Preparation of Acid Derivatives
Synthesis of Acid Chlorides, Anhydrides, Esters, and Amides from Carboxylic Acids.
By chemca Team • Updated Jan 2026
Carboxylic acids ($RCOOH$) can be converted into four main types of derivatives where the $-OH$ group is replaced by another functional group ($Cl, OCOR, OR', NH_2$).
1. Preparation of Acid Chlorides ($RCOCl$)
From Carboxylic Acids
The hydroxyl group of a carboxylic acid is replaced by chlorine using $PCl_5, PCl_3,$ or $SOCl_2$.
A. Reaction with Thionyl Chloride ($SOCl_2$):
$$ R-COOH + SOCl_2 \xrightarrow{\Delta} R-COCl + SO_2 \uparrow + HCl \uparrow $$
Preferred Method: This is the best method because the byproducts ($SO_2$ and $HCl$) are gases and escape easily, leaving behind pure Acid Chloride.
B. Reaction with Phosphorus Chlorides:
$$ R-COOH + PCl_5 \rightarrow R-COCl + POCl_3 + HCl $$
$$ 3R-COOH + PCl_3 \rightarrow 3R-COCl + H_3PO_3 $$
2. Preparation of Acid Anhydrides ($(RCO)_2O$)
Dehydration & Substitution
A. Dehydration of Carboxylic Acids:
Heating carboxylic acids with strong dehydrating agents like Phosphorus Pentoxide ($P_2O_5$) or Conc. $H_2SO_4$.
$$ 2CH_3COOH \xrightarrow{P_2O_5, \Delta} (CH_3CO)_2O + H_2O $$
B. From Acid Chlorides:
Reaction of Acid Chloride with Sodium Carboxylate (Nucleophilic Acyl Substitution).
$$ R-COCl + R-COO^-Na^+ \xrightarrow{\Delta} (RCO)_2O + NaCl $$
3. Preparation of Esters ($RCOOR'$)
Esterification Reactions
A. Fischer Esterification:
Direct reaction of Carboxylic Acid with Alcohol in the presence of an acid catalyst (Conc. $H_2SO_4$ or Dry $HCl$).
$$ R-COOH + R'-OH \xrightleftharpoons{H^+} R-COOR' + H_2O $$
B. From Acid Chlorides/Anhydrides (Alcoholysis):
Reaction with alcohols. Pyridine is added to neutralize the acid formed.
$$ R-COCl + R'-OH \xrightarrow{\text{Pyridine}} R-COOR' + HCl $$
$$ (RCO)_2O + R'-OH \xrightarrow{\text{Pyridine}} R-COOR' + R-COOH $$
4. Preparation of Amides ($RCONH_2$)
Ammonolysis
A. From Carboxylic Acids (Thermal):
Reaction with Ammonia forms Ammonium salt, which on heating loses water to form Amide.
$$ R-COOH + NH_3 \rightarrow R-COO^-NH_4^+ \xrightarrow{\Delta, -H_2O} R-CONH_2 $$
B. From Acid Chlorides, Anhydrides, or Esters:
Direct reaction with Ammonia (Ammonolysis).
$$ R-COCl + 2NH_3 \rightarrow R-CONH_2 + NH_4Cl $$
$$ R-COOR' + NH_3 \rightarrow R-CONH_2 + R'OH $$
C. From Nitriles (Partial Hydrolysis):
Controlled hydrolysis of nitriles yields amides.
$$ R-CN \xrightarrow{H_2O / OH^- (\text{Partial})} R-CONH_2 $$
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