Structure and Properties

The carboxylic group (-COOH) is the functional group of the carboxylic acids. It is kind of a combined carbonyl and hydroxyl group. The carbon atom in the group has sp² hybridization and is in the center of a flat triangle with a carbon atom (or hydrogen, in one case) and two oxygen atoms at the corners. The carbon atom has a double bond to one oxygen atom and a single bond to the other. This other oxygen atom is also bonded to a hydrogen atom. It is this hydrogen atom that gives these compounds their acidic properties and the "acid" part of their names.

Structure of the carboxylic group. [64str01.JPG]

The bond angles and arrangement of the atoms in the carboxylic group, the oxygen atoms in particular, make carboxylic acids polar molecules. Not only are the molecules polar, but they also contain a hydrogen atom bonded to an oxygen atom. Because of this their intermolecular bonds are hydrogen bonds. At least the end of the molecule where the carboxylic group is, has hydrogen bonds. As with other types of polar organic molecules, the polar functional group is only one part of the molecule. Depending on the size and structure of the molecule, it might be just a small part.

Structure and model of carboxylic acid group. [64str02.JPG]

Consequently, the solubility of carboxylic acids in water depends on the size of the molecules. The small ones, with five carbon atoms or fewer, are pretty soluble in water. The larger ones are less soluble and the very large ones are practically insoluble in water.

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