Esters: Structure, Nomenclature, and Chemical Reactions

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Topic 8 — Key Facts for Kenyatta University (Kenya) Core concept: Esters (R–COOR’) are derived from carboxylic acids by substituting the –OH with –OR’ group; they have the functional group –COO– and are named as alkyl alkanoates (e.g., ethyl acetate = ethyl ethanoate) High-yield point: Esters undergo hydrolysis (acid-catalysed reversible, alkaline irreversible giving carboxylate salt + alcohol); transesterification (alcohol + ester → new ester + new alcohol); and reduction (LiAlH₄ → two alcohols) ⚡ Exam tip: The acid-catalysed hydrolysis of an ester is reversible and follows the mechanism of nucleophilic acyl substitution; alkaline hydrolysis (saponification) is NOT reversible because the carboxylate anion is resonance-stabilised and cannot reform the ester

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Esters: Carboxylic Acid Derivatives

Esters are derivatives of carboxylic acids in which the acidic –OH group is replaced by an alkoxy group (–OR’). They result from the condensation of a carboxylic acid and an alcohol via the Fischer esterification reaction.

The general formula is R–COOR’, where R is the acyl group (from the acid) and R’ is the alkyl group (from the alcohol).

Nomenclature of Esters

IUPAC naming: The alkyl group (from the alcohol) is named first, followed by the acyl group with the suffix -oate.

Examples:

Structure	Common Name	IUPAC Name
CH₃COOCH₃	Methyl acetate	Methyl ethanoate
CH₃COOCH₂CH₃	Ethyl acetate	Ethyl ethanoate
CH₃COOCH₂CH₂CH₂CH₃	Butyl acetate	Butyl ethanoate
HCOOCH₃	Methyl formate	Methyl methanoate
CH₃CH₂COOCH₂CH₃	Ethyl propionate	Ethyl propanoate
C₆H₅COOCH₂CH₃	Ethyl benzoate	Ethyl benzoate

Key naming principle: The alkyl prefix comes from the alcohol (methyl, ethyl, etc.); the suffix comes from the acid (acetate, propionate, etc.).

Physical Properties

Boiling Points: Esters have lower boiling points than carboxylic acids or alcohols of similar molecular weight because they cannot hydrogen bond with each other (no –OH group).

Compound	MW	BP (°C)	Intermolecular Forces
Ethyl acetate	88	77	Dipole-dipole + London
Butyric acid	88	163	H-bonding dimer
1-pentanol	88	138	H-bonding

Solubility:

• Small esters (C₁–C₄) are moderately soluble in water
• Solubility decreases as molecular size increases
• Esters are good solvents for non-polar and moderately polar compounds

Odour: Esters are responsible for many fruit flavours and aromas:

Ester	Flavour/Aroma
Methyl butanoate	Apple
Pentyl acetate	Banana
Octyl acetate	Orange
Isobutyl methanoate	Raspberry
Ethyl butanoate	Pineapple
Isoamyl acetate	Pear

⚡ Exam Tip: Many artificial fruit flavours are simple esters. Isoamyl acetate (from bananas) is a classic example of the relationship between chemical structure and flavour. This type of question is frequently tested.

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🔴 Extended — Deep Study (3mo+)

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Chemical Reactions of Esters

1. Hydrolysis

A. Acid-Catalysed Hydrolysis: Esters + water + acid catalyst (H₂SO₄) → carboxylic acid + alcohol:

CH₃COOC₂H₅ + H₂O ⇌ CH₃COOH + C₂H₅OH

Characteristics:

• Reversible — product equilibrium can be shifted by removing water or using excess alcohol
• The mechanism is the reverse of Fischer esterification
• The rate is increased by acid catalyst
• The mechanism involves protonation of the carbonyl oxygen → nucleophilic attack by water → tetrahedral intermediate → proton transfer and loss of alcohol → carboxylic acid

B. Alkaline Hydrolysis (Saponification): Esters + aqueous NaOH (or KOH) → carboxylate salt + alcohol:

CH₃COOC₂H₅ + NaOH → CH₃COO⁻Na⁺ + C₂H₅OH

Characteristics:

• Irreversible — because the carboxylate anion is resonance-stabilised
• The alkyl group (R’) is released as an alcohol
• The reaction is driven by formation of the stable carboxylate anion
• One mole of ester reacts with one mole of base

⚡ Exam Tip: Saponification is the basis of soap-making. Triglycerides (esters of glycerol and fatty acids) are hydrolysed by NaOH to give glycerol and sodium fatty acid salts (soap). The reaction with NaOH/KOH is essentially irreversible.

2. Transesterification

An ester reacts with an alcohol (different from the one in the ester) to give a new ester and a new alcohol:

R–COOR' + R''–OH → R–COOR'' + R'–OH

Conditions: Acid catalyst (or base) at elevated temperature

Mechanism: Same as acid-catalysed hydrolysis, but with a different alcohol as the nucleophile instead of water.

