Esters: Esterification and Hydrolysis

Esters are the compounds behind nearly every pleasant smell in the natural world — from pear drops to ripe bananas to rose perfume. They are also the workhorse molecules of industry: polyester fabric, plasticisers, solvents, fragrances and flavours are all built on ester chemistry.

An ester is a condensation product of a carboxylic acid and an alcohol. This lesson covers the OCR A-Level Chemistry A (H432) specification point 6.2.1 (d): formation, nomenclature, hydrolysis and uses of esters.

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1. Structure and Naming of Esters

An ester has the general formula R–COO–R', where R comes from the acid and R' comes from the alcohol. The functional group –COO– contains a C=O and an adjacent C–O–C linkage.

1.1 Naming rules

Ester names come in two parts: the alcohol part comes first, then the acid part with the suffix -oate.

Rule: [alkyl from alcohol] [acyl from acid with -oate ending]

Acid + alcohol	Ester	Name
Ethanoic acid + methanol	CH₃COOCH₃	Methyl ethanoate
Ethanoic acid + ethanol	CH₃COOCH₂CH₃	Ethyl ethanoate
Methanoic acid + ethanol	HCOOCH₂CH₃	Ethyl methanoate
Propanoic acid + methanol	CH₃CH₂COOCH₃	Methyl propanoate
Butanoic acid + ethanol	CH₃(CH₂)₂COOCH₂CH₃	Ethyl butanoate

Exam Tip: Students often get this backwards. The name reads: "alcohol alkyl group" + "acid minus -oic + -oate". Methyl ethanoate is the ester from methanol and ethanoic acid, not the other way around.

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2. Esterification: Making an Ester

Esterification is the condensation reaction between a carboxylic acid and an alcohol to give an ester and water.

2.1 Overall equation

$CH_3COOH + C_2H_5OH \rightleftharpoons CH_3COOC_2H_5 + H_2O$CH3​COOH+C2​H5​OH⇌CH3​COOC2​H5​+H2​O

Ethanoic acid + ethanol gives ethyl ethanoate, catalysed by concentrated H₂SO₄.

Note the equilibrium arrows — the reaction is reversible, and usually sits around 65–70% conversion before equilibrium is reached. To push it towards the ester, industrial chemists use excess alcohol or remove the water (Le Chatelier's principle).

2.2 Conditions

• Reagents: Carboxylic acid + alcohol
• Catalyst: A few drops of concentrated H₂SO₄ (acts as both catalyst and dehydrating agent — removing water also pushes equilibrium towards ester).
• Temperature: Warm under reflux for 20–30 minutes.

Why reflux? The reaction is slow at room temperature, but both the reactants and the ester are volatile and would otherwise boil away. A reflux condenser allows heating without losing material.

2.3 Mechanism (brief)

You are not required to know the detailed esterification mechanism for OCR H432, but you should understand:

1. H⁺ from H₂SO₄ protonates the C=O of the acid, making the carbonyl carbon even more electrophilic.
2. The alcohol's lone pair attacks the δ+ C.
3. After proton shuffling, water is eliminated, giving the ester.

2.4 Observations and isolation

When you carry out the practical:

• Smell is the most obvious sign — a fruity, sweet smell develops as the ester forms.
• To isolate the ester: distil off the ester (lower bp than the acid), wash with aqueous Na₂CO₃ to remove unreacted acid, then dry with anhydrous MgSO₄ before a final distillation.

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3. Hydrolysis of Esters

Hydrolysis is the reverse of esterification — the ester is broken apart by water to give an alcohol and either a carboxylic acid (acid hydrolysis) or a carboxylate salt (base hydrolysis). OCR requires both.

3.1 Acid hydrolysis

$CH_3COOC_2H_5 + H_2O \rightleftharpoons CH_3COOH + C_2H_5OH$CH3​COOC2​H5​+H2​O⇌CH3​COOH+C2​H5​OH

• Conditions: Dilute H₂SO₄ (or HCl) catalyst; reflux.
• Products: Carboxylic acid + alcohol.
• Reversible: Note the ⇌ arrows — acid hydrolysis is an equilibrium, so yields are typically only 50–70%.

To drive the equilibrium towards hydrolysis, use a large excess of water.

3.2 Base hydrolysis (saponification)

$CH_3COOC_2H_5 + NaOH \longrightarrow CH_3COO^-Na^+ + C_2H_5OH$CH3​COOC2​H5​+NaOH⟶CH3​COO−Na++C2​H5​OH

• Conditions: Aqueous NaOH; reflux.
• Products: The sodium carboxylate (not the free acid) and the alcohol.
• Irreversible: Note the single arrow — because the carboxylate is such a weak base, it will not recombine with the alcohol.

Base hydrolysis is preferred for making pure products because it goes to completion. It is also the process by which natural fats and oils (which are esters of glycerol with long-chain fatty acids) are converted to soap — hence the traditional name saponification (from Latin sapo, soap).

graph TD
  A[Ester R-COO-R'] --> B{Hydrolysis conditions}
  B -->|Acid: dilute H2SO4, reflux| C["Carboxylic acid + alcohol<br/>Reversible 50-70%"]
  B -->|Base: aq NaOH, reflux| D["Carboxylate salt + alcohol<br/>Irreversible 100%"]
  D --> E[Acidify with HCl to get free acid]

3.3 Getting the free acid from saponification

If you want the free carboxylic acid rather than the sodium salt, you must acidify the reaction mixture with HCl after hydrolysis:

$CH_3COO^-Na^+ + HCl \longrightarrow CH_3COOH + NaCl$CH3​COO−Na++HCl⟶CH3​COOH+NaCl

This is a common OCR exam step — do not forget it when asked for the "products" of saponification followed by acidification.

