Bonding carboxylic acids

Polarity of functional group Due to the presence of the polar 0-H and C=0 bonds, carboxylic acids are polar compounds. 

Carboxylic acids with six or fewer carbon atoms are soluble in water because of the polarity of the acid functional group and the ability of the acidic hydrogen atom to hydrogen bond. Carboxylic acids with more than six carbon atoms react with and dissolve in either aqueous sodium bicarbonate or aqueous sodium hydroxide solution. 

Small-ring molecules, most often hydrogen-bonding carboxylic acid hosts, form the basis of a new family of inclusion compounds"4 "6127. 

O—H6 hydrogen-bonded carboxylic acids 2500-3300 variable, broad... 

The structures of three cocrystals of caffeine having a 1 1 stoichiometry with various hydroxy-2-naphthoic acids have been reported [62], The anticipated imidazole-carboxylic acid supramolecular synthon was observed in caffeine cocrystals containing l-hydroxy-2-naphthoic acid and 3-hydroxy-2-naphthoic acid, while a hydrogen-bonded carboxylic acid dimer (and no hydroxyl-caffeine heterosynthon) was observed in the caffeine cocrystal with 6-hydroxy-2-naphthoic acid. 

ITPE is due to the formation of strong intermolecular ionic crosslinks, which act as a compatibiliser. The IR spectra [75] of Zn-mHDPE and Zn-XNBR in the range of 1750-1250 cm 1 are shown in Figure 5.14. The absence of the band corresponding to hydrogen bonded carboxylic acid pairs (1700-1720 cm 1) indicate almost complete neutralisation of the acid groups in both polymers. 

Because of two hydrogen bonds, carboxylic acids show a very strong attractive force between two molecules that persists even in the gas phase. Show this hydrogen bonding between two carboxylic acid molecules. 

As an example, we consider H NMR measurements on a single crystal of benzoic acid [ 14], carried out to investigate tunnelling dynamics in hydrogen bonded carboxylic acid dimers. 

The presence of hydrogen bonded carboxylic acid dimers in the solid was demonstrated from the lH DQ MAS NMR spectrum, with the 1H "1H distance determined to be 2.79 0.9 A. The spectral changes as a function of temperature... 

Zhang, R., and Lifshits, C., Ab initio calculations of hydrogen-bonded carboxylic acid cluster systems Dimer evaporations, J. Phys. Chem. 100, 960-966 (1996). 

The partial IR spectrum of acid Subfraction 1 shows IR absorption at 3460 cm because of the pyrrolic nitrogen N-H absorption of carba-zole-like compounds. Amide carbonyl absorption appears at 1685 cm" The partial IR spectrum of acid Subfraction 2 shows the same two IR bands and additional bands at 3585 cm and 1650 cm owing to phenols and a second amide type. The partial IR spectrum of acid Subfraction 3 shows phenol absorption at 3585 cm S pyrrolic nitrogen absorption at 3460 cm S and strong carbonyl absorption at 1695 cm and 1725 cm characteristic of carboxylic acid dimers and monomers. In addition, absorption of hydrogen-bonded carboxylic acid and phenolic hydroxyl groups can be seen in the region of 3500-2300 cm" ... 

Figure 3-18 shows the spectra obtained recently by Millikan and Pitzer, of gaseous and solid formic acid (1410, 1411). They find band widths for monomer (gas phase), dimer (gas phase), high polymer (glass, 20-77°K), and solid (20-77°K) of, respectively 50 cm", 500 cm, 600 cm", and 400 cm". Figure 3-19 contrzists the Pg band structures for the various deuterated formic acids (gas phase) (1411). The comparisons made by BratoX, Hadii, and Sheppard (270) include those spectra shown in Fig. 3-20. The spectral similarity in the p, region for a variety of H bonded carboxylic acids in GCU solution is apparent. Here the band widths are about 400 cm" (979).

Fig. 3 Hydrogen bonded carboxylic acid... amide dimers in the benzamide succinic acid cocrystal. (From Ref. . )...

Because of the strong hydrogen bonds, carboxylic acid molecules form dimers. 

The boiling points of carboxylic acids are high relative to other classes of compounds due to hydrogen bonding carboxylic acid molecules can hydrogen bond in two places and as a result often exist as dimers. Lower molecular weight carboxylic acids are water soluble. 

Figure 2.1.58 Crystal packing of 73 (Pcan) (Ref. [105]) (a) along [100] exhibiting elliptical channels besides not hydrogen bonded carboxylic acid groups and (b) along [001] with interlocked cleavage plane.

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