14.5:
Physical Properties of Carboxylic Acid Derivatives
Physical properties depend on the strength of intermolecular forces, with ionic forces being the strongest and dispersion forces the weakest.
Since esters and acid chlorides cannot function as hydrogen bond donors, they fail to engage in intermolecular hydrogen bonding. Consequently, they have the lowest boiling points.
In comparison, acid anhydrides are large polar molecules and experience strong dispersion forces leading to higher boiling points.
Nitriles have even higher boiling points resulting from strong dipolar interactions between the cyano groups.
Among the derivatives, amides have the highest boiling points.
Primary and secondary amides form strong hydrogen bonds and have higher boiling points than tertiary amides, where the primary interaction is the dipole-dipole interaction.
The melting point of amides follows the same trend as their boiling points.
Esters, amides, and nitriles with less than four carbons are highly polar, making them water-soluble.
However, the water solubility decreases as the length of the non-polar alkyl group increases, making longer chains soluble in less polar solvents.
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.