Ortho-unsubstituted benzoic acids

Under similar conditions, ortho-unsubstituted benzoic acids undergo double alkenylation at the both ortho-positions and subsequent decarboxylation to form 1,3-distyrylbenzenes (Scheme 4.29). 

More widely applicable acylation of ortho-unsubstituted benzoic acids using a catalytic amount of magnetite nanoparticle and aliphatic carboxylic acids as acyl source can be realized (Scheme 4.36) [41]. 

Substituent effects on the A,u I reaction have been studied by Bender and Chen55. These authors measured the rates of hydrolysis of a series of 4-substituted 2,6-dimethylbenzoates in 9.70 M sulphuric acid at 25°C, and found that the values for the first-order coefficients with 4-methoxy, 4-methyl, 4-unsubstituted and 4-bromo-compounds (5.0, 0.37, 0.033 and 0.01 x I0 4 sec-1, respectively) are satisfactorily correlated by the Hammett equation, following cr+ with a slope p = —3.22. Since the esters are not fully protonated in 9.70 M H2SOj, part of this factor is due to the effects of the 4-substituent on the protonation equilibrium, p for the protonation of substituted benzoic acids is about — l35, but is likely to be considerably smaller for di-ortho-substituted compounds, since the conjugative interaction of the p-substituents with the protonated carboxyl group requires coplanarity with the ring. 

If we compare the acid strengths Ka) of a series of substituted benzoic acids with the strength of benzoic acid itself (Table 26-4), we see that there are considerable variations with the nature of the substituent and its ring position, ortho, meta, or para. Thus all three nitrobenzoic acids are appreciably stronger than benzoic acid in the order ortho para > meta. A methoxy substituent in the ortho or meta position has a smaller acid-strengthening effect, and in the para position decreases the acid strength relative to benzoic acid. Rate effects also are produced by different substituents, as is evident from the data in Table 26-5 for basic hydrolysis of some substituted ethyl benzoates. A nitro substituent increases the rate, whereas methyl and methoxy substituents decrease the rate relative to that of the unsubstituted ester. 

Hapten antibody interaction of N=36 ortho-, meta- and para-substituted benzoic acids [K relative to K of unsubstituted compound), was correlated with following parameter combinations B° and... 

---