Theory — Esters
An ester contains the functional group R–CO–OR' (also written as –COOR' or –CO₂R'). Two parts: the acyl portion (R–CO–) comes from a carboxylic acid; the alkoxy portion (–OR') comes from an alcohol. Esters are formed by condensing an acid and an alcohol with loss of water (Fischer esterification, the topic of the previous lab). They are broken back down by hydrolysis. Almost everything else esters do is a variation on those two themes — making the C–O bond, or breaking it.
1. Naming esters
The IUPAC name has TWO parts: alkyl (from the alcohol) + alkanoate (from the carboxylic acid). The alcohol part comes first, separated by a space.
- CH₃COOCH₃ → methyl ethanoate (methyl acetate). Acid = ethanoic; alcohol = methanol.
- CH₃COOC₂H₅ → ethyl ethanoate (ethyl acetate, the most common organic solvent).
- CH₃CH₂CH₂COOC₂H₅ → ethyl butanoate (smells of pineapple).
- CH₃COOCH₂CH₂CH(CH₃)₂ → 3-methylbutyl ethanoate (isoamyl acetate, banana oil).
- C₆H₅COOCH₃ → methyl benzoate (component of ylang-ylang fragrance).
For cyclic esters (lactones), the parent ring is named with the suffix -olide, or with the older Greek-letter convention: γ-butyrolactone (5-membered), δ-valerolactone (6-membered). Greek letters indicate which carbon (γ = C3, δ = C4) carries the OH that closed onto the COOH.
2. Esters and smell — the everyday chemistry
Many simple low-molecular-weight esters have characteristic fruity or floral fragrances. They are responsible for the natural smells of fruits and flowers and are the basis of synthetic flavourings:
| Ester | Structure | Smell / source |
|---|---|---|
| Ethyl ethanoate | CH₃COOC₂H₅ | Pear drops; nail polish remover (also a solvent) |
| Ethyl butanoate | CH₃CH₂CH₂COOC₂H₅ | Pineapple |
| 3-Methylbutyl ethanoate (isoamyl acetate) | CH₃COOCH₂CH₂CH(CH₃)₂ | Banana |
| Octyl ethanoate | CH₃COOC₈H₁₇ | Orange |
| Methyl butanoate | CH₃CH₂CH₂COOCH₃ | Apple |
| Benzyl ethanoate | CH₃COOCH₂C₆H₅ | Jasmine, pear |
| Methyl salicylate | 2-OH-C₆H₄-COOCH₃ | Wintergreen (in muscle rub liniments) |
The trend: small esters (C5–C10 total) are volatile and fruity; medium esters (C10–C20) are floral or oily; large esters (C20+) are waxy and odourless (e.g. cetyl palmitate; the natural waxes of leaves and skin).
3. The major reactions of esters
(a) Hydrolysis — acid catalysed. An ester reacts with water under acid catalysis to give back the carboxylic acid and the alcohol — the exact reverse of Fischer esterification. Mechanism: protonation of C=O, addition of water, proton shuffles, loss of alcohol, deprotonation. Six steps, all reversible. Equilibrium is balanced; to drive forward, use excess water or remove the alcohol.
Same six-step mechanism as Fischer, in reverse
Equilibrium-controlled. H₂O excess OR R'-OH removed drives forward
(b) Saponification — base hydrolysis. NaOH hydrolyses an ester at room temperature with a COMPLETELY different mechanism from acid catalysis. Hydroxide attacks the C=O directly, no protonation needed. Tetrahedral intermediate forms, R'-O− leaves, the carboxylic acid is immediately deprotonated by the second equivalent of OH− to give the carboxylate. The carboxylate cannot be re-attacked by R'-OH (carboxylate is too electron-rich), so saponification is irreversible. This is why traditional soap-making uses NaOH on animal fat (a triglyceride): the fat is hydrolysed quantitatively to glycerol + sodium fatty acids = soap.
Mechanism: OH− attacks C, tetrahedral, R'-O− leaves, R-COOH immediately deprotonated
IRREVERSIBLE because carboxylate is too unreactive for R'-OH to re-attack
(c) Reduction. LiAlH₄ in dry THF reduces an ester all the way to TWO alcohols — the alkoxide R'-O− leaves the carbonyl, then the carbonyl carbon is reduced further to the primary alcohol. To stop at the aldehyde requires DIBAL-H at −78°C with one equivalent — the controlled stoichiometry and low temperature trap the tetrahedral intermediate before it collapses to the aldehyde.
