Mechanism of Alkaline Hydrolysis of Some HO-π-COOAr Acyl Derivatives

Abstract

To gain knowledge on the role played by the nature of the bridge interposed between hydroxyl and carbonyl groups in esters of the title type, in principle able to hydrolyze through dissociative pathways via the conjugate base of the substrate (E1cB mechanism), we have studied the alkaline hydrolyses of 2,4-dinitrophenyl esters in which the pi-system is a biphenyl, azobenzene, benzylideneaniline, or stilbene skeleton. Kinetic data, such as reactivity comparisons and Arrhenius parameters, show that these substrates react through the usual, associative, B(Ac)2 mechanism. This outcome is discussed and interpreted from both structural and energetic standpoints. The data suggest that a value of 0.0 is the most appropriate assignment of the sigma(p)() value for the benzylidenamino substituent (C(6)H(5)CH=N-).

Occurrence of an Elongated p-Oxo Ketene Intermediate in the Dissociative Alkaline Hydrolysis of Aryl (2E,4E)-5-(4‘-Hydroxyphenyl)pentadienoates

“…In other words we have further enlarged the π-conjugated backbone, these substrates possessing indeed three π units in the bridge. In all cases our results indicate that these substrates react through the usual associative B Ac 2 mechanism, and this outcome has been tentatively ascribed to an exceedingly large loss of aromaticity on going from substrate to the transition state when two p -phenylene units are present in the π-conjugated bridge.…”

The Alkaline Hydrolysis of Aryl (2E)-3-(4‘-Hydroxyphenylazo)propenoates. A Kinetic Study

“…Furthermore, aiming to increase our knowledge on the factors that drive the mechanism in ester hydrolysis ( inter alia , stability of the putative unsaturated intermediate), we have also investigated other esters possessing different molecular architecture such as two aromatic π-systems linked by Z = Y π-conjugated systems, with Z and Y being N or CH. In all cases, these substrates react through the usual associative B Ac 2 mechanism, and this result could be rationalized invoking an exceedingly large loss of aromaticity on going from substrate to the transition state when two p -phenylene units are present in the π-conjugated bridge.…”

A New, “Elongated” Oxo Ketene Intermediate in the Dissociative Hydrolysis of 2,4-Dinitrophenyl (2E,4E)-5-(4‘-Hydroxyphenyl)pentadienoate

“…The hydrolyses of esters 5 and 6 were carried out under pseudo-first-order conditions in 40% dioxane/water (v/v) at 25 °C and ionic strength held constant (0.1 M) with added potassium chloride, as previously described. 4 The progress of the reactions was followed by monitoring the change of absorbance due to disappearance of the substrate or liberation of products. The products of the reactions in alkaline solution were identified as the anions of 2,4-dinitrophenol and the appropriate acid.…”

“…To get a better knowledge of the role played by the nature of the interposed bridge on the hydrolytic mechanism, we have recently undertaken a kinetic study on the alkaline hydrolysis of the 2,4-dinitrophenyl ester 4 , and we have found that this species, provided with three π-conjugated systems in the bridge, hydrolyzes through the usual associative pathway. This finding has been tentatively rationalized by taking into account that two p -phenylene units are present in the bridge of 4 and, therefore, the loss of aromaticity involved in reaching the rate-determining transition state of the E1cB path could be large enough to make this route unfavorable.…”