14.5: Physical Properties of Carboxylic Acid Derivatives
Intermolecular forces dictate several physical properties such as boiling points, melting points, solubilities, and so forth. They are classified into four types: ionic forces, hydrogen bonds, dipole–dipole forces, and dispersion forces. Ionic forces are the strongest, while dispersion forces are the weakest.
Among the carboxylic acid derivatives, the boiling points of acid chlorides and esters are very similar and are the lowest in the series. Acid anhydrides have slightly higher boiling points, followed by nitriles. Amides have the highest boiling points due to their ability to form intermolecular hydrogen bonds. Within amides, primary amides have two N–H bonds, each representing a potential hydrogen bonding site. Secondary amides have only one N–H bond, limiting hydrogen bonding to one site. In contrast, tertiary amides lack N–H bonds and do not participate in hydrogen bonding; however, they are held by dipole–dipole forces. As a result, primary and secondary amides have higher boiling points than tertiary amides. A similar trend is observed with the melting points of amides.
In terms of solubility, esters, amides, and nitriles with less than four carbons are highly polar and soluble in water. In contrast, corresponding derivatives with more than four carbons are less polar and soluble in nonpolar solvents like ethers, chlorinated alkanes, and aromatic hydrocarbons.