2,4-dihydroxybenzoic acid, decarboxylation

Further evidence regarding the mechanism was provided by LynnandBoums643 , who found a pH-dependent carbon-13 isotope effect in the decarboxylation of 2,4-dihydroxybenzoic acid in acetate buffers. The dependence was interpreted in favour of the A-SE2 mechanism, for an increase in acetate concentration would increase kL t and hence partitioning of the intermediate so that k 2 becomes more rate-determining. 

Exercise 26-10 Explain why gallic acid decarboxylates on heating more readily than benzoic acid. Would you expect 2,5-dihydroxybenzoic acid to decarboxylate as readily as its 2,4 isomer Explain. 

The solvent isotope effect in the decarboxylation of 2,4-dihydroxy-benzoic acid is ( atcoohJhjO /( ArcooD)D,o = 6.25. The solvent isotope effect on the acid ionization constant of 2,4-dihydroxybenzoic acid has been determined separately, by potentiometric methods ... 

Kinetic carbon-13 isotope effects in the decarboxylation of 2,4-dihydroxybenzoic acid at 85 °C [247, 249]... 

These results indicate that proton transfer occurs in the ratedetermining step and there is little carbon—carbon bond cleavage in the transition state. Clear-cut evidence for a two-step mechanism is supplied by Lynn and Bourns results of variable carbon-13 isotope effects in the decarboxylation of 2,4-dihydroxybenzoic acid in acetate buffers [249] (Table 22). Slow proton transfer occurs in the first step and C—C bond cleavage takes place in the second step. At high concentrations of the buffer base, the rate of reversal of the first step becomes comparable to the (relatively fast) rate of the second step and, consequently, the second step becomes partially rate-determining which causes a weak carbon isotope effect. The most reasonable structure of the intermediate is that of the sigma complex. 

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... 

In less than one minute, half of the 2,4-dihydroxybenzoic acid is decomposed already at 160 °C in the microreactor setup [39, 40]. This demonstrates the need to find optimal process parameters for temperature and residence time to achieve efficient high-p,T operation, which is achieved by detailed parameter variation (e.g., as statistical analysis. Design of Experiments, DoE). In this way, the best operation point was foimd (200 °C 40 bar 2000 mL h 16 s). There is also the question of substrate selectivity, e.g., the isomeric product 2,6-dihydroxybenzoic acid exhibits lower decarboxylation rates. 

Carbon isotope effects can be used in a straightforward way to determine whether a bond cleaves in the rate-limiting step. For example the isotope effect on the decarboxylation of 2,4-dihydroxybenzoic acid [30a] varies with the nature and concentration of acid catalyst suggesting a change in rate-determining step (Eqn. 49). 

The degradation, by decarboxylation, of a-naphthaleneacetic acid has been shown to be wavelength dependent. Short wavelengths are the more destructive and the decarboxylation follows pseudo-first order kinetics. The photodegradation of 2,4-dihydroxybenzoic acid in water catalysed by titanium dioxide has been reported in some detail. ... 

Bromoresorcinol has been prepared by the monobromination of resorcinol monobenzoate and subsequent hydrolysis,1 from 2-bromo-5-aminophenol by the diazo reaction,2 by treating resorcinol with dichlorourea and potassium bromide,3 and by the bromination of 2,4-dihydroxybenzoic acid followed by decarboxylation.4 The above procedure is based particularly upon the observations of Rice.4... 

The copolymerization of 5-[(4-fluorophenyl)sulfonyl]-2-fiuorobenz-oic acid with bis-(4-hydroxyphenyl)-sulfone results in carboxylated PES. However, during polycondensation, partial decarboxylation occurs. The copolymerization of 2,5-dihydroxybenzoic acid with bis-(4-fluorophenyl)-sulfone results in a PES with quantitative decarboxylation. ... 

Preparation by decarboxylation of 3-benzoyl-5-chloro-2,4-dihydroxybenzoic acid with concentrated hydrochloric acid in refluxing dilute acetic acid for 24 h [1203], (30%) [1204],... 

Also obtained by decarboxylation of 5-acetyl-3-benzoyl-2,4-dihydroxybenzoic acid on heating in a test-tube at 220-225° for 1 h [234],... 

Also obtained by decarboxylation of 3-acetyl-5-bromo-2,4-dihydroxybenzoic acid with dilute acetic acid containing few drops of concentrated hydrochloric acid, at reflux (40%) [1982,1983]. 

Obtained by decarboxylation of 3-acetyl-5-chloro-2,4-dihydroxybenzoic acid [1982,1983]. 

Also obtained by decarboxylation of 2-acetyl-3,5-dihydroxybenzoic acid with copper powder in quinoline at 220-230° [2319]. 

Habibi D, Nematollahi D, Meshldnghalam S, Varmaghani E (2014) An unexpected oxidative decarboxylation reaction of 2,3-dihydroxybenzoic acid in the synthesis of new dibenzylte-trahydroquinoxalinediones. Tetrahedron 70 4361-4366. doi 10.1016/j.teL2014.04.077 Haddadin MJ, Issidorides CH (1976) Application of benzolurazan oxide to the synthesis of heteroaromatic A-oxides. Heterocycles 4(4) 767-816. doi 10.3987/R-1976-04-0767 Haddadin MJ, Agopian G, Issidorides CH (1971) Synthesis and photolysis of some substituted quinoxaline di-A-oxides. J Org Chem 36(4) 514-518. doi 10.1021/jo00803a005 Haddadin MJ, Bitar H, Issidorides CH (1979) Synthesis and some reactions of o-nitrosoaniline. 

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