Organic Chemistry · Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution Reactions
Chemistry of Esters
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In 30 seconds
An Ester RCO-OR', acyl group + alkoxy group Full entry → has the structure RCO-OR': an acyl group bonded to an alkoxy group. Esters are the most familiar carboxylic acid derivatives — they produce the smells of fruits (banana, pineapple, rum), make up the fats and oils in food, and appear in biodiesel and many drugs. Their reactivity sits below anhydrides but above amides: alkoxide (RO-) is a mediocre leaving group, so esters undergo nucleophilic acyl substitution more slowly than acid chlorides or anhydrides, but far faster than amides. The key reactions are hydrolysis (acidic, reversible; basic/Saponification Basic hydrolysis of an ester to carboxylate salt + alcohol Full entry →, irreversible), Aminolysis Ester + amine → amide + alcohol Full entry → (to amides), reduction (to alcohols), Transesterification Exchange of the alkoxy group using another alcohol Full entry →, and reaction with Grignard reagents (to tertiary alcohols). Because esters are stable, easy to make, and easy to convert, they are among the most used intermediates in organic synthesis.
Why this matters
- Fats, oils, and soaps: Triglycerides are triesters of glycerol with fatty acids. Saponification — basic hydrolysis — of a fat or oil gives glycerol and fatty-acid salts: soap. Soap-making is literally ester chemistry.
- Flavor and fragrance chemistry: Isoamyl acetate smells like banana, ethyl butyrate like pineapple, methyl salicylate like wintergreen. The food and perfume industries are built on esters.
- Biodiesel: Vegetable oils (triglycerides) are converted by transesterification with methanol into fatty acid methyl esters — biodiesel — plus glycerol. This reaction connects organic chemistry to renewable energy.
- Drug design and metabolism: Aspirin is an ester (of salicylic acid's phenol); many prodrugs are esters designed to hydrolyze at a controlled rate; local anesthetics like benzocaine are esters. Hydrolysis rate governs drug duration.
- Exams: Saponification stoichiometry (3:1 soap:Triglyceride Triester of glycerol with three fatty acids Full entry →), transesterification products, LiAlH₄ reduction products (two alcohols!), and acid vs. base hydrolysis direction are classic problems.
The college version
Core Concepts
Structure, naming, and physical properties
An ester is named as an alkyl alkanoate: the alkyl group from the alcohol comes first, then the acid name with "-oic acid" changed to "-oate" (ethyl acetate = ethanol + acetic acid; methyl benzoate = methanol + benzoic acid). Esters are polar but cannot hydrogen-bond to each other (no –OH), so they boil lower than the parent acids — ethyl acetate boils at 77 °C versus acetic acid at 118 °C. Small esters are volatile liquids with pleasant odors; this volatility is why fruits smell.
Synthesis: three routes plus transesterification
- Fischer esterification Acid + alcohol ⇌ ester + water (acid-catalyzed) Full entry →: RCOOH + R'OH H+⟶ RCOOR' + H2O. Equilibrium (K ≈ 3–4); drive with excess alcohol or water removal.
- Acid chloride or anhydride + alcohol: RCOCl + R'OH → RCOOR' + HCl (base traps HCl); anhydrides give ester + carboxylic acid.
- Carboxylate salt + alkyl halide (SN2): RCOO- + R'X → RCOOR' + X-. Great for methyl and primary alkyl groups; no acid needed.
- Transesterification: RCOOR' + R''OH H+ or OH-⟶ RCOOR'' + R'OH. The alcohol exchanges; driven by excess R''OH. This is the biodiesel reaction.
Hydrolysis: reversible in acid, irreversible in base
- Acidic hydrolysis is the reverse of Fischer esterification: RCOOR' + H2O ⇌ RCOOH + R'OH. It reaches equilibrium.
- Basic hydrolysis (saponification) is not an equilibrium: RCOOR' + OH- → RCOO- + R'OH. The carboxylate salt cannot be attacked by alkoxide to re-form the ester, so the reaction runs to completion. This irreversibility is why soaps are made with NaOH/KOH and why base is the standard way to fully break down esters.
