Benzoic acid, labelled with isotopic

Benzimidazole derivatives, III, 161 Benzimidazole rule of rotation, I, 21 1,3-Benzodioxan, 8,8 -methyIenebis(6- nitro-), III, xvii Benzoic acid, cellulose esters, I, 320 labelled with isotopic C, III, 231 starch esters, I, 302, 303 —, 2,4-dihydroxy-, effect on conductivity of boric acid, IV, 195 —, 2,4,5-trihydroxy-, effect on conductivity of boric acid, IV, 195 —, 3,4,5-trihydroxy-. See Gallic acid. 

A cleai-cut answer was provided by Irving Roberts and Harold C. Urey of Columbia University in 1938. They prepared methanol that had been enriched in the rnass-18 isotope of oxygen. When this sfflnple of methanol was esterified with benzoic acid, the methyl benzoate product contained all the label that was originally present in the methanol. 

Evidence in support of the mechanism shown in figure 21.4 comes from isotope-labeling experiments. When 180-labeled methanol reacts with benzoic acid, the methyl benzoate produced is found to be l80-labeled but the water produced is uniabeled. Thus, it is the C-OH bond of the carboxylic acid that is broken during the reaction rather than the CO—H bond and the RO-H bond of the alcohol that is broken rather than the R-OH bond. 

Gross and coworkers129 also studied the unimolecular dissociation of protonated acy-lanilines, viz. A-[2-(benzoyloxy)phenyl]benzamides formed via both fast-atom bombardment (FAB) and electrospray ionization (ESI). They found that cyclization occurs upon the loss of a molecule of benzoic acid, and that a similar process occurs for the molecular ion under El. This gas-phase reaction is analogous to a solution reaction leading to phenyl-benzoxazoles. The proposed cyclization process, for which concurrent mechanisms were proposed (Scheme 38 depicts only the displacement reaction route), was corroborated by accurate mass measurements, tandem mass spectrometric experiments with comparison with reference ions, isotopic labeling and theoretical calculations. 

Isotopes may be conveniently used to obtain additional information about reaction mechanisms. A particular example is the formation of an ester from benzoic acid and methyl alcohol. By labeling the methyl alcohol with it can be shown that the carbon-oxygen bond in the alcohol remains intact during the course of the reaction, implying that an OH group is lost from the acid [13]. 

Upon Incubation of membranes with differently labelled phenylalanine plus cinnamic acid, benzoic acid was labelled predominantly by the Isotope which was applied as marker of phenylalanine. 

Chromatography of various isotopomers of benzoic acid shows a moderate effect of number of deuterium atoms on the separation factor however, an important effect is exerted by the position of the label (Table 3). Isotopomers containing deuterium in the ortho positions display lower isotopic separation than isotopomers with deuterium in meta or para position. The isotope effects associated with the ortho positions are minimal, probably because interactions between ortho C- H or C- H bonds and the stationary phase are minimized by the steric shielding of the carboxyl group. 

This question was resolved using isotopic labeling. For example, Fischer esterification of benzoic acid with methanol that had been enriched with the isotope of oxygen gave labeled methyl benzoate. ... 

NMR spectroscopy of urine combined with labelling with stable isotopes such as and has been used to monitor the silent process of deaceteylation and subsequent reacetylation (futile deacteylation) in the rat as this has implications for the toxicity of paracetamol and phenacetin. The metabolites of 5-fluorouracil in plasma and urine have been analysed using i F NMR spectroscopy in patients receiving chemotherapy. NMR has been employed to study the pharmacokinetics of benzoic acid in relation to liver function. The metabolism of hydrazine in rats has been probed using NMR spectroscopy of urine. 

Explain why benzoic acid with an isotopic label in the hydroxyl oxygen atom can be prepared, but that it cannot be used in mechanistic studies in aqueous solutions. 

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