Carboxylic acid hydrogen bonding

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. 

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,... 

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. 

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... 

Three main concepts influence acidity when comparing methanol with formic acid (1) the carbonyl group polarizes the 0-H bond, (2) the OH of carboxylic acids hydrogen bonds to a greater extent, and (3) the conjugate base (formate, 9) is more stable than methoxide (2), driving the equilibrium to the right. These concepts are the same as those introduced in Chapter 2 to explain acid-base reactions and the importance of the acidity constant, K. ... 

Oxalic acid crystallizes in three different forms as anhydrous a form, as anhydrous p form, and as a dihydrate. In the unit cell of the p-oxalic acid form, there are four cyclic dimers with two parallel hydrogen bonds similar to those found in carboxylic acids. Hydrogen bonds in the a-oxalic acid form are in open-chain structures separated from one another by the O—C=0 groups. The crystalline structure of the dihydrate is similar to the p form but with two molecules of water included, thus forming cyclic structure with four nonequivalent hydrogen bonds. 

Carboxylic acids hydrogen bond strongly to water—they can act as both H-bond donors and acceptors (4.46), and the lower acids are miscible with water. In nonpolar solvents, such as benzene, they self-associate through hydrogen bonding to give dimers (4.47). 

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