Carboxylic acids, resonance structures

The reaction is less selective than the related benzoylation reaction (/pMe = 30.2, cf. 626), thereby indicating a greater charge on the electrophile this is in complete agreement with the greater ease of nuclophilic substitution of sulphonic acids and derivatives compared to carboxylic acids and derivatives and may be rationalized from a consideration of resonance structures. The effect of substituents on the reactivity of the sulphonyl chloride follows from the effect of stabilizing the aryl-sulphonium ion formed in the ionisation step (81) or from the effect on the preequilibrium step (79). 

In para-amino benzoic acid, there is another resonance structure right next to the six-sided ring. It is a carboxyl group, shown with a single bond between carbons, and a double bond between the carbon and the oxygen. This is also a place where the electron can bounce around between the three nuclei. 

In order to co clarify the role of complex formation, the new data on stability constants should be accumulated, being collected at strictly similar conditions. It should be also mentioned that any analysis of equilibrium in solutions involving anions of polybasic hydroxy carboxylic acids requires the data on the deprotonation constants of the acid in question. This information would be crucial for conclusions regarding the presence and stability of mixed complexes in the system. Valuable knowledge about the structure of complex compounds present in solutions (and in precursors as well, see later) may be gained by means of vibrational spectroscopy (IR and Raman spectra) and nuclear magnetic resonance. 

Irradiation of matrix-isolated imidazole-2-carboxylic acid gave the 2,3-dihydro-imidazol-2-ylidene-C02 complex (31) characterized by IR spectroscopy and calculated to lie 15.9 kcal mol above the starting material. A series of non-aromatic nucleophilic carbenes (32) were prepared by desulfurization of the corresponding thiones by molten potassium in boiling THF. The most hindered of the series (32 R = Bu) is stable indefinitely under exclusion of air and water and can be distilled without decomposition. The less hindered carbenes slowly dimerize to the corresponding alkenes. Stable aminoxy- and aminothiocarbenes (33 X = O, S) were prepared by deprotonation of iminium salts with lithium amide bases. The carbene carbon resonance appears at 260-297 ppm in the NMR spectrum and an X-ray structure determination of an aminooxycarbene indicated that electron donation from the nitrogen is more important than that from oxygen. These carbenes do not dimerize. 

Although phenoxlde ion has more number of resonating structures than carbojQ late Ion, carboxylic acid is a stronger acid than phenol. Why ... 

Problem 16.4 Write resonance structures for the COOH group and show how these and orbital hybridization account for (a) polarity and dipole moments (1.7-1.9D) of carboxylic acids (b) their low reactivity toward nucleophilic additions, as compared to carbonyl compounds. ... 

C—OH = 1.36 0.01 A, and these values have been supported by several infrared and microwave spectroscopic investigations. Nearly the same values have been reported also for many other carboxylic acids. The bond numbers calculated from these bond lengths are about 1.85 and 1.15, respectively that is, the presence of the hydrogen atom causes structure A to make an 85 percent and structure B only a 15 percent contribution. Essentially the same resonance is found in esters for methyl formate, for example, the bond lengths in the carboxylate group are88 C=0 = 1.22 0.03 A and C—OCH3 = 1.37 0.04 A. 

Structures III and IV assist ionisation of the C-X bond, whereas structure II facilitates nucleophilic addition and consequently a bimolecular displacement of X. The various derivatives of carboxylic acids form a series with varying degrees of resonance stabilisation decreasing in the following order ... 

Thiazolidine-4-carboxylic acid containing peptides exhibit an enhanced polar character when compared with most of the side-chain protected cysteine peptides. 139 The anomalously low nucleophilicity of its imino group (pATa = 6.24) in comparison to that of proline (pXa = 10.60) is due to the resonance stabilization of its unprotonated form 187188 This supposed resonance stabilization is supported by the X-ray crystal structure of 4-thiaproline.1 88 ... 

While the benzene ring is the most familiar example of the necessity for modifying the Lewis structure language by the addition of the resonance concept, there are many others. The carboxylic acids, for example, are much stronger acids than the alcohols this difference musthedne largely-ln greater-stability of... 

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