R-COOR' + 1 DIBAL-H (−78°C) → R-CHO + R'-OH [partial; aldehyde]
LiAlH₄ gives 1° alcohol. DIBAL-H (cold, 1 equiv) gives aldehyde
(d) Reaction with Grignard reagents. 2 equivalents of R''MgX add to an ester to give a 3° alcohol (after aqueous workup). The first attack gives a tetrahedral intermediate that collapses to a ketone; the ketone is more reactive than the original ester, so the second equivalent of Grignard immediately adds to it. Result: a tertiary alcohol with TWO identical R'' groups attached. Stopping at the ketone requires very controlled conditions (Weinreb amide chemistry).
First R'' adds, gives ketone (in situ), second R'' adds to that ketone
Always 3° alcohol with TWO identical R'' groups; cannot stop at ketone with normal Grignards
(e) Transesterification. An ester reacts with a different alcohol under acid or base catalysis to swap the alkoxy group: R-COOR' + R''-OH ⇆ R-COOR'' + R'-OH. Industrially huge: this is how biodiesel is made (triglycerides + methanol → fatty acid methyl esters + glycerol). It is the reverse mechanism of Fischer esterification, with R'-OH leaving and R''-OH attacking.
(f) Claisen condensation. The α-hydrogens of an ester (next to C=O) are weakly acidic (pKa ≈ 25). With a strong base (NaOEt for ethyl esters, LDA in modern lab work), one ester is deprotonated to give an enolate; this attacks a second ester to give a tetrahedral intermediate that collapses by losing R'-O−. Net: two esters condense to give a β-keto ester. Ethyl acetate + NaOEt → ethyl acetoacetate (a foundational compound in organic synthesis).
Step 1: NaOEt deprotonates α-C of one ester → enolate
Step 2: enolate attacks C=O of the other ester → tetrahedral → loss of OEt−
Product is a β-keto ester. Foundation for ketone synthesis via decarboxylation
4. Esters in industry and biology
Polyesters (PET, PLA). Polymerisation of a diol with a dicarboxylic acid (or diester) gives a polyester. PET = polyethylene terephthalate, made from terephthalic acid + ethylene glycol; produced at >50 million tonnes per year worldwide for plastic bottles, fibres (Dacron, Terylene) and films (Mylar). PLA = polylactic acid, made by ring-opening polymerisation of lactide (the cyclic diester of lactic acid); biodegradable and compostable, used in 3D printing filament and disposable cutlery.
Triglycerides — fats and oils. Animal fats and vegetable oils are tri-esters of glycerol with three long-chain fatty acids. Saturated fats (animal: butter, lard) tend to be solids; unsaturated fats (plant: olive oil, sunflower) are liquids. Saponification with NaOH gives glycerol + sodium fatty acid salts = soap.
Aspirin. Acetylsalicylic acid — an ester of salicylic acid with acetic acid. The acetyl group masks the phenol OH (less GI irritation). In the body, esterases hydrolyse aspirin back to salicylic acid (the active drug).
Acetylcholine. The neurotransmitter at the neuromuscular junction. An ester of choline with acetic acid; hydrolysed in milliseconds by acetylcholinesterase to terminate the signal. Nerve agents (sarin, VX) and organophosphate insecticides work by inhibiting this esterase, leading to acetylcholine accumulation and continuous nerve firing.
Vinyl esters and acrylates. Methyl methacrylate (MMA), an α,β-unsaturated ester, polymerises to give poly(methyl methacrylate) — Plexiglas, Lucite, dental composites. Vinyl acetate gives poly(vinyl acetate) (PVA glue, latex paint).
5. Diagnostic tests (see Section IV)
| Test | Positive result | Detects | Distinguishes from |
|---|---|---|---|
| NaHCO₃ effervescence | NO bubbles | Ester (no acidic H) | Carboxylic acid (positive) |
| Hydroxamic acid + FeCl₃ | Magenta-red colour | Ester (and amide) | Carboxylic acid (no colour) |
| Saponification + acidification | Carboxylic acid recovered | Confirms ester structure | Useful confirmatory test |
| Fragrance recognition | Specific fruity scent | Identifies specific ester (banana, apple etc.) | Ketones, aldehydes (different smell) |
| IR spectroscopy | Strong C=O band at 1735–1750 cm⁻¹ | Ester | Acid (1700–1725, broader); amide (1640–1690); ketone (1705–1720) |
Instructions
This lab's Simulation section has four parts. Complete them in order.
Prerequisite: Complete (or be familiar with) Lab Skills & Safety, Mechanisms, Aldehydes & Ketones, and especially Carboxylic Acids before starting this lab — the chemistry of esters is the natural continuation of that thread.
Simulation
Four interactive parts. Use the ↺ Reset Simulation button at any time to clear all answers and start over.
Eight esters. For each: (a) IUPAC name, (b) parent carboxylic acid, (c) parent alcohol.
Six reactions of esters. For each: read the prompt, click the reagent in the dispenser shelf to add it to the flask, then click the predicted product.
Eight conceptual problems on hydrolysis mechanism, saponification irreversibility, reduction selectivity, and Claisen chemistry.
Round 1 — SDS interpretation
Four key reagents used in ester chemistry. Each has 4 questions.
Round 2 — Microscale diagnostic tests
Six unknown samples. For each, run the indicated test and identify the functional group.
Team Questions
Discuss with your team before answering.
Example Lab Notebook Entry
Use the format below as a template.
Esters — Lab Notebook Entry
Submitted by: [Student Name]
Course: Organic Chemistry I · Section: 201-A · Date: April 27, 2026
Objective
To recognise esters by structure and IUPAC name; to identify the parent carboxylic acid and parent alcohol of any ester; to predict the products of hydrolysis (acid and base), reduction (LiAlH₄ vs DIBAL-H), Grignard addition, transesterification, and Claisen condensation; to interpret SDS information for common ester-related reagents; and to identify esters by diagnostic microscale tests including the hydroxamic acid/FeCl₃ test, NaHCO₃ effervescence (negative for esters), and saponification confirmation.
Naming summary (Section I results)
| Structure | IUPAC name | Parent acid | Parent alcohol |
|---|---|---|---|
| CH₃COOCH₃ | Methyl ethanoate (methyl acetate) | Ethanoic (acetic) | Methanol |
| CH₃COOC₂H₅ | Ethyl ethanoate (ethyl acetate) | Ethanoic (acetic) | Ethanol |
| CH₃CH₂CH₂COOC₂H₅ | Ethyl butanoate | Butanoic | Ethanol |
| CH₃COOCH₂CH₂CH(CH₃)₂ | 3-Methylbutyl ethanoate (isoamyl acetate) | Ethanoic | 3-Methylbutan-1-ol (isoamyl alcohol) |
| C₆H₅COOCH₃ | Methyl benzoate | Benzoic | Methanol |
| CH₃COOCH₂C₆H₅ | Benzyl ethanoate | Ethanoic | Benzyl alcohol (phenylmethanol) |
| CH₃CH₂CH₂COOCH₃ | Methyl butanoate | Butanoic | Methanol |
| γ-butyrolactone (cyclic) | Oxolan-2-one (γ-butyrolactone, GBL) | 4-Hydroxybutanoic (intramolecular) | Same molecule, intramolecular OH |
Reaction bench observations (Section II)
| Ester | Reagent | Product (predicted) | Class of product |
|---|---|---|---|
| Ethyl acetate | H₂O / cat. H₂SO₄, Δ | Acetic acid + ethanol (equilibrium) | Reversible hydrolysis |
| Methyl benzoate | NaOH (aq), Δ | Sodium benzoate + methanol | Irreversible saponification |
| Ethyl acetate + methanol | cat. NaOMe | Methyl acetate + ethanol | Transesterification |
| Ethyl propanoate | LiAlH₄ / dry THF, then H₂O | Propan-1-ol + ethanol | Reduction to two alcohols |
| Methyl benzoate | DIBAL-H, 1 equiv, −78°C | Benzaldehyde + methanol | Partial reduction to aldehyde |
| Methyl benzoate | 2 equiv PhMgBr, then H₂O | Triphenylmethanol + methanol | 3° alcohol |
Microscale test results (Section IV, Round 2)
| Unknown | Test applied | Observation | Identified as |
|---|---|---|---|
| Sample 1 | NaHCO₃ (aq) | NO effervescence; sample insoluble (organic layer) | Ester (NOT carboxylic acid) |
| Sample 2 | Hydroxamic acid + FeCl₃ | Magenta-red coloured complex | Ester (or amide) confirmed |
| Sample 3 | NaOH saponification + acidify with HCl | Recovers a carboxylic acid that gives NaHCO₃+ on retest | Confirmed ester structure |
| Sample 4 | Fragrance test («sniff» the volatile) | Banana smell | 3-Methylbutyl ethanoate (isoamyl acetate) |
| Sample 5 | Boiling-point comparison with parent acid + alcohol | Lower b.p. than the parent acid; no broad O-H stretch in IR | Ester (no O-H) |
| Sample 6 | IR spectroscopy | Strong C=O at 1742 cm⁻¹; no broad O-H | Ester (typical 1735–1750 range) |
Discussion
Esters sit between carboxylic acids and ethers in reactivity: they have a polarised C=O like acids, but they are far less acidic (no acidic H on oxygen). The two faces of ester reactivity — nucleophilic attack at C and proton chemistry on the α-C — cover most of their behaviour. Hydrolysis (in either direction, acid or base catalysed) is the most common transformation. The acid-catalysed pathway is precisely the reverse of Fischer esterification and is reversible, while saponification is mechanistically distinct (no protonation; direct OH− attack) and is irreversible because the carboxylate product is too unreactive to be re-attacked by R'-OH.
Reduction of esters showed two contrasting selectivity profiles. LiAlH₄ in THF reduces all the way to the primary alcohol (with the alkoxide R'-O− leaving the carbonyl C and being protonated separately on workup). DIBAL-H at −78°C with controlled stoichiometry stops at the aldehyde because the bulky DIBAL-H aluminium acts as a Lewis acid that traps the tetrahedral intermediate, preventing collapse before the second equivalent of hydride can act. This selectivity is essential in synthesis, where converting an ester to an aldehyde without going further is often required.
The Claisen condensation extended the carbonyl α-acidity theme from the Aldehydes & Ketones lab. Esters are slightly less acidic at the α-position (pKa ≈ 25 vs ≈ 20 for ketones) because the alkoxy oxygen donates electron density into the C=O, partially deactivating the α-C. Strong bases (NaOEt for ethyl esters; LDA for general ester deprotonation) generate the enolate, which attacks a second ester to give a β-keto ester after loss of R'-O−. The product (ethyl acetoacetate is the iconic example) has a much more acidic α-H (pKa ≈ 11) and is the foundation for the acetoacetic ester synthesis — alkylation, decarboxylation, and ketone production.
The diagnostic tests in Section IV clearly separated esters from their close neighbours: the NaHCO₃ effervescence test gives a NEGATIVE result for esters (no acidic H to lose) but positive for carboxylic acids, providing a clean distinction. The hydroxamic acid/FeCl₃ test gives a magenta colour for esters (and amides) but not for free carboxylic acids. IR spectroscopy is the most diagnostic of all: esters show a strong C=O stretch at 1735–1750 cm⁻¹, distinct from carboxylic acids (1700–1725, broader, often with a broad O-H stretch around 3000), amides (1640–1690), and ketones (1705–1720). The IR carbonyl region alone often allows assignment of carbonyl class without any chemistry at all.
The industrial and biological context emphasised in the theory section made the ester functional group personally relevant: the smells of fruits and flowers, polyester clothing, plastic bottles, aspirin, neurotransmission, and biodiesel are all ester chemistry. The reagents in the SDS round — concentrated H₂SO₄ (Fischer catalyst), DIBAL-H (pyrophoric), methyl methacrylate (MMA, Plexiglas precursor, sensitiser), and ethyl acetate itself (the most common organic solvent) — cover both lab and industrial scales of ester chemistry.
Conclusion
The ester group brings together the electrophilic C=O and the nucleophilic α-C in a structure with low acidity, moderate stability, and rich chemistry. Hydrolysis (both directions), reduction (with selectivity options), Grignard addition (forced to 3° alcohol), transesterification (industrial scale for biodiesel), and Claisen condensation (foundation of β-keto ester chemistry) make esters one of the most central functional groups in organic synthesis. Diagnostic testing — especially IR spectroscopy and the hydroxamic acid/FeCl₃ test — allows clean identification.
References
1. Clayden, J.; Greeves, N.; Warren, S. Organic Chemistry, 2nd ed., Oxford University Press, 2012, Chs 12, 26, 28.
2. Smith, M. B.; March, J. March's Advanced Organic Chemistry, 7th ed., Wiley, 2013, Chs 16, 19.
3. IUPAC. Recommendations on Organic Nomenclature, 2013.
4. Sigma-Aldrich SDS for ethyl acetate (CAS 141-78-6), methyl methacrylate (CAS 80-62-6), DIBAL-H (CAS 1191-15-7), conc. H₂SO₄ (CAS 7664-93-9), accessed online March 2026.
Practice Questions
Work through each before peeking at the hint.