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Chapter 20: Carboxylic Acid Derivatives. Nucleophilic Acyl Substitution



Summary | Carboxylic Acids Derivatives | Nucleophilic Acyl Substitution | Reactions of Carboxylic Acid Derivatives | Spectroscopic Analysis | Self Assessment | Quiz |


Carboxylic Acid Derivatives

Chapter 20: Carboxylic Acid Derivatives. Nucleophilic Acyl Substitution


Nomenclature:

Acyl Chlorides
Functional group suffix = oyl chloride (review)
Anhydrides
Functional group suffix = alkanoic anhydride (review)
Esters
Functional group suffix = alkyl -oate (review
Functional group prefix = alkoxycarbonyl-  or   carbalkoxy-
Cyclic esters are called lactones
Carboxylic Acids
Functional group suffix = -oic acid (review
Functional group prefix = carboxy-
Amides
Functional group suffix = amide (review
Functional group prefix = carbamoyl-
Cyclic amides are called lactams
Nitriles
Functional group suffix = nitrile or -onitrile (review
Functional group prefix = cyano-
Structure:
resonance in carboxylic acid derivatives
  • It also has implications for structure... Look at the amide in the table above.... 
  • What do you notice about the geometry at the N atom? planar
  • 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 p system (see image to the right, 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

Reactivity:

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

Study Tip:
Note that unlike aldehydes and ketones, this reactivity of carboxylic acids retains the carbonyl group, C=O.

The observed reactivity order is shown below:

Acid derivative reactivity order

This reactivity order is important. You should be able to understand, rationalize and use it.
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 substituent 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 electrophilicity is increased, \ more reactive
  • the ability of the substituent to function as a leaving group.

There are 3 resonance structures to consider for carboxylic acid derivatives.
carboxylic acid derivatives resoance contributors 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 stabilizing effect.

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

electrostatic potential of acetyl chloride
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. 
electrostatic potential of acetic anhydride 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. 
electrostatic potential of methyl acetate
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. 
electrostatic potential of acetamide
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. 
electrostatic potential of acetonitrile
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. 

 
Derivative Substituent
Electronic Effect
Leaving Group Ability Relative Reactivity
Acyl chloride -Cl
withdrawing group (inductive)
very good
1 (most)
Anhydride -OC=OR
donating
good
2
Ester -OR
strongly donating
poor
=3
Acid -OH
strongly donating
poor
=3
Amide -NH2
very strongly donating
very poor
4
Carboxylate -O-
very, very strongly donating
appalling !
5 (least)

It is also useful to appreciate where aldehydes and ketones fit into the reactivity scale towards nucleophiles:

acyl halides > anhydrides > aldehydes > ketones > esters = carboxylic acids > amides

 


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