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4. Uses of Esters

Esters dominate several industries:

Use	Example
Flavourings and perfumes	3-methylbutyl ethanoate (banana), pentyl ethanoate (pear), octyl ethanoate (orange)
Solvents	Ethyl ethanoate — nail varnish remover, decaffeination
Plasticisers	Phthalate esters — soften PVC
Polyesters	Terylene (PET) — clothing, bottles (see Lesson 9)
Biodiesel	Methyl esters of long-chain fatty acids — transesterification of vegetable oils

4.1 Fats and oils as triesters

Natural fats and oils are triesters of glycerol (propane-1,2,3-triol) with three long-chain fatty acids. Saponification with NaOH gives three sodium carboxylate salts (soap) plus glycerol. This is how soap has been made for millennia, and is the reason traditional lye soap is still produced from wood ash (K₂CO₃) and animal fat.

The distinction between a fat (solid at room temperature) and an oil (liquid at room temperature) comes from the fatty-acid chains: fats are built from mostly saturated long-chain acids (e.g. stearic acid, no C=C double bonds) and pack closely into a solid; oils are built from unsaturated acids (e.g. oleic acid, one cis C=C bond; linoleic acid, two cis C=C bonds) whose bent geometry prevents close packing and gives a liquid. Industrial hydrogenation of vegetable oils (over a Ni catalyst, see Lesson 9 of the basic-organic course) reduces some of the C=C bonds, converting the oil into a semi-solid spread (margarine). Partial hydrogenation can also isomerise residual cis C=C bonds to trans, generating the "trans fats" whose use has been restricted in many countries since 2010 because of their cardiovascular risk.

4.2 Biodiesel — transesterification

When a fat is heated with methanol and a basic catalyst (NaOH), each ester linkage is exchanged: the glycerol leaves and methanol takes its place. The products are three methyl esters of long-chain fatty acids — biodiesel — plus free glycerol as a by-product. The chemistry is transesterification (an ester is converted into a different ester, swapping the alcohol part), and the methyl esters have viscosity and combustion behaviour similar to diesel from crude oil, so they can be burned directly in a diesel engine without modification. Biodiesel made from used cooking oil is a small but growing fraction of the European road-fuel mix (~7% by volume). The same reaction, run with ethanol instead of methanol, gives ethyl ester biodiesel — slightly higher cetane number but harder to wash free of soap by-products. UK pump diesel is typically blended with up to 7% fatty acid methyl ester (FAME) biodiesel — the "B7" specification on the pump.

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5. Physical Properties of Esters

Esters are polar molecules (because of C=O) but they cannot form hydrogen bonds between themselves — neither oxygen is bonded to H. So their boiling points are comparable to ketones of similar size and considerably lower than the parent acid or alcohol.

Short-chain esters are pleasant-smelling volatile liquids; longer esters are oils; very long esters (such as fats) are waxy solids. Esters are generally poorly soluble in water because the hydrocarbon part dominates.

Compound	M_r	Boiling point (°C)	Strongest IMF
Methyl ethanoate CH₃COOCH₃	74	57	Permanent dipole
Ethanoic acid CH₃COOH	60	118	H-bonding (dimer)
Propan-1-ol C₃H₇OH	60	97	H-bonding
Pentane C₅H₁₂	72	36	London only

Methyl ethanoate (M_r 74) boils at 57 °C — significantly higher than pentane (M_r 72, b.p. 36 °C) because of the permanent dipole, but significantly lower than ethanoic acid or propan-1-ol of similar M_r because esters cannot donate hydrogen bonds.

Key Insight: Esters can accept H-bonds from water (the C=O oxygen has lone pairs), but they cannot donate H-bonds to each other (no O–H). This is exactly the same pattern as aldehydes and ketones in Lesson 1.

5.1 Smells and uses revisited

Short-chain esters (C₃-C₆) are notably fruity-smelling and so are widely used as artificial flavourings. The "pear drops" smell of school chemistry is 3-methylbutyl ethanoate; the "banana" of food labels is pentyl ethanoate. The volatility of esters (low M_r + only dipole-dipole forces) is what makes them effective perfume top notes — they evaporate quickly from skin to give an immediate scent.

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6. Worked Example — Drawing an Ester from Acid + Alcohol

Q: Draw the ester formed from propanoic acid + butan-1-ol, and name it.

Approach:

1. Identify the acid OH: propanoic acid is CH₃CH₂COOH. Mentally remove the –OH from –COOH.
2. Identify the alcohol H: butan-1-ol is CH₃CH₂CH₂CH₂OH. Mentally remove the H from the –OH.
3. Join the two fragments with an oxygen between them.

$CH_3CH_2-COOH + HO-CH_2CH_2CH_2CH_3 \rightleftharpoons CH_3CH_2-COO-CH_2CH_2CH_2CH_3 + H_2O$CH3​CH2​−COOH+HO−CH2​CH2​CH2​CH3​⇌CH3​CH2​−COO−CH2​CH2​CH2​CH3​+H2​O

Naming: alkyl (from alcohol) first, then -oate (from acid):

• Butyl (from butan-1-ol)
• Propanoate (from propanoic acid)

Name: butyl propanoate. This is a fruity-smelling liquid used in pineapple flavouring.

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7. Worked Example — Going