Carboxylic acid protonation

It will be similar for an acyl-CoA. However, for a carboxylic acid, protonated on oxygen, the pRa will be much higher ( 22-25) and for carboxylate anions even higher ( 29-32).100... 

The proton spectrum of phenylacetic acid (C6H5 CH2 C02H Fig. 3.47) exhibits three absorptions in the ratio 1 2 5 due to the carboxylic acid, methylene and phenyl protons respectively. The carboxylic acid proton has been offset by... 

Grignard reagents are not compatible with carboxylic acids proton transfer converts the Grignard reagent to the corresponding hydrocarbon. 

Tolonen et al. [167] described a simple and efficient method for determination of labile protons in drug metabolites using postcolumn infusion of deuterium oxide in LC/MS experiments with ESI and TOF-MS. The number of exchangeable protons in analytes hydroxyl, amine, thiol, and carboxylic acid protons can easily be determined by comparing the increase in m/z values after H/D-exchange occurring on line between... 

Apparently, the carboxylic acid proton of pKa-4.8 assists readily the halogen expulsion as compared with the H atom of the CH3 group (pKa - 48). In order to support this conclusion, the H of the COOH was replaced by a methyl group to give methyl 2-halopropi-onates. The comparative kinetic data are shown in Table 13. The methyl haloesters pyrolyze much slower than the free acids and they probably eliminate HX through a normal four-membered transition state. 

From Appendix 1, we know that >K, the dissociation constant associated with carboxylic acid protons, is approximately 4.75. Furthermore, from Appendix 1, we know that pATa, the dissociation constant associated with protonated amines, is approximately 10. Finally, remembering that pATa = logATa, the Keq for this reaction is approximately... 

Carboxylic Acid Protons Because carboxylic acid protons are bonded to an oxygen next to a carbonyl group, they have considerable positive character. They are strongly deshielded and absorb at chemical shifts greater than 510. Carboxylic acids frequently exist as hydrogen-bonded dimers (shown at left), with moderate rates of proton exchange that broaden the absorption of the acid proton. 

Carboxylic acid protons are the most deshielded protons we have encountered, absorbing between 610 and 613. Depending on the solvent and the concentration, this acid proton peak may be sharp or broad, but it is always unsplit because of proton exchange. 

A linear relationship between isotropic 2H chemical shift (6H) and 0---0 distance (r0...o) has also been established [77] for several metal phosphates and minerals. Similarly, for carboxylic acid protons, SH has been shown [78] to depend linearly on r0...0, and for several trihydrogen selenites, SH was shown [79] to correlate linearly with r0...0 and rH...0 distances. 

The XH NMR spectrum for fumaric acid contains two lines, assigned to the olefinic and carboxylic acid protons, the latter of which is characterised by the same O- -O distance and has the same Siso value as the intermolecular hydrogen bond in maleic acid. 

Hydrogens a to one carbonyl group are weakly acidic. Hydrogens a to two carbonyl groups are much more acidic, but they are not as acidic as carboxylic acid protons. 

Minor species. For an amino acid such as alanine, the major species in solution at pH 7 is the zwitterionic form. Assume a p.ST value of 8 for the amino group and a pAT value of 3 for the carboxylic acid and estimate the ratio of the concentration of neutral amino acid species (with the carboxylic acid protonated and the amino group neutral) to that of the zwitterionic species at pH 7. 

The catalyst 6 forms an imminium species (not shown) via a nucleophilic attack of the Lewis basic amine at the carbonyl moiety of substrate 7. The imminium ion is deprotonated to give enamine 8. Coordination of the electrophile 9, which can be an aldehyde, ketone or azodicarboxylate, leads to the transition state 10. In this way, the facial orientation of the substrates is more or less fixed in a cyclic conformation, depending on the steric- and electronic properties of the constellation. The electron-donating properties of the nitrogen atom then directs the addition to the electrophile that is saturated by the Bronsted acid, in this case the carboxylic acidic proton. 

The rate constant (36) and (37) as a function of temperature correlated well with the experimental data obtained for the carboxylic acid protons of crystalline perprotobenzoic acid and ring-deuterobenzoic acids by nuclear magnetic resonance 7) [75] and inelastic neutron scattering (for an analysis of the experiment see Refs. 76 and 77). It should be noted that some of the major parameters of the model (for instance,. /) allowed the direct determination by fluorescence line narrowing technique. 

Methylation of hydrolyzed or methanolized copolymers. One gram samples of hydrolyzed or methanolized copolymers were suspended or dissolved in dry benzene and the mixtures were treated with diazomethane until methylation was complete. This was indicated by the complete dissolution of the copolymers and by the presence of the yellow color of diazomethane in the reaction mixtures for 8 hrs. The reaction mixtures were poured into methanol to precipitate the products, which were reprecipitated twice from benzene solution into methanol before being dried in vacuo. Completely methylated copolymers were soluble in CCli, but copolymers containing very small amounts of acid units were not. This simple test was employed routinely to ensure the completeness of methylation. The 1H-NMR spectra of the copolymers in C D6 solution were examined for the presence of carboxylic acid proton resonances at 5=11.5 ppm as an additional criterion for the completeness of methylation. 

The carboxylic acid protons which are extremely labile hydrogens exchange with many other types of labile hydrogen to produce an averaged chemical shift for the protons involved in the exchange. 

Values of and are mixed acid ionization constants for loss of first and second carboxylic acid protons, respectively. Values of reported by Miles and co-workers. ifj and A j are thermodynamic dissociation constants reported by McCoy. Values of 23 calculated from unpublished temperature jump data of L. D. Rich, E. M. Eyring, and... 

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