Reduction and reaction with Grignard reagents
- LiAlH₄ reduction gives two alcohols — the primary alcohol from the acyl side (RCH2OH) and the alcohol from the alkoxy side (R'OH):
RCOOR' LiAlH4⟶ RCH2OH + R'OH
- Grignard addition: An ester reacts with two equivalents of R''MgX to give a tertiary alcohol: the first equivalent converts the ester to a ketone, which reacts with the second to give R-C(OH)(R'')2. A powerful way to build carbon skeletons.
Aminolysis and the amide connection
An ester reacts with ammonia or an amine to give an amide plus the alcohol: RCOOR' + R''NH2 → RCONHR'' + R'OH. This works because amides are more stable than esters (nitrogen's lone pair donates into the carbonyl more effectively than oxygen's). It is the standard way to convert an ester to an amide when starting from the acid would require harsher conditions.
How It Works / Step-by-Step Process
Saponification mechanism (basic hydrolysis):
- Hydroxide attacks the ester's acyl carbon; the C=O π electrons move to oxygen, forming a tetrahedral intermediate.
- The intermediate collapses: an oxygen lone pair reforms the C=O while the C-OR' bond breaks; alkoxide R'O- departs.
- The alkoxide abstracts a proton from water, giving the alcohol R'OH and regenerating hydroxide, which continues the cycle. The carboxylate salt RCOO-M+ accumulates because it is resonance-stabilized and cannot re-form the ester.
Net: ester + OH- → carboxylate + alcohol, driven to completion.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Ester | Ether | Ester: RCO-OR' (has a carbonyl); ether: R-O-R' (no carbonyl). Esters hydrolyze and reduce; ethers are much more inert |
| Acidic hydrolysis | Basic hydrolysis (saponification) | Acidic: reversible equilibrium (acid + alcohol). Basic: irreversible (carboxylate salt cannot re-esterify) |
| Ester + 1 equiv Grignard | Ester + 2 equiv Grignard | One equivalent gives a ketone (which reacts further); two give the tertiary alcohol. Stopping at the ketone needs different reagents (e.g., Weinreb amides, covered later) |
| LiAlH₄ reduction of an ester | Reduction of a ketone/aldehyde | Ester gives TWO alcohols (acyl + alkoxy sides); a ketone gives one secondary alcohol |
| Fischer esterification direction | "Ester + water → nothing" | It is an equilibrium: water hydrolyzes the ester back unless removed or overwhelmed by excess alcohol |
| Triglyceride "3 esters" | "3 fatty acids + glycerol mixed" | In the fat they are bonded as esters; only after hydrolysis do free fatty acids (or soaps) and glycerol exist separately |
Eli explains
The same idea, in plain words
Explain it like I’m 10
An ester is like a Lego car made of two parts snapped together: an engine block (the acid part) and a driver's seat (the alcohol part). You can swap the seat for a different one (transesterification), pull the car apart with water (hydrolysis — in soap-making, with a strong cleaner), or bolt on two extra wheels (Grignard). The engine block is always the same piece; only the seat changes.
Worked example
Example 1: Saponification stoichiometry of a triglyceride
Glyceryl tristearate (a fat, 891.5 g/mol) is hydrolyzed with excess NaOH to give three molecules of sodium stearate (soap, 306.5 g/mol) and one molecule of glycerol. What mass of soap forms from 25.0 g of the fat?
Balanced reaction first:
triglyceride + 3NaOH ⟶ 3 sodium stearate + glycerol
Dimensional analysis with the 3:1 mole ratio:
25.0 g fat × 1 mol fat891.5 g × 3 mol soap1 mol fat × 306.5 g soap1 mol soap = 25.8 g soap
Answer: 25.8 g of soap (theoretical). The "×3" is the key step — each fat molecule carries three ester bonds and produces three soap molecules.
Example 2: LiAlH₄ reduction products
What products form when methyl benzoate, C6H5COOCH3 (136.15 g/mol), is reduced with excess LiAlH4?
