Trihydroxybenzoic acids

A subsequent kinetic investigation of the effect of added hydrochloric acid upon the rate of decarboxylation of 2,4,6-trihydroxybenzoic acid was analysed in terms of three possible mechanisms640 ... 

Figure 10-4. 2,4,6-Trihydroxybenzoic acid, 4,5-dihydroxy-m-benzenedisul-fonic acid.

All dyes are prone to fading caused mainly by chemical oxidation when exposed to light. The formation of low molecular mass products has been evidenced, such as 2,4-dihydroxybenzoic acid and 2,4,6-trihydroxybenzoic acid from morin degradation, in old fustic dyed samples [169] and from luteolin in weld and dyer s broom [170],... 

Ring protonation has been looked for in some other phenolic systems also. Thus a C-protonated cation is observed at low temperature when 2,4,6-trihydroxybenzoic acid is dissolved in 70% perchloric acid and pure fluorosulphuric acid (Birchall et al., 1964). A... 

Hydroxy acids have been protected as acetals which are l,3-dioxan-4-ones. Numerous examples of such dioxa-nones were reported, and they have been widely used in synthetic organic chemistry. In particular, dioxanone triflates prepared from 2,4,6-trihydroxybenzoic acid or analogs were used for several transition metal-catalyzed crosscouplings. A Suzuki coupling <2006EJ01678> and a Stille coupling <2005JOC3686> provide illustrations of this principle (Scheme 93). 

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. 

General acid catalysis by anilinium and pyridinium ions has been observed in the decarboxylation of 2,4,6-trihydroxybenzoic acid [246] and 4-aminosalicylic acid [245]. Acetic acid acts as a general catalyst in the reactions of 4-aminosalicylic acid and 2-hydroxyl-l-naphthoic acid [82, 245]. In all of these examples, reaction rates obey the equation... 

According to Schubert and Gardner s results [239], the rate coefficient of decarboxylation of 2,4,6-trihydroxybenzoic acid remains unchanged at ft = ftArcooH = K. kfi in 10 % (1 M) to 38 % aqueous perchloric acid. In the case of 2,4-dihydroxybenzoic acid, however, k rises to values much... 

A similar change of the rate-determining step from C protonation to carbon—carbon bond cleavage occurs also in the decarboxylation mechanism of 2,4,6-trihydroxybenzoic acid, for a carbon isotope effect of ki2/kl3 = 1-041 in 8.35 M perchloric acid has been reported [248]. There is no primary carbon isotope effect in the same reaction in 10-3 M perchloric acid kn/k 3 = 1.006. 

Clarke, Clarke and Mosher claim that 2,3,6-trihydroxybenzoic acid VIII) is 10 times, and 2,4,6-trihydroxybenzoic acid IX) 8 times, more active than sodium salicylate in rheumatic fever, but Hamilton and Bywaters found IX) to be inactive in three patients receiving 1-8 g in 24 hours . 

Electron-withdrawing substituents of -M type must diminish the basicity of the aromatic ring, naturally hinders direct observation of arenium ions due to the reduction of their stability. Besides, the -M type substituents are basic centra themselves, so in similar cases the proton is often attached to the substituent rather than to the aromatic ring. This has been observed, for instance, upon protonation of acetophenone, benzophenone and their derivatives benzoic acid nitrobenzene and nitromesitylene The formation of arenium ions from compounds containing -M type substituents can only be observed when simultaneously the molecule contains strong electron-releasing substituents. An example is the formation of ion (8) when 2,4,6-trihydroxybenzoic acid is dissolved in 70% HCIO or HS03F >. 

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