Carboxylic acids contain carbon, oxygen and hydrogen. Many naturally occurring acids are carboxylic acids, such as the acid that causes nettles to “sting” and the acid in vinegar. This is called thionic acid. It is created when alcohol reacts with oxygen (oxidizes).
Carboxylic acids with low molecular weights dissolve in water because the carboxyl group forms several hydrogen bonds with water. A carboxylic acid acts both as a hydrogen bond donor through its hydroxyl hydrogen atom and as a hydrogen bond acceptor through the lone pair electrons of both oxygen atoms. The solubility of carboxylic acids, like that of alcohols, decreases with increasing chain length because long nonpolar hydrocarbon chains dominate the physical properties of the acid.
Carboxylic acids dissolve in common alcohol solvents such as ethanol. This solubility results from intermolecular hydrogen bonds between solute and solvent, and from van der Waals attractions between the ethyl group of ethanol and the nonpolar tail of the carboxylic acid. Nonpolar solvents, such as chloroform, are also excellent solvents for carboxylic acids. In these solvents, the carboxylic acids exist as relatively nonpolar hydrogen-bonded dimers that are compatible with the solvent.
Carboxylic acids are characterized by the strong absorption due to the carbonyl group in the infrared spectra of these compounds. The absorption occurs in the same region as the carbonyl groups of aldehydes and ketones, but the absorption for carboxylic acids occurs at slightly higher wavenumber, and tends to be somewhat broadened. The O—H bond of carboxylic acids absorbs in the same region as that for alcohols. However, the absorption is very much broader for carboxylic acids, and it overlaps the C—H absorptions.
Some carboxylic acids are found in fats and oils from animals and plants. They are called fatty acids. When they react with alcohol, they create compounds called esters, which give flowers their scent. Some expensive perfumes are still made by distilling the scent from flowers and preserving it in alcohol.
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