作者:Julio Casado、Guillermo Gonz�lez-Alatorre、Carmen Izquierdo、Christian Brunner
DOI:10.1002/(sici)1097-4601(1996)28:4<307::aid-kin9>3.0.co;2-z
日期:——
The kinetics of the nitrosation of methyl, ethyl, propyl, butyl, and allyl urea were studied by conventional and stopped-flow spectrophotometry in the presence or absence of acetate or mono-, di-, or trichloroacetate anions. In the presence of a large excess of urea, the observed rate equation wasv = [urea][nitrite][H+](2)/K-a + [H+](theta + xi K-R[carboxylic acid]/K-R + [H+])where K-a is the acidity constant of nitrous acid and K-R that of the carboxylic acid. The ureas exhibited the 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 agreement with the values 4.1-5.5 predicted by a semiclassical transition state 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[GRAPHICS]stretching (2700-2250 cm(-1)) in the protonated urea. This result, the observed catalysis by carboxylate ions and the value of the Bronsted parameter beta(0.45) show the rate-controlling step of these reactions to be the transfer of a proton from the protonated N-alkyl-N-nitrosourea to the solvent or to the organic anion, if present. The observed order of substrate reactivities is explicable in terms of the capacity 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 hydrophobic alkyl chain of the nitrosourea. (C) 1996 John Wiley & Sons, Inc.