Carboxylic proton transfer

These equations tell us that the reverse process proton transfer from acids to bicarbon ate to form carbon dioxide will be favorable when of the acid exceeds 4 3 X 10 (pK, < 6 4) Among compounds containing carbon hydrogen and oxygen only car boxylic acids are acidic enough to meet this requirement They dissolve m aqueous sodium bicarbonate with the evolution of carbon dioxide This behavior is the basis of a qualitative test for carboxylic acids... 

Step 1 The carboxylic acid is protonated on its carbonyl oxygen The proton donor shown in the equation for this step is an aUtyloxonium ion formed by proton transfer from the acid catalyst to the alcohol... 

Step 4 Proton transfer steps yield an alcohol and a carboxylate anion... 

Step 6 Proton transfer processes yielding ammonium ion and the carboxylic acid ... 

Reaction (5.N) describes the nylon salt nylon equilibrium. Reactions (5.0) and (5.P) show proton transfer with water between carboxyl and amine groups. Since proton transfer equilibria are involved, the self-ionization of water, reaction (5.Q), must also be included. Especially in the presence of acidic catalysts, reactions (5.R) and (5.S) are the equilibria of the acid-catalyzed intermediate described in general in reaction (5.G). The main point in including all of these equilibria is to indicate that the precise concentration of A and B... 

Acidic hydrolysis of 14 occurs via protonation of the nitrogen followed by attack of water on the resulting cationic intermediate. Proton transfer followed by ring-opening affords cation 15, which is trapped by a second equivalent of water. Another proton transfer followed by loss of the amino group affords protonated carboxylic acid 16, which loses to provide the carboxylic acid product. 

Finally a proton transfer leads to formation of carboxylate anion 3. Of particular interest is the benzilic acid rearrangement of cyclic diketones such as 4, since it... 

The Zwitterions of Amino Acids. Now that we have decided what aspects are important, we may mention some other types of proton transfers. Consider, for example, the positive ion NHJ CHjCOOH of glycine. If we transfer the proton from the carboxyl group to a distant water molecule, we have... 

The ionization of (E)-diazo methyl ethers is catalyzed by the general acid mechanism, as shown by Broxton and Stray (1980, 1982) using acetic acid and six other aliphatic and aromatic carboxylic acids. The observation of general acid catalysis is evidence that proton transfer occurs in the rate-determining part of the reaction (Scheme 6-5). The Bronsted a value is 0.32, which indicates that in the transition state the proton is still closer to the carboxylic acid than to the oxygen atom of the methanol to be formed. If the benzene ring of the diazo ether (Ar in Scheme 6-5) contains a carboxy group in the 2-position, intramolecular acid catalysis is observed (Broxton and McLeish, 1983). 

However, on heating to about 200°C, a thermodynamically more favorable reaction takes place. The proton transfer is reversed, and the amine acts as a nucleophile as it attacks the carbon atom of the carboxyl group in a condensation reaction ... 

Figure 15. Disproved isokinetic relationship for the proton transfer from nitro-methane to various carboxylic acid anions (156). Symbols as in Figure 13.

In order to account for the inability of many enzymes to bind the protonated form of the basic inhibitors or permanently cationic ones better than uncharged analogs (for example, yS-o-galactosidase from E. coli, and P-v>-glucosidase from almonds), it was proposed that the enzyme could proton-ate the inhibitor at the active site by a cationic acid (for example, protonated histidine). If proton transfer cannot occur, the attractive forces due to the carboxylate would be canceled by the repulsion from the cationic acid. Experimental evidence for this proposal is, however, still lacking. In fi-D-gn-lactosidase from E. coli, a tyrosine is presumed to be responsible for the protonation of substrates. ... 

In molecules with both an acidic and a basic function, there is in principle the possibility of an intramolecular proton transfer leading to betaines (see above). Due to the values of the functional groups, pyridine and carboxylic acid, in [66] no intramolecular proton transfer takes place and no betaine is formed. 

Benzoic acid is a weak carboxylic acid that undergoes proton transfer with water. When a 0.125 M aqueous solution of benzoic acid (Cg H5 CO2 H) reaches equilibrium, [H3 O ] = 0.0028 M. What is. eq... 

As described in Chapter a carboxylic acid can be represented as R CO2 H, in which the symbol R can be as simple as a hydrogen atom or a methyl group (CH3 ) or as elaborate as a structure containing dozens of atoms. The R group of a carboxylic acid almost always contains C—bonds, but hydrogen atoms bonded to carbon are not acidic. These atoms do not participate in aqueous proton transfer reactions. 

All carboxylic acids are weak. In an aqueous solution at equilibrium, a small fraction of the carboxylic acid molecules have undergone proton transfer to water molecules, generating hydronium ions and anions that contain the—CO2 carboyylate CH3 CO2 H((317)-FH2 0(/) CH3 CO2 (atj) + H3 0 (aq)... 

The detailed mechanism of this enantioselective transformation remains under investigation.178 It is known that the acidic carboxylic group is crucial, and the cyclization is believed to occur via the enamine derived from the catalyst and the exocyclic ketone. A computational study suggested that the proton transfer occurs through a TS very similar to that described for the proline-catalyzed aldol reaction (see page 132).179... 

Two methods for converting carboxylic acids to esters fall into the mechanistic group under discussion the reaction of carboxylic acids with diazo compounds, especially diazomethane and alkylation of carboxylate anions by halides or sulfonates. The esterification of carboxylic acids with diazomethane is a very fast and clean reaction.41 The alkylating agent is the extremely reactive methyldiazonium ion, which is generated by proton transfer from the carboxylic acid to diazomethane. The collapse of the resulting ion pair with loss of nitrogen is extremely rapid. 

Proton transfer reactions are often the first step in many reactions that alcohols, ethers, aldehydes, ketones, esters, amides, and carboxylic acids undergo. 

The red shift of 3-amino-phthalate fluorescence when the solvent is changed from water to DMSO or another aprotic solvent is due to different hydrogen bonding above all, however, it is due to proton transfer from the amino group of the excited AP<—> to the neighboring carboxylate yielding the species (-)AP(-), so that the emitters are 51 a in water and 52 a in DMSO 1U> ... 

The observed hyperbolic dependences suggest a mechanism that involves a pre-equilibrium binding of two pyridine carboxylates to the Fem of lm, followed by the intramolecular proton transfer from the coordinated acid (Scheme 3). This option has been supported by measurements of the binding constants for py and related ligands. The data in Fig. 8 agree with the mechanism in Scheme 3. The reactive intermediate for robust lm is the diaxially coordinated species, one of the two ligands being picolinic acid (L). 

---