Protonation of carboxylic acids

Babbit, P.C., et al. A functionally diverse enzyme superfamily that abstracts the a protons of carboxylic acids. Science 267 1159-1161, 1995. 

A question that may justifiably be raised here is whether these proofs of exclusive carbonyl protonation of carboxylic acids in concentrated and anhydrous acids necessarily imply the dominance of this form in dilute acid. Evidence that this is not so for amides has been discussed on pages 328 ff. It is possible that the alternative protonated form of carboxylic acids [201] is dominant in aqueous acid, but as the overall extents of protonation are small, it is not detectable by any spectroscopic method. Unlike amides, carboxylic acids become measurably protonated in quite concentrated acid (>60% sulphuric acid), which would tend to favour the formation of the protonated form with a delocalized charge. The form [201]... 

A variety of 1,3-dicationic carboxonium superelectrophiles have been generated from protonation of carboxylic acids and their derivatives. For example, carboxonium-carbenium dications have been proposed in the superacid promoted reaction of cinnamic acid (50) and related... 

Table 13.1 collects chemical-shift information for protons of various types. Within each type, methyl (CH3) protons are more shielded than methylene (CH2) protons, and methylene protons are more shielded than methine (CH) protons. These differences are small—only about 0.7 ppm separates a methyl proton from a methine proton of the same type. Overall, proton chemical shifts among common organic compounds encompass a range of about 12 ppm. The protons in alkanes are the most shielded, and O—H protons of carboxylic acids are the least shielded. 

The OH proton of carboxylic acids fails to show this same variation of chemical shift with concentration. The reason stems from the fact that carboxylic acids are more strongly associated by hydrogen bonding and associate in solution as dimers. Since the dimers are not easily dissociated on dilution, the chemical shift of the OH proton does not change appreciably. 

The acidic protons of carboxylic acids are highly deshielded and absorb far downfield in the d 10—12 region. 

These results and discussion in favor of the p-toluene sulfonic acid (pTSA) as an effective catalyst for polyesterification of dimer acid and butanediol strongly supports the proton-catalyzed nature of polyesterification. Usually, the proton-catalyzed mechanism for esterification is extrapolated to proton-catalyzed polyesterification [104]. The polyesterification of dimer acid and butanediol involves protonation of the dicarboxylic acid by the reaction of protonated species with the hydroxy group of glycol to yield the polyester. The proton catalyzing the protonation of carboxylic acid is provided by the carboxyl group of the monomer, i.e., dimer acid, and by pTSA in absence and presence of added catalyst, respectively. 

De Robertis A, De Stefano C, Rigano C, Sammartano S (1990) Thermodynamic parameters of the protonation of carboxylic acids in aqueous tetraethylammonium iodide solutions. J Solut Chem 19 569-587... 

De Robertis A, De Stefano C, Foti C (1999) Medium effects on the protonation of carboxylic acids at different temperatures. J Chem Eng Data 44 262 270 Gorzsas A, Getty K, Andersson I Petteisson L (2004) Speciation in the aqueous H+/HjVO / HjOj/citrate system of biomedical interest. Dalton Trans 34 2873-2882 Ohman LO, Sjoberg S (1983) Equilibrium and structural studies of silicon(IV) and aluminium(lll). Part 9. A potentiometric study of mono- and poly-nuclear aluminium(III) citrates. J Chem Soc Dalton Trans 8 2513-2517... 

First, the competing reaction of nucleophiles with the acidic proton of carboxylic acids is eliminated. 

PAA/POE400 hybrid films without neutralization also show ionic conduction, which is derived from the protons of carboxylic acids. The ionic conductivity is one or two orders lower than PAA-PAANa/POE4oo systems. A.C. impedance measurements showed an increase in interface resistance when sodium amalgam was used as active electrodes. This indicates the reaction of carboxylic acids to active electrodes. Therefore, the PAAM/PAA/POE400 is not suitable for secondary battery systems constructed from active electrodes such as alkali metals. 

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