Para-methoxy-benzoic acid

When substituents can also be involved in the resonance effects, changes in acidity become more marked. Consider hydroxy- and methoxy-benzoic acid derivatives. The pATa values are found to be 3.0, 4.1, and 4.6 for the ortho, meta, and para hydroxy derivatives respectively, and 4.1, 4.1, and 4.5 respectively for the corresponding methoxy derivatives. 

The effect of a para-alkoxy substitution of benzoic acid is also clear the pKa-values and Tmax.(exotherm)-values are increasing going from benzoic acid, to 4-methoxy-benzoic acid and to 4-ethoxy-benzoic acid. Besides, the dH-values of the last two mentioned acids are clearly lower than all the other dH-values. Such a difference in Tmax.(exotherm)-value is not present between benzoic acid and 3-methoxy-benzoic acid (meta-alkoxy substitution). The reason for the decreased epoxide reactivity due to para-alkoxy substitution might be the conjugated mesomeric structure which causes an extra negative charge on the carbonyl-group. 

Careful measurements reveal that para-methoxyben-zoic acid is less acidic than benzoic acid, while meta-methoxy-benzoic acid is more acidic than benzoic acid. Explain these observations. 

Sodium in liquid ammonia and ethanol reduced benzoic acid to 1,4-dihy-drobenzoic acid. Reduction of p-toluic acid was more complicated and afforded a mixture of cis- and rranj-l,2,3,4-tetrahydro-p-toluic acid and cis-and tra j-l,4-dihydrotoluic acid. m-Methoxybenzoic acid yielded 1,2,3,4-tetrahydro-5-methoxybenzoic acid, and 3,4,5-trimethoxybenzoic acid gave l,4-dihydro-3,5-dimethoxybenzoic acid in 87% yield (after hydrogenolysis of the methoxy group para to carboxyl) [984. In the case of 4 -methoxy-biphenyl-4-carboxylic acid, sodium in isoamyl alcohol at 130° reduced completely only the ring with the carboxylic group, thus giving 92% yield of 4-(p-methoxyphenyl)cyclohexanecarboxylic acid [955]. 

This does not occur in the case of catalyst and reactants here described. With Bronsted-type catalysis, the reaction between the benzoyl cation, Ph-C" =0, and the hydroxy group in phenol is quicker than the electrophilic substitution in the ring. This hypothesis has been also confirmed by running the reaction between anisole and benzoic acid in this case the prevailing products were (4-methoxy)phenylmethanone (the product of para-C-benzoylation) and methylbenzoate (obtained by esterification between anisole and benzoic acid, with the co-production of phenol), with minor amounts of phenylbenzoate, phenol, 2-methylphenol and 4-methylphenol. Therefore, when the 0 atom is not available for the esterification due to the presence of the substituent, the direct C-acylation becomes the more favored reaction. 

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. 

The preparation of a number of medium ring benzoic acid lactones was achieved by treatment of compounds such as VIII/176 with an excess of meta-chloroperoxybenzoic acid in dichloromethane, Scheme VIII/33 [103]. However, this oxidation reaction is not general for the synthesis of aromatic lactones. If the same reaction conditions are used as in the conversion of VIII/176 to VIII/177, the methoxy derivative VIII/178 is not transformed into the corresponding lactone. Instead the cyclic carbonate VIII/183 was isolated in a yield of 50 %. The proposed mechanism of this abnormal reaction is shown in Scheme VIII/33. From model compounds, the methoxyl group in the para-position to the center of oxidation seems to be important for the formation of VIII/183 [103]. The carbonate VIII/183 is unstable in aqueous alkaline medium and decomposes to the spiro compound, VIII/185, Scheme VIII/33 [103]. For an analogous reaction, see ref. [104]. 

The chemical information available through LFER is primarily the reaction constant p, but this value depends upon the substituent constants selected for the construction of the LFER. The u values available are ct, ct", ct" or and Ui, these quantities are listed, for many substituents, in Tables 7-1, 7-3, 7-4, 7-6, and 7-7. A reasonable approach is to plot log k against the substituent constant defined by a standard reaction that is expected to be most like the reaction under study. It is also reasonable to plot log k against several of the ct quantities, seeking the best correlation. [In choosing between two types of substituent constants, it is necessary to make use of substituents for which the two scales (say ct and rr, for example) are not themselves correlated, for otherwise both LFERs will be acceptable. ] The ct or o constants should be applicable to reactions that do not combine reaction sites and para substituents of the + and — type (push-pull systems capable of through resonance) for example, one would not expect ct" or o to provide good correlations for reactions of phenols or anilines substituted with nitro or cyano or for reactions of benzoic acids substituted with amino or methoxy. 

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