Luciano Do Amaral

Effects of α-substitution on the rate of chloromercuriolactonization of phenyl esters of γδ-unsaturated acids

Second-order rate constants for the reaction of several phenyl esters of α-alkyl-and/or α-aryl-substituted allylacetic acids with mercury(II) chloride have been determined in 50% aqueous ethanol. It was found that α-substitution increases the rate of reaction. The rate constants can be correlated with the dual substituent parameter expressions (i) and (ii). In the systems studied the steric coefficient is much larger log k2= log k2o+ρ*σ*+δEs(i) log k2= log k2o+ρ*σ*+ψν(ii) than the polar one, suggesting that the effect of the substituents on the rate of the reaction is essentially steric in nature, with a small additional contribution from the polar effect.

Kinetics and mechanism of the formation of Girard T hydrazones from naphthaldehydes

In aqueous solution, Girard T hydrazone formation from naphthaldehydes does not proceed to completion under the conditions used. The corresponding equilibrium constants have been determined and employed to calculate the rate of the reaction at completion. For all the reactions studies, the pH–rate profiles, extrapolated to zero buffer concentration, show one break at pH 4–5, characteristic of a change in the rate-determining step from aminomethanol dehydration to aminomethanol formation on going from pH 7 to pH 1. The formation of the aminomethanol is subject to catalysis by hydronium ion and by carboxylic acids present in the buffers used to maintain pH. Bronsted α values for this catalysis vary from 0.24 to 0.26. We suggest a stepwise preassociation mechanism for the reaction catalysed by carboxylic acids.

Kinetics and mechanism for the formation of phenylhydrazone from benzaldehydes, naphthaldehydes, and formyl-1,6-methano[10]annulenes

Formation of phenylhydrazone from benzaldehyde, 4-chlorobenzaldehyde, 1-naphthaldehyde, 2-naphthaldehyde, 2-formyl-1,6-methano[10]annulene in 50% aqueous ethanol (v/v) at 25.0 °C and ionic strength 0.20M occurs in two steps, (i) the formation of an aminomethanol intermediate and (ii) the dehydration of the aminomethanol to give the products of reaction. Formation of the aminomethanol intermediate is the rate-determining step below pH 5–6 and dehydration of the aminomethanol is the rate-determining step above pH 5-6. Both the reactions are subject to hydronium ion catalysis. The values of the catalytic constants are very similar with all the aldehydes. Formation of the aminomethanol is subject to general acid catalysis by carboxylic acids. The α Broonsted exponents for catalysis of the intermediate formation reaction for the formyl-1,6-methano[10]annulenes are higher than those for the benzaldehydes and smaller than those for the naphthaldehydes. Comparison of the general behaviour of the formyl-1,6-methano[10]annulenes with the benzaldehydes and naphthaldehydes provides strong evidence of the aromatic character of 1,6-methano[10]annulene ring.