Phenolic products, meta orientation

A kinetic model is presented by Pohlman and McColl (1989) to describe the initial and rapid redox processes between polyhydroxyphenolic acid and soil or Mn oxide suspensions. The oxidation process of polyhydroxybenzoic acid by soil and Mn oxides follows second-order kinetics. The rate constants derived from the model are similar in magnitude in both suspensions for the organic reductants studied (Table 8-16). Polyhydroxyphenolic acids with para- and ortho-OH groups are rapidly oxidized by Mn oxides with spectral evidence suggesting that the reaction leads to polymeric humic products probably via semiquinone or benzoquinone derivatives. By contrast, polyhydroxyphenolic acids with meta-oriented phenolic-OH groups are not oxidized by soil or Mn oxide suspensions within a 120-min reaction period. Presumably these compounds are not capable of being oxidized to semiquinone or benzoquinone intermediates. The rapid disappearance of polyhydroxyphenolic acids is accompanied by formation of Mn and colored humic products in both soil and Mn oxide suspensions. These results provide further evidence that abiotic oxidation of certain organics can lead to the formation of humic substances and the mobilization of Mn in nature. 

The general meta orientation of phenolic products obtained at high metal ion concentrations, irrespective of the type of substituent, is diflB-cult to reconcile with any established mode of aromatic substitution. It seems logical that in dilute metal ion solution such a species as FeO which would be formed in the initial step of the autoxidation of ferrous ion could be the hydroxylating species. Such complexes for cuprous and... 

A brief account of aromatic substitution may be usefully given here as it will assist the student in predicting the orientation of disubstituted benzene derivatives produced in the different substitution reactions. For the nitration of nitrobenzene the substance must be heated with a mixture of fuming nitric acid and concentrated sulphuric acid the product is largely ni-dinitrobenzene (about 90 per cent.), accompanied by a little o-dinitrobenzene (about 5 per cent.) which is eliminated in the recrystallisation process. On the other hand phenol can be easily nitrated with dilute nitric acid to yield a mixture of ortho and para nitrophenols. It may be said, therefore, that orientation is meta with the... 

It is clear from the proposed mechanism that the initial step will be facilitated by the presence of a second hydroxyl group in the phenol, oriented meta to the first. In view of the importance of polyphenolic molecules in natural product chemistry, this synthesis takes on a special significance. Thus, the reaction of dihydric phenols with citral (65) has been used to synthesize a diverse range of natural products. However, such compounds can, and in some cases do, give rise to a mixture of isomeric products. Olivetol (64), for instance, yields a mixture of mono- and bis-chromenes of which cannabichromene (66) is a significant component (71JCS(C)796). On the other hand, the phenol (67) affords a h igh yield of isoevodionol (68) on reaction with 4,4-dimethoxy-2-methylbutan-2-ol (71JCS(C)8U). 

Later, Titov( ) proposed a different reaction scheme in vdiich NO" " reacts with the aromatic to form a nitroso compound which rearranges to a nitrophenol. This mechanism will be discussed later. Bennett and Youle(7,8,9) also found phenolic impurities amounting to about 2% of the yield in the nitration of several different aromatic compounds. They observed that the hydroxyl group enters the aromatic molecule according to the normal orientation rule for the substituent originally present and that the amount of by-products is greater for meta-directing substituents. 

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