Kinetics and mechanism of the aminolysis of phenyl and 4-nitrophenyl chloroformates in aqueous solution

The reactions of secondary alicyclic amines with phenyl and 4-nitrophenyl chloroformates (PClF and NPClF, respectively) are subjected to a kinetic investigation in aqueous solution, 25.0 degrees C, ionic strength 0.2 (KCI). The reactions are followed spectrophotometrically at 210-270 nm (PClF) and at 310-400 nm (NPClF). Under amine excess, pseudo-first-order rate coefficients (k(obsd)) are obtained. From linear plots of k(obsd) VS free amine concentration, the second-order rate coefficients (k(N)) for aminolysis are obtained. For the aminolysis of both substrates, linear Bronsted-type plots (log k(N) vs amine pK(a)) of slopes 0.23 (PClF) and 0.26 (NPClF) are found. The values of the slopes are consistent with stepwise mechanisms where the formation of a zwitterionic tetrahedral intermediate (T+/-) is the rate-determining step (k(1) step). In contrast, the aminolysis (anilines) of the same substrates in acetonitrile are concerted, which is attributed to destabilization of T+/- in the latter solvent due to a faster expulsion of the amine from T+/- in acetonitrile compared to water. The values of k(1) are larger for the title reactions compared to the same aminolysis of the corresponding thionochloroformates, and this is attributed to the relatively hard character of these amines which prefer to bind to the harder carbonyl group (relative to thiocarbonyl): There is no change in mechanism by the change of S- by O- in T+/-, which should destabilize this intermediate. By comparison with the stepwise pyridinolysis of methyl chloroformate, it is concluded that the changes of methoxy by phenoxy (or 4-nitrophenoxy) and a pyridine by an alicyclic amine in T+/- do not greatly affect the stability of these intermediates.