Properties and Reactions of Esters
Physical trends, Hydrolysis (Saponification), Reduction, and Condensation.
By chemca Team • Updated Jan 2026
Esters ($RCOOR'$) are pleasant-smelling compounds widely used in flavors and fragrances. Their chemical reactivity is dominated by nucleophilic acyl substitution at the carbonyl carbon and reactions involving the $\alpha$-hydrogen.
1. Physical Properties
- Odor: Lower esters have pleasant, fruity smells (e.g., Isoamyl acetate smells like banana).
- Boiling Point: Esters cannot form intermolecular Hydrogen bonds with each other. Thus, their boiling points are lower than isomeric acids and alcohols, but comparable to aldehydes/ketones of similar mass.
- Solubility: Lower members are soluble in water due to H-bonding with water molecules. Solubility decreases as the alkyl chain grows.
2. Hydrolysis Reactions
Cleavage of Ester Bond
A. Acidic Hydrolysis:
Reversible reaction catalyzed by dilute acid ($H^+$).
$$ RCOOR' + H_2O \xrightleftharpoons{H^+} \underset{\text{Carboxylic Acid}}{RCOOH} + \underset{\text{Alcohol}}{R'OH} $$
B. Alkaline Hydrolysis (Saponification):
Irreversible reaction with aqueous base ($NaOH/KOH$).
$$ RCOOR' + NaOH \xrightarrow{\Delta} \underset{\text{Sodium Salt (Soap)}}{RCOO^-Na^+} + R'OH $$
Why Irreversible? The carboxylic acid formed immediately reacts with the base to form a stable carboxylate salt, preventing the reverse reaction.
3. Nucleophilic Acyl Substitution
Replacement of Alkoxy Group
A. Ammonolysis:
Reaction with Ammonia ($NH_3$) or amines to form Amides.
$$ RCOOR' + NH_3 \rightarrow \underset{\text{Amide}}{RCONH_2} + R'OH $$
B. Transesterification (Alcoholysis):
Reaction with an excess of another alcohol ($R''OH$) in the presence of acid or base catalyst to exchange the alkoxy group.
$$ RCOOR' + R''OH \xrightleftharpoons{H^+ \text{ or } RO^-} RCOOR'' + R'OH $$
Used in Biodiesel production.
4. Reduction Reactions
Conversion to Alcohols
Esters are reduced to a mixture of two alcohols: one corresponding to the acyl group and the other to the alkoxy group.
A. Catalytic Hydrogenation ($H_2/Ni$) or $LiAlH_4$:
$$ RCOOR' + 4[H] \xrightarrow{LiAlH_4} RCH_2OH + R'OH $$
B. Bouveault-Blanc Reduction:
Reduction using Sodium and Ethanol ($Na / C_2H_5OH$).
$$ RCOOR' \xrightarrow{Na / C_2H_5OH} RCH_2OH + R'OH $$
5. Claisen Condensation
Self-Condensation
Esters having at least one $\alpha$-hydrogen atom undergo self-condensation in the presence of a strong base like Sodium Ethoxide ($C_2H_5ONa$) to form $\beta$-Keto Esters.
$$ 2 \underset{\text{Ethyl Acetate}}{CH_3COOC_2H_5} \xrightarrow{1. C_2H_5ONa, 2. H^+} \underset{\text{Ethyl Acetoacetate ($\beta$-Keto Ester)}}{CH_3COCH_2COOC_2H_5} + C_2H_5OH $$
Mechanism: The base abstracts an $\alpha$-proton to form an enolate ion, which then attacks the carbonyl carbon of another ester molecule (Nucleophilic Acyl Substitution).
Knowledge Check
Test your understanding of Ester Reactions
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