Ch20: Structure of Carboxylic Acid Derivatives

Chapter 20: Carboxylic Acid Derivatives. Nucleophilic Acyl Substitution

Structure of Carboxylic Acid Derivatives

The carbonyl group consists of an O atom bonded to a C atom via a double bond in a planar, sp² hybridisation model similar to that of a ketone or an alkene.
The heteroatom group is connected to this C=O unit via a σ bond.
To see these features, compare the JMOL images to the below.
JMOL images of the other carboxylic acid derivatives can be found on the previous page.


carboxylic acid	aldehyde	alkene

The resonance interaction of the carbonyl C=O with the lone pair of the adjacent heteroatom (structure III) has important implications on the reactivity

resonance in carboxylic acid derivatives

It also has implications for structure... Look at the JMOL image of the amide to the right.

QUESTION What do you notice about the geometry at the N atom ? ANSWER

Amines and ammonia are usually pyramidal (want to see ?)
The planar sp² N system allows the N lone pair to align with the C=O π system (see image below, with the other bonds omitted for clarity)
The resonance interaction in the amide results in the C-N bond having some double bond character (shorter, restricted rotation)

orbital picture of amide resonance

amide

Nitriles are slightly different to the other derivatives in that they involved a triple bond. Nitriles consists of a N atom bonded to a C atom via a triple bond in a linear, sp hybridisation model similar to that of an alkyne. Compare the JMOL images to the right.
	nitrile	alkyne

© Dr. Ian Hunt, Department of Chemistry