Reason: The acyl side (C6H5CO-) becomes benzyl alcohol, C6H5CH2OH (108.14 g/mol); the alkoxy side (-OCH3) becomes methanol.
Answer: benzyl alcohol + methanol; 1 mol ester → 1 mol of each. From 5.00 g of ester:
5.00 g × 1 mol136.15 g × 108.14 g1 mol = 3.97 g benzyl alcohol
Students who answer "just benzyl alcohol" miss the alkoxy half — a classic trap.
Example 3: Transesterification for biodiesel
Methyl palmitate, CH3(CH2)14COOCH3 (270.45 g/mol), is treated with excess ethanol and catalytic acid. What is the product?
Reason: Transesterification exchanges the alkoxy group: –OCH₃ is replaced by –OCH₂CH₃, releasing methanol.
Answer: ethyl palmitate, CH3(CH2)14COOCH2CH3, plus methanol. Excess ethanol drives the equilibrium right — exactly how biodiesel plants convert oil triglycerides to ethyl or methyl esters.
Key takeaways
- Esters: RCO-OR'; named alkyl alkanoate (ethyl acetate, methyl benzoate).
- Reactivity: acid chloride > anhydride > thioester > ester > amide.
- Fischer esterification: acid + alcohol ⇌ ester + water, H+, K ≈ 3–4.
- Acidic hydrolysis is reversible; basic hydrolysis (saponification) is irreversible (the carboxylate salt cannot re-esterify).
- Saponification: 1 triglyceride + 3 OH- → 3 fatty-acid salts (soap) + 1 glycerol.
- LiAlH4 reduction: ester → primary alcohol (acyl side) + alcohol (alkoxy side) — two products.
- Grignard: 2 equiv R''MgX → tertiary alcohol.
- Aminolysis: ester + amine → amide + alcohol.
- Transesterification: ester + different alcohol ⇌ different ester + alcohol (biodiesel).
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Write the acid-catalyzed and base-catalyzed hydrolysis of ethyl acetate. Which is reversible, and why?
Show answer
Acidic: CH3COOCH2CH3 + H2O ⇌ CH3COOH + CH3CH2OH — reversible. Basic: CH3COOCH2CH3 + OH- → CH3COO- + CH3CH2OH — irreversible, because the resonance-stabilized carboxylate cannot be attacked by alkoxide to re-form the ester.
How many moles of soap and glycerol are produced from one mole of a triglyceride?
Show answer
One mole of triglyceride gives three moles of soap (fatty-acid salts) and one mole of glycerol.
What two alcohols result from LiAlH4 reduction of methyl butanoate?
Show answer
1-butanol (CH3CH2CH2CH2OH, from the acyl side) and methanol (from the alkoxy side).
Why does an ester react with an amine to give an amide, rather than the reverse?
Show answer
Because the amide is thermodynamically more stable: nitrogen's lone pair donates into the carbonyl more effectively than oxygen's, so the equilibrium favors amide + alcohol. (The amine is also the better nucleophile.)
What is the product of ethyl benzoate with two equivalents of methylmagnesium bromide (followed by aqueous workup)?
Show answer
2-phenyl-2-butanol, C6H5C(OH)(CH3)2 — a tertiary alcohol bearing the phenyl group and two methyl groups; methanol is the byproduct.
Why do esters boil at lower temperatures than the carboxylic acids they come from?
Show answer
Esters have no –OH, so they cannot hydrogen-bond to each other; only dipole–dipole interactions hold them together. Carboxylic acids form strong hydrogen-bonded dimers, which is why the acid boils much higher.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Ester
- RCO-OR', acyl group + alkoxy group
- Fischer esterification
- Acid + alcohol ⇌ ester + water (acid-catalyzed)
- Saponification
- Basic hydrolysis of an ester to carboxylate salt + alcohol
- Transesterification
- Exchange of the alkoxy group using another alcohol
- Aminolysis
- Ester + amine → amide + alcohol
- LiAlH₄ reduction of esters
- Hydride reduction to two alcohols
- Triglyceride
- Triester of glycerol with three fatty acids
Sources & references
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