Properties of Carboxylic Acids

PROPERTIES OF CARBOXYLIC ACIDS

As you know, the carbon and oxygen in the carbonyl group are both sp2 hybridized bonds, which give the group its trigonal shape. The hydroxyl group is also sp2 hybridized, leaving one of its lone pair of electrons to combine with the pi-bond, which leads to a resonance structure. Figure 48 shows this resonance structure:

Figure 48.

The melting point of carboxylic acids is not easily memorized as it does not vary with molecular size. The boiling point, however, will increase with molecular weight. When it comes to the melting point of fatty acids, the melting point of saturated fatty acids will be generally greater than that of unsaturated fatty acids so that fatty acids will be solid at room temperature and unsaturated fatty acids will be liquid. This is because of the way the solids are packed in unsaturated fatty acids versus saturated fatty acids.

Carboxylic acids are weak acids but are much stronger than their comparable alcohols. The acidity is increased when there are electronegative side chains near the carboxyl group, such as chlorine or bromine, which make more acidic compounds. This becomes acidic because of the resonance of the carboxyl group, which leads to a negative charge on the hydroxyl oxygen molecule at the end of the carboxylic acid.