Carboxylic acids, hydrogenation

Acids require vigorous conditions for successful reductions on a synthetic scale, but they can be reduced to the alcohol in small yield over rhodium even at ambient conditions. This fractional reduction is without utility but it is sufficient to cause errors in absorption measurements when a carboxylic acid is used as a solvent. At 150 C and 2000 psig, RhjOj becomes a useful catalyst for carboxylic acid hydrogenation 13). 

Figure 4-14 The reactant, the first transition state, and the initial enolate. The 4-/3 hydrogen (Ha) is transferred to Asp38, while the carboxylic acid hydrogen (Hb) from Asp" is partially transferred to the 3-carbonyl oxygen.

In the case of carboxylic acids, hydrogen hooding can induce liquid-cryslal phases hy lengthening the molecular unit through dimerization ... 

When an acyl group replaces the hydrogen of alcohols, carboxylic acids, hydrogen halides, ammonia or amines, we have the acyl compounds known as esters, anhydrides, halides, and amides, respectively. 

Some of the simplest selective systems are based on hydrogen bonding interactions between molecules. Carboxylic acids hydrogen bond to form dimers (Fig. 7-24), although this process is relatively unselective in the absence of other interactions. The interaction is described as self-complementary - the carboxylic acid functionality contains both the carbonyl oxygen atom as a donor and the hydrogen atom as the acceptor. 

Dark fermentation is a catabolism anaerobic bacteria convert sugars and proteins to carboxylic acids, hydrogen, carbon dioxide, and solvents. This biological chemical reaction is carried out under anaerobic condition in which the presence of oxygen is prohibited. 

On an industrial scale, the traditional method for cleavage of carbon-carbon double bonds is ozonolysis, used for the manufacture of azelaic acid and nonanoic acids from oleic acid, and of butane tetracarboxylic acid from tetrahydrophthalic anhydride. The process is effectively a quantitative and mild process.178 However, it is capital and energy intensive. The intermediate ozonide is worked up either reductively or oxidatively to produce the aldehyde, ketone or carboxylic acid. Hydrogen peroxide is the common oxidizing agent used in the second step.179-181 Oxygen can also be used either alone182 or in combination with zeolites.183 Reviews on ozonolysis are available and the reader is directed to reference 184 for further information. 

Several interesting cyclic configurations are observed. The very short carboxylic acid hydrogen bond C-OH... 0 = C of 1.475 A occurs in L-glutamic acid B-form, as part of a trimer system. Cyclic trimers involving the components of three-center bonds are also observed in the A-form of glutamic acid. In this pattern,... 

Solid state 2H NMR has been used to obtain detailed structural information for the amide and carboxylic acid hydrogen sites in a single crystal of the model peptide N-acetyl-D,L-valine [134]. Both the amide and carboxylic acid hydrogens are involved in intermolecular hydrogen bonds. The results were compared with experimental data obtained for acetylanilide [135] and ab initio calculations for glycylglycine [136]. 

For protons attached to atoms other than carbon the chemical shifts of protons attached to oxygen increase with increasing acidity of the O-H group thus 5=1—6 ppm for alcohols, 4-12 ppm for phenols and 10-14 ppm for carboxylic acids. Hydrogens bound to nitrogen (1° and 2° amines) are found at 5 = 3—8 ppm. The approximate chemical shift regions are shown in Fig. 29.5. 

Acetic acid shows all the characteristic properties of carboxylic acids. Hydrogen gas and acetate salts are formed upon reaction with metals and it undergoes neutralization reactions with bases and basic salts. 

That such a delicate balance exists between tautomers is not an obvious prediction based solely on ApK s. The 10 pK unit difference in 9 benzoate favors the amidine-carboxylic acid form by 0.6 eV. However, a simple electrostatic calculation [123] for a positive and negative charge at a salt bridge distance of 3.8 A translates into a stabilization energy of-0.50 eV in the solvent THF, which nearly offsets the stabilization of the amidine-carboxylic acid tautomer derived from the ApfQ. Electron-rich carboxylates such as benzoate are sufficiently basic that the amidine-carboxylic acid hydrogen bond interaction prevails while the interface retains its ionic nature for more acidic carboxylic acids and various sulfonic acids. 

Ponec et al. also reported this surface carboxylate species on their partially reduced iron oxide [24]. These results strongly suggest that the surface carboxylate species exists as an intermediate in the carboxylic acid hydrogenation. 

Atrazine (the template) has a triazine ring and two amino groups, all of which form hydrogen bonds with appropriate residues in aprotic solvents (Fig. 6.2). Accordingly, methacrylic acid, having a carboxylic acid (hydrogen-bonding site), is chosen as the functional monomer. Acrylic acid is also useful. The two types of interactions at the two amino... 

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