Dissociative ElcB mechanism

A dissociative elimination-addition pathway has also been proposed to account for the kinetics of alkaline hydrolysis of 2,4-dinitrophenyl 4 -hydroxyphenylpropionitrile in 40% (v/v) dioxane-water, although participation of the associative Bac2 mechanism cannot be ruled out since it may be facilitated by the electronic effect of the triple bond. Formation of intermediate (15), having a conjugated and cumulated double-bond system, should favour the ElcB mechanism and thereby account for the contrasting entropies of activation found for hydrolysis of (14) and the corresponding 4 -methoxyphenylpropionate. 

Kinetic data indicate that the hydrolysis of 5-2,4-dinitrophenyl 4,-hydroxythioben-zoate (61) in mild alkaline solutions (pH 8-11) most likely follows a dissociative, ElcB pathway, through a p-oxoketene intermediate, whereas at higher pH values an associative mechanism carries the reaction flux (Scheme 18). LFER relationships obtained from a kinetic study on the alkaline hydrolyses of substituted 5-aryl 4 -hydroxythiobenzoates seem to suggest that the associative pathway is a concerted, one-step process, rather than the classical mechanism via a tetrahedral intermediate.51... 

Mechanism 20.2 is classified as Elcb (elimination-unimolecular, conjugate base) because the conjugate base of the reactant undergoes unimolecular dissociation. 

The dissociative mechanism of acyl transfer ElcB), which prevails for the alkaline hydrolysis of 2,4-dinitrophenyl 4-hydroxybenzoate, could not be detected in a series of extended analogues. Thus, kinetic studies of the alkaline hydrolysis of the 2,4-dinitrophenyl esters of 4-(4 -hydroxyphenyl)benzoic acid (24), 4-(4 -hydroxyphenyl) azobenzoate (26 Z = Y = N), Al-(4 -hydroxybenzylidene)-4-aminobenzoate (26 Z = CH, Y = N) and 4 -hydroxy-fran5-stilbene-4-carboxylate (26 Z = Y = CH) showed that these compounds did not form the corresponding ElcB intermediates (25) and (27), but instead reacted through the usual Bac2 mechanism. ... 

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