Ultraviolet absorption carboxylic acids

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

The molecular absorption intensity of polar compounds is usually small, but highly sensitive detection can be obtained after pre- or post-column derivatiza-tions. The use of ultraviolet absorption or fluorescence-active counter-ions makes it possible to achieve highly sensitive detection of polar compounds and enhance the capability of ion-pair liquid chromatography. For example, N,N-dimethylprotriptyline has been used as a counter-ion for carboxylic acids12 and picric acid for quaternary amines13 in normal-phase ion-pair partition liquid chromatography. Phenethylammonium, cetylpyridinium, l-phenethyl-2-pyco-linium, and naphthalene-2-sulfonic acid have been used for sulfonic acid and alkyl amines detection.14,15 Ion-pair post-column extraction was applied on-line for fluorescence detection.16... 

The carboxyl function does absorb ultraviolet radiation, but the wavelengths at which this occurs are appreciably shorter than for carbonyl compounds such as aldehydes and ketones, and, in fact, are out of the range of most commercial ultraviolet spectrometers. Some idea of how the hydroxyl substituent modifies the absorption properties of the carbonyl group in carboxylic acids can be seen from Table 18-2, in which are listed the wavelengths of maximum light absorption (Amax) and the extinction coefficients at maximum absorption (emax) of several carboxylic acids, aldehydes, and ketones. 

Wavelengths tor Maximum Ultraviolet Absorption of Some Carboxylic Acids, Aldehydes, and Ketones (n--- n )... 

The carbonyl group. Acetone (in cyclohexane solution) exhibits two absorption bands one appears at 190nm (e 1860) and corresponds to the n - n transition, while the second is at 280 nm (e 13) and corresponds to the n - n transition. The absorption maxima of these bands are solvent-dependent. Ultraviolet spectra of saturated aldehydes, carboxylic acids, esters and lactones exhibit a similar absorption profile, and in general are of little diagnostic value. 

Comparison of the ultraviolet absorption and ionization constants of aminopyrazine, methylaminopyrazine, and dimethylaminopyra-zine indicates that aminopyrazine exists as such and not in the tautomeric imino form.147 Aminopyrazines, like their pyridine analogs, form diazonium salts, which readily decompose to the corresponding pyrazinones. For example, nitrous acid treatment of aminopyrazine318 and 2-aminopyrazine-5-carboxylic acid210 gives the corresponding pyrazinones in 30 and 59% yield, respectively. The diazonium salt from aminopyrazine cannot be converted into bromo-pyrazine under the conditions of the Sandmeyer reaction. 

LOD, limit of detection NR, not reported MDC. mircodiffusion cell TCA, trichloroacetic acid AAS. atomic absorption spectrometry UV, ultraviolet absorbance detection ECD, electron capture detection NPD, nitrogen phosphorus detection GC-MSD, gas chromatography-mass selective detection GC, gas chromatography ATC, 2-amino-1hiazoline-4-carboxylic acid. 

Ludwig. This compound has an empirical formula corresponding to structure (1) and shows the ultraviolet and infrared" absorptions of a pyrrole-3-carboxylic ester. Its acetylation gives a tetra-O-acetyl derivative." Oxidation with lead tetraacetate yields ethyl 5-formyl-2-methyl-pyrrole-3-carboxylate (4), identical with the compound prepared in a different way. Oxidation with potassium permanganate in alkaline solution at low temperature yields 3-(ethoxycarbonyl)-2-methylpyrrole-5-carboxylic acid (7) which can be transformed " into the diethyl ester (8), identical... 

In these and other papers by Anslow and coworkers the frequency at which absorption is complete, is used to calculate the energy required for the presumed related dissociation process (e.g. of a proton from a carboxyl group, fission of the S—S bond in cystine), using an equation relating the mass of the ion produced to the frequency of complete absorption (see also Anslow, 1932a, b). As the frequency at which complete dissociation commences has been obtained with complete disregard for the well-established selective absorption of saturated carboxylic acids and aminocarboxylic acids in the vacuum ultraviolet below 2000 A. (see Platt and Klevens, 1944, for a review of the available data), and the more easily accessible short wave bands of the aromatic amino acids (see Section III, 1), and as the theoretical basis of the equation concerned is open to serious criticism, this particular aspect of the work will not be considered further. 

The new degradation product (27) was isolated preparatively, and its structure was determined by the classical methods of analytical and physical chemistry. The most remarkable characteristic was the strong absorption in the ultraviolet (u.v.) spectrum at 232 nm (log e = 3.75), indicating the presence of an a,j8-unsaturated carboxylic acid. Disaccha-... 

The apparent ionization constants and ultraviolet absorption spectra of the three isomeric pyridine monocarboxylic acids were determined at 25 in aqueous KCI. As expected, it was confirmed that these acids exist in aqueous isoelectric solutions mainly in the dipolar ion form. Similar data for monocarboxylic and monosulfonic acids and polyvalent acids were obtained more recently. The dissociation constants and electrophoretic mobilities of fifteen substituted alkyl-, nitro-, and chloropyiidine carboxylic acids were also determined. The ultraviolet absorption spectra of a series of dihydropyridine-3, S-dicarboxylic acids were correlated with electronic and steric factors. ... 

Ultraviolet spectmm of carboxylic acids and related derivatives also had been studied. Their ultraviolet absorption characteristics can be given as follows ... 

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