14.2 Derivatives of Carboxylic Acid
🎯 Learning Outcomes
- Show the preparation of acid derivatives from carboxylic acids.
- Explain the comparative physical properties of acid derivatives.
- Explain the comparative chemical properties of acid derivatives (hydrolysis, ammonolysis, amines–RNH2, alcoholysis, and reduction only).
- Claisen condensation and Hofmann bromamide reaction.
- Describe the amphoteric nature of amide and relative reactivity of acid derivatives.
Introduction & Nomenclature
Carboxylic acids give rise to a family of compounds called acid derivatives, formed when the –OH part of the carboxyl group (–COOH) is replaced by another group.
1. Acid Halides or Acyl Halides (R–COX)
These are the derivatives of carboxylic acids in which the –OH part of the carboxyl group is replaced by a halo (–Cl, –Br or –I) group. The most reactive, most common and least expensive compounds among the acid halides are acid chlorides.
Nomenclature: The names of acid halides are derived from IUPAC or common names of the corresponding acids by replacing the suffix –ic acid with –yl halide. The secondary suffix for the acid chloride group is –oyl chloride. e.g.
| Formula | IUPAC name | Common name |
|---|---|---|
| HCOCl | Methanoyl chloride | Formyl chloride |
| CH3COCl | Ethanoyl chloride | Acetyl chloride |
| CH3CH2COCl | Propanoyl chloride | Propionyl chloride |
2. Acid Amide or Amide (–CONH2)
Acid amides can be regarded as derivatives of carboxylic acids in which the –OH part of the carboxylic group is replaced by the –NH2 group.
Nomenclature: Acid amides are named by replacing the suffix –ic acid of the common name or –oic acid of the IUPAC name of the corresponding acid with the suffix ‘amide’. Or the secondary suffix for the functional group amide is –amide. e.g.
| Formula | IUPAC name | Common name |
|---|---|---|
| HCONH2 | Methanamide | Formamide |
| CH3CONH2 | Ethanamide | Acetamide |
| CH3CH2CONH2 | Propanamide | Propionamide |
3. Ester (–COOR)
An ester is considered a derivative of a carboxylic acid whose –OH group is replaced by an alkoxy group (–OR).
Nomenclature: While writing the IUPAC name of an ester, the secondary suffix –oate is used, and the alkyl group is written as a prefix.
4. Acid Anhydride (–CO–O–CO–)
Acid anhydrides are considered to be derived from carboxylic acids, formed by the removal of water from two molecules of the acid.
Nomenclature: The acid anhydrides are named by replacing the suffix ‘acid’ of the IUPAC or common name of the carboxylic acids with ‘anhydride’. Some examples are:
Preparation of Derivatives from Carboxylic Acids
1. Preparation of Acid Chloride
(studied in Carboxylic Acids chapter already)
Acyl chlorides can be prepared by the reaction of carboxylic acids with phosphorus pentachloride, phosphorus trichloride or thionyl chloride in the presence of pyridine.
2. Preparation of Acid Anhydride
(studied in Carboxylic Acids chapter already)
Acid anhydrides can be prepared by heating carboxylic acids in the presence of P2O5. e.g.
3. By Treatment of Carboxylic Acids with Acid Chlorides
4. Preparation of Esters
(studied in the Alcohols chapter already)
Esters can be prepared by the reaction of carboxylic acids with alcohols in the presence of concentrated sulphuric acid. This is called the Esterification reaction.
5. Preparation of Acid Amide
Comparative Physical Properties
| State | Boiling Point | Solubility | |
|---|---|---|---|
| Acid chlorides | Lower members – colourless liquids with pungent smell. Higher members – colourless solids. |
Less than that of parent acids due to absence of hydrogen bonding. | Soluble in organic solvents. Can’t be used in water as they undergo hydrolysis. |
| Acid anhydrides | Lower members – colourless liquids with pungent smell. Higher members – colourless solids. |
Higher than that of parent acid due to larger relative molecular mass. | Soluble in organic solvents. Can’t be used in water as they undergo hydrolysis. |
| Esters | Lower members – colourless liquids with pleasant fruity smell. Higher members – colourless, odourless solids. |
Lower than that of acid with comparable molecular wt. due to lack of H-bonding. | Soluble in organic solvents. Soluble in water as well. |
| Amides | First member – methanamide – liquid. All other – colourless, odourless crystalline solids. |
Higher than corresponding acid due to availability of 2 H atoms for H-bonding. | Soluble in organic solvents. Lower members are soluble in water due to H-bond formation. |
Comparative Chemical Properties
Acid chlorides are the most reactive derivatives of carboxylic acids. These mainly give the following reactions.
Reactions of Acid Chloride
Reactions of Acid Anhydride
Reactions of Ester (Alcoholysis)
Reactions of Amide
Claisen Condensation
Esters containing α-hydrogen undergo self-condensation in the presence of a strong base such as sodium ethoxide to form β-keto esters; such a reaction is called Claisen condensation.
Hofmann Bromamide Degradation Reaction
Amides react with bromine in the presence of alkali to form a primary amine carrying one carbon less than the parent amide. This is also called a degradation reaction.
Amphoteric Nature of Amide
Amides behave both as weak acids and weak bases, i.e., they are amphoteric in nature.
Amides react with reactive metals like Na, K, etc. to release hydrogen. Similarly, they also react with mercuric oxide, producing salt and water. These reactions show the acidic nature.
Amides are Lewis bases as they have a lone pair of electrons on the N atom. But these are weaker bases than amines, as the electrons are delocalized due to resonance. These react with strong acids to form salts — showing their basic nature.
Relative Reactivity of Acid Derivatives
The relative reactivity of acid derivatives is in the following order:
Acid chlorides > Acid anhydrides > Esters > Amides
📘 Chapter Summary
- Acid derivatives form when the –OH of –COOH is replaced: by –X (acid halide), –NH2 (amide), –OR (ester), or –O–CO– (anhydride).
- Acid chlorides come from RCOOH + PCl5/PCl3/SOCl2; anhydrides from RCOOH + P2O5 or from an acid + acid chloride; esters via Fischer esterification (RCOOH + R’OH/H+); amides via the ammonium carboxylate salt on heating.
- Physically: acid chlorides & anhydrides have pungent smells and lower/higher boiling points than the parent acid respectively; esters smell fruity and are the most water-soluble besides amides; amides are high-melting crystalline solids (extensive H-bonding).
- All four derivatives undergo hydrolysis, ammonolysis/alcoholysis, and reduction — but acid chlorides do so fastest and amides slowest.
- Reactivity order: acid chlorides > acid anhydrides > esters > amides.
- Claisen condensation: esters with α-hydrogens self-condense (NaOC2H5) into β-keto esters.
- Hofmann bromamide reaction: RCONH2 + Br2/NaOH → a primary amine with one carbon fewer than the amide.
- Amides are amphoteric — weakly acidic (react with Na) and weakly basic (react with strong acids), though weaker bases than amines due to resonance delocalisation of the N lone pair.