Decarboxylation of aromatic acids with methyl, methoxy and hydroxy substituents

2 DECARBOXYLATION OF AROMATIC ACIDS WITH METHYL, METHOXY AND HYDROXY SUBSTITUENTS 

Some work has been done on the kinetics of decarboxylation of aromatic acids in non-aqueous solutions at high temperatures [233]. Usually, the reactions are much faster in aqueous solutions [239] particularly if they are acid catalyzed. Therefore, emphasis in this article will be on decarboxylation in aqueous solution. A brief review has been published in 1968 [240]. In almost all cases, rates can be conveniently followed with the aid of the UV spectrophotometric method. 

6-Trihydroxybenzoic acid and some substituted salicyclic acids are decarboxylated much faster and their rates can be studied using dilute aqueous solutions of strong acids. In these examples, the first-order rate coefficient, k = rate/Cs, is found to be proportional to the fraction of non-ionized acid, ArCOOH, in the acid—base equilibrium of the substrate [239, 244, 245], viz. 

Decarboxylation in water. Rate coefficients and Arrhenius parameters, ju = 0.1 M (KC1) 

Values of the rate coefficients ft ArcooH and fc for various acids are collected in Table 21. According to eqn. (65), the decarboxylation mechanism is either a unimolecular decay of the acid, ArCOOH, or a bimolecular reaction of the anion, ArCOO , with H30+. 

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