Radical Chemistry benzoic acid

Fhoto loaned by Frau Hiickei, GSttingen, Germany Wohler in Later Life,4 Professor of chemistry at GcSttingen. Famous for his researches on cyanogen, cyanuric acid, and the radical of benzoic acid, and on the metals titanium, aluminum, yttrium, beryllium, and vanadium. German translator of Berzelius Textbook of Chemistry and Hisinger s Mineral Geography. ... 

The simple phenol system has been discussed here at some length, because material balance is obtained and mechanistic details are fairly well understood. However, according to the data in Tables 3.5 and 3.6, there is a very noticeable gap in the material balance in the case of the hydroxylated benzoic acids, although some aspects such as the acid-catalyzed water elimination, in salicylate also more pronounced in the case of the para-OH-adduct radical, are very similar (Mark and von Sonntag, unpubl.). Interestingly, addition of Fe(III) to oxidize the intermediates also did not improve the material balance (Tables 3.5 and 3.6). From this, it follows that the underlying chemistry of the salicylate and the other hydroxybenzoate systems are at present not yet adequately understood, and the... 

O-H deprotonation of arylalkanol (I), benzoic acid (II) and arylalkanoic acid (III) radical cations eventually leads to radical zwitterions where the electron hole is localized on the aromatic system and the negative charge resides on the oxygen atom originally bonded to the acidic proton (Scheme 65). From the radical zwitterions an oxygen centered radical can be formed by intramolecular electron transfer from the side-chain -0 to the aromatic jr-system, whose chemistry will be discussed below. 

The relation between the constant of the para acid and that of the ortho or meta acid varies with the nature of the substituent. While p-nitrobenzoic acid is a slightly stronger acid than m-nitro-benzoic acid, the constant of p-chlorobenzoic acid is only about one-half that of the meta acid. The case of p-hydroxybenzoic acid is a striking one while o-hydroxybenzoic acid and m-hy-droxybenzoic acid are more highly ionized than benzoic acid, the constant of the para acid is less than half that of benzoic acid. A satisfactory explanation of such facts as these would, no doubt, materially advance organic chemistry. The effect of a phenyl radical on a carboxyl group in a side-chain, is shown by the constants for phenylacetic acid, hydrocinnamic acid, and cinnamic acid. 

Hydrogen peroxide can generate the extremely reactive hydroxyl radical (OH ) via Fenton chemistry which can be monitored by the detection assay based on the hydroxylation of benzoic acid (Gutteridge, 1987 Hunt etal., 1988, 1992a, 1993), shown in Fig. 7. 

For a number of different substituted benzenes, the substituents are involved themselves in the redox chemistry. Among these substituent-active compounds, we find phenolic substances, aromatic amines, chalcogenide substituted benzenes, benzyUc substances, benzoic adds, thiobenzoic adds and benzyl alcohols. As has already been discussed, phenol radical cations, aniline radical cations and toluene radical cations are more or less acidic and can deprotonate to form the corresponding neutral radical. Chedcogenide substituted benzenes (S, Se and Te) are usually characterized by the fact that their redox properties are determined by the chalcogenides rather than by the substituent pattern. 

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