Example — Biodiesel Production: Triglyceride (vegetable oil) + methanol → fatty acid methyl ester (biodiesel) + glycerol:

Triglyceride + 3CH₃OH → 3 Fatty acid methyl ester + Glycerol

⚡ Exam Tip: Transesterification is used industrially to convert vegetable oils (which are triglycerides) to biodiesel (fatty acid methyl esters, FAME). This is a key renewable fuels topic that is increasingly covered in Kenyatta University chemistry courses.

3. Reduction Reactions

LiAlH₄ Reduction: Esters + LiAlH₄ → two primary alcohols:

R–COOR' + 4[H] → R–CH₂OH + R'–CH₂OH

LiAlH₄ is a strong reducing agent that reduces esters completely to alcohols. This reaction is used industrially to convert fats (triglycerides) to fatty alcohols:

Triglyceride + 4[AlH₄⁻] → Glycerol + 3 Fatty alcohol

⚡ NaBH₄ does not reduce esters. Only LiAlH₄ can reduce esters to alcohols. This is a commonly tested distinction.

Catalytic Hydrogenation: Esters can be hydrogenated under high pressure and temperature with copper chromite catalysts to give fatty alcohols (used in cosmetics and detergents).

4. Aminolysis

Esters react with amines to give amides:

R–COOR' + R''–NH₂ → R–CONH–R'' + R'–OH

Mechanism: The amine acts as a nucleophile, attacking the carbonyl carbon. The alkoxy group leaves as a leaving group.

Example:

CH₃COOC₂H₅ + CH₃NH₂ → CH₃CONHCH₃ + C₂H₅OH

⚡ Exam Tip: This is the basis of amide bond formation in peptide synthesis. In biological systems, this reaction is enzyme-catalysed (using ATP to activate the carboxyl group).

5. Reaction with Grignard Reagents

Esters + 2 equivalents of Grignard reagent → tertiary alcohol:

R–COOR' + 2R''–MgBr → R–C(R'')₂–OH + R'–OMgBr

After acidic workup: R–C(R”)₂–OH (tertiary alcohol)

Mechanism:

• First equivalent attacks the carbonyl → tetrahedral intermediate → eliminates alkoxide → gives ketone
• Second equivalent attacks the ketone → after workup gives tertiary alcohol

Examples:

CH₃COOC₂H₅ + 2CH₃MgBr → (CH₃)₃C–OH + EtOH + MgBr₂   (tert-butanol from ethyl acetate + 2 equivalents methylmagnesium bromide)

⚡ Exam Tip: Esters require 2 equivalents of Grignard reagent to give a tertiary alcohol. Acid chlorides also require 2 equivalents. Aldehydes require 1 equivalent for a secondary alcohol. Ketones require 2 equivalents for a tertiary alcohol.

6. The Claisen Condensation

Ester + ester (same or different) in the presence of a strong base (NaOEt) → β-keto ester:

2CH₃COOC₂H₅ + 2NaOEt → CH₃COCH₂COOEt + 2C₂H₅OH + 2Na⁺

Mechanism:

1. One ester forms an enolate (at the α-position)
2. The enolate attacks the carbonyl carbon of the second ester
3. Elimination of ethoxide gives the β-keto ester product

Crossed Claisen: When two different esters are used, one must not have α-hydrogens (to avoid mixtures).

⚡ Exam Tip: Only esters with α-hydrogens can undergo the Claisen condensation. Ethyl ethanoate has α-hydrogens; ethyl benzoate does not. Ethyl formate has no α-hydrogens either.

Saponification Value

The saponification value (SV) of a fat or oil is defined as the milligrams of KOH required to saponify 1 gram of the fat or oil:

SV = (Molecular weight of KOH × 56.1) / Mean molecular weight of fatty acids in the triglyceride

• Higher SV means smaller average molecular weight of fatty acids
• Coconut oil: SV ~250 (high proportion of short-chain fatty acids)
• Olive oil: SV ~185 (high proportion of oleic acid)

⚡ Use: SV is used to determine the average chain length of fatty acids in a fat sample — a key analytical technique in food chemistry.

Waxes

Waxes are esters of long-chain fatty acids (C₁₆–C₃₀) with long-chain alcohols (C₁₆–C₃₀):

CH₃(CH₂)₁₄COO(CH₂)₁₅CH₃   (Cetyl palmitate, beeswax)

They are硬的, water-resistant solids used for protective coatings on leaves, fruits, and animal fur.

Common natural waxes:

• Beeswax: Cetyl palmitate
• Carnauba wax: Myricyl cerotate (from Brazilian palm)
• Lanolin: Cholesterol and lanocerin esters (from sheep wool)

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