14.4:
Structures of Carboxylic Acid Derivatives
Carboxylic acid derivatives contain an acyl group, such as CH3CO, bonded to a heteroatom like Cl, O, or N.
The carbonyl carbon and oxygen are sp2 hybridized, resulting in a trigonal planar geometry with bond angles approximating 120° and a carbon–oxygen bond length of 1.21 Å.
The derivatives are resonance stabilized, where the extent of stabilization depends on the electronegativity of the heteroatom.
Since nitrogen is less electronegative than chlorine or oxygen, it can readily donate its lone pair and accommodate the positive charge better, making amides the most stable.
The carbon–nitrogen bond in amides exhibits a partial double bond character, with the nitrogen atom being sp2 hybridized and planar.
Consequently, in secondary and tertiary amides, rotation about the carbon–nitrogen bond gives E and Z conformers, where the equilibrium favors the less sterically hindered Z conformer.
Finally, in nitriles, both carbon and nitrogen are sp hybridized. They adopt a linear geometry where the carbon–nitrogen bond length is 1.16 Å.
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.