The mechanism of aromatic side-chain reactions with special reference, to the polar effects of substituents. Part XIII. Kinetic examination of the reaction of aryl isoCyanates with methyl alcohol

Abstract

The velocity of the reaction R-N:C:O + MeOII→NHR·CO 2Me has been determined in di-n-butyl ether solution at 20° for various aryl isocyanates. The reaction is shown to be a base-catalysed addition of the alcohol to the C:O group of the isocyanate (see p. 716, footnote) and, in the presence of a constant concentration of a tertiary base, follows second-order kinetics. In the absence of a base catalyst the "spontaneous" reaction is autocatalysed by the weakly basic urethane product. Unlike the heterocyclic and aliphatic tertiary bases, the dialkylanilines of comparable basic strength have almost no catalytic effect. Calculation of the repulsion energies between non-bonding groups in the formation of the intermediate reaction-complex (I) between the linear -N:C:O group and the bases triethylamine, pyridine, and dimethylaniline, shows that these are much greater in the case of the dialkyl-(Chemical Equation Presented) aniline molecule, which is flattened by resonance, and are of such a magnitude as to confirm the suggestion that steric effects alone are sufficient to explain the lack of catalytic effect in this class of base. The experimental velocity order R = p-NO2· C6H4 ≫ C6H5 > p-Me·C6H4 > p-MeO·C6H 4 ≫ cycloC6H11 is in agreement with the view that the reaction velocity is increased by increased conjugation of the unshared electron pair on the nitrogen in the isocyanate group with the group R.