Azulene-1-carboxylic acid, decarboxylation

The observed first-order rate coefficient of the decarboxylation of azulene-l-carboxylic acid [10] at 25 °C as a function of [H+] at a constant ionic strength of / = 0.5 M follows eqn. (13), which is a special case of eqn. (5), viz. 

Rate coefficients, DjO solvent isotope effects, and carbon-13 isotope effects in the decarboxylation of azulene-l-carboxylic acid and 2,4-dihydroxybenzoic acid in solutions of moderately concentrated strong acids... 

The constancy of k in the acidity region above 3.5 M HC1 indicates a change of the rate-determining step. In that region, the transition state formed by carbon—carbon bond cleavage in X contains as many protons as the main form of the substrate in the solution, HS, and consequently, the rate must be independent of [H30+] or h0. Additional evidence for carbon—carbon bond cleavage in the slow step in solutions with H0 values below —1.3 may be obtained from results of carbon-13 [247] (Table 22) and D20 solvent isotope effect [259] studies (Table 24). The value of h2o/ d2o decreases from 2.0 in 1 M HC1 to 1.3 in 4 M HC1, and remains constant if the HC1 concentration is increased further, in a very similar way as in the example of the decarboxylation of azulene-1-carboxylic acid. 

C17H22O3, Mr 306.36, cryst., mp. 158-160°C. Flowers of chamomile (Matricaria chamomilla, Astera-ceae) contain ca. 0.15% of proazulenes, especially M., which can be obtained by chloroform extraction. When extracts containing M. are heated, elimination of water and acetic acid occurs to furnish chamazulene-carboxylic acid which, as an a-aryl-carboxylic acid, easily undergoes decarboxylation to chamazulene (see azulenes). 

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