bond with the water molecule to which the proton will be transferred, and the degree to which the formation of such bonds is hindered by the hyd rophobic alkyl chain of the nitrosourea. (C) 1996 John Wiley & Sons, I nc.