Chapter 20: Carboxylic Acid Derivatives. Nucleophilic Acyl Substitution

Reactivity of Carboxylic Acid Derivatives

Carboxylic acid derivatives react tend to react via nucleophilic acyl substitution where the group on the acyl unit, R-C=O undergoes substitution:

The observed reactivity order is shown below:

It is useful to view the carboxylic acid derivatives as an acyl group, R-C=O,  with a different substituent attached.  The important features of the carboxylic acid derivatives that influence their reactivity are governed by this substitutent in the following ways:  the effect the substituent has on the electrophilicity of the carbonyl C (review substituent effects ?) if the substituent is electron donating, then the electrophilicity is reduced, \ less reactive if the substituent is electron withdrawing, the the electrophilicity is increased, \ more reactive the ability of the substitutent to function as a leaving group.

I  and II are similar to those of aldehydes and ketones, but there is also a third possibility III where a lone pair on the heteroatom Z is able to donate electrons to the adjacent positive center. The stronger this electron donation from Z the less positive the carbonyl C and the less electrophilic the carbonyl group.  The ability of Z to donate electrons is linked to its electronegativity...the more electronegative Z is, the less the stabilising effect.

Use the following series of electrostatic potential  maps to look at the electrophilicity of the carbonyl C in a example of each the more common carboxylic acid derivatives. Note how the blue colour gradually reduces in intensity down the series.

	The image shows the electrostatic potential for ethanoyl chloride.  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.
	The image shows the electrostatic potential for ethanoic anhydride.  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.
	The image shows the electrostatic potential for methyl ethanoate.  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.
	The image shows the electrostatic potential for ethanamide.  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.
	The image shows the electrostatic potential for acetonitrile.  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.

Question

• Why is the O atom in an anhydride a poorer electron donor than that is esters or acids ?