14.4 : Structures of Carboxylic Acid Derivatives

Structure of Carboxylic Acid Derivatives

Carboxylic acid derivatives contain an acyl group attached to a heteroatom such as chlorine, oxygen, or nitrogen. The carbonyl carbon and oxygen are both sp²-hybridized with an unhybridized p orbital.

The three sp² orbitals of the carbonyl carbon form three σ bonds, one each with the carbonyl oxygen, the α carbon, and the heteroatom, whereas the other two sp² orbitals of the carbonyl oxygen are occupied by the lone pairs. Further, the unhybridized p orbital on the carbonyl oxygen and carbon overlap, forming a π bond.

The three atoms attached to the carbonyl carbon lie on the same plane, exhibiting a trigonal planar geometry. Their bond angles are approximately 120° with a carbon–oxygen bond length of 1.21 Å.

Resonance Stabilization

In acid derivatives, the electrons are delocalized across the carbonyl carbon, oxygen, and the heteroatom. Thus, all acid derivatives are stabilized by resonance, where the extent of stabilization depends on the electronegativity of the heteroatom.

Since nitrogen is less electronegative than oxygen, it readily shares its lone pair of electrons with the electron-withdrawing carbonyl group forming a π bond between the carbonyl carbon and nitrogen. Consequently, the positive charge is better accommodated on the nitrogen, stabilizing amides to a greater extent.

As a result, the carbon–nitrogen bond develops a partial double bond character, where the nitrogen atom is sp²-hybridized and planar.

The planar geometry facilitates rotation about the carbon–nitrogen bond, allowing secondary and tertiary amides to adopt E and Z conformations. The Z conformation is favored, as the van der Waals repulsion between bulky groups is minimized.

The presence of a partial double bond restricts free rotation about the carbon–nitrogen bond by imposing a high rotational energy barrier of 71 kJ/mol. Thus, the interconversion between the E and Z conformers is slow around the partial carbon–nitrogen double bond compared to rotation about a carbon–carbon single bond.

Structure of Nitriles

All carboxylic acid derivatives contain a carbonyl group. However, nitriles have a cyano group where a carbon atom is triply bonded to a nitrogen atom, and both atoms are sp hybridized. They show a linear geometry with a bond angle of 180° and a carbon–nitrogen bond length of 1.16 Å.