Phenolic acids general explanation

As reported in the literature, the acylation of aromatic hydrocarbons can be carried out by using zeolites as catalysts and carboxylic acids or acyl chlorides as acylating agents. Thus toluene can be acylated by carboxylic acids in the liquid phase in the presence of cation exchanged Y-zeolites (ref. 1). The acylation of phenol or phenol derivatives is also reported. The acylation of anisole by carboxylic acids and acyl chlorides was obtained in the presence of various zeolites in the liquid phase (ref. 2). The acylation of phenol by acetic acid was also carried out with silicalite (ref. 3) or HZSM5 (ref. 4). The para isomer has been generally favoured except in the latter case in which ortho-hydroxyacetophenone was obtained preferentially. One possible explanation for the high ortho-selectivity in the case of the acylation of phenol by acetic acid is that phenylacetate could be an intermediate from which ortho-hydroxyacetophenone would be formed intramolecularly. 

Still more recent work by the same authors has suggested an alternative possibility. It has been generally assumed that L-ascorbic acid has no effect on the polyphenolase system other than its effect as a reducing agent for the o-quinone formed by the oxidation of the phenols. It has now been shown that ascorbic acid itself has an inhibitory action on the polyphenolase enzyme. When polyphenolase prepared from potato was treated with ascorbic acid under anaerobic conditions, and the ascorbic acid subsequently removed by dialysis, the activity of the enzyme was very considerably reduced. The enzyme after such treatment could not be reactivated by the addition of cupric salts and appeared to bo irreversibly inactivated. It was also shown that neither dehydroascorbic acid nor the further oxidation products of dehydroascorbic acid were responsible for this result. There is at present no explanation of the mechanism of this inhibitory action of ascorbic acid, but it is quite clear that, if these results are confirmed, other explanations are possible of why these enzymes do not exert their full potential effect in vivo. 

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