THE MECHANISM OF WEAKLY BASE-CATALYZED REACTIONS

It is probably unrealistic, at this time, to demand that a single mechanism be general for all of the reactions under consideration. What should be stressed is the fact that the number of pertinent examples is at present extremely limited and the further fact that a variety of mechanisms, which can account for all the present experimental observations, is available. It is even possible that the advent of new data will expand rather than constrict the mechanistic possibilities. One promising approach is the direct measurement of the rates of some of the individual steps, and such work is now in progress46. 

The observed second-order rate coefficients can, in all cases, be treated empirically as consisting of the sum of a second-order rate coefficient (for the uncatalyzed reaction) and the product of a third-order rate coefficient and the catalyst concentration (for the catalyzed reaction), as indicated by eqn. (4). If the catalyst is a base this constitutes base catalysis in a formal sense, at least. Nevertheless, 

CATALYTIC COEFFICIENTS IN THE REACTION OF 2,4-DINITROCHLOROBENZENE AND ALLYLAMINE IN 2-PHENYLETHANOL AT 24.8 0.1 °C 

Similar solvent effects are observed in the reaction of 2,4-dinitrochlorobenzene and n-butylamine at 24.8 °C6. These results are summarized in Table 3. The reac- 

Although individual runs for the first set of experiments follow the second-order rate law, the observed second-order rate coefficients, k, are strongly dependent on the initial amine concentrations, with the rate increasing regularly as the amine concentration increases. Nevertheless, for all of the measurements, a plot of k versus the initial amine concentration is linear, and the data can be fitted withegn.(4), with k equal to 1.87 x 10-4 l.mole-1. sec-1 and k equal to 5.63 x 10-412. mole-2. sec-1. 

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The mechanism of high-temperature, base-catalyzed esterification involves cyclic anhydride formation by the polycarboxylic acid, followed by base-catalyzed reaction of anhydride groups with cellulosic hydroxyls. Fumaric acid, which cannot form a cyclic anhydride because of the trans arrangement of the two carboxyl groups, fails to react with cotton in the presence of weak bases and heat. 

It can be shown that the mechanism expressed in equation (41) always appears as a first-order reaction catalyzed by bases (proton acceptors) in general, when the hydrogen ion concentration is so great that [RH] [ ]. If 1 takes up protons to form mainly SH, we measure the velocity of the protolytic reaction between RH and B, i.e., the velocity of dissociation of the very weak acid RH. If I takes up protons to form RH, the apparent basic catalysis is a disguised acid catalysis of the ion I. If neither of these possibilities is especially favored, we get an apparent basic catalysis which is the result of both a protolytic reaction between RH and the bases and between the acids and I. From the kinetic experiments, it is impossible to decide which of the three possible cases we have in a given reaction. 

The complex then reacts with the alcohol in a manner similar to that postulated by the Baker mechanism for the base-catalyzed reaction. The kinetics involving this square root law is not valid for the cupric acetate-or zinc naphthenate-catalyzed reaction of these tertiary isocyanates. It seems that metal salts of strong acids and of weak acids conform to different mechanisms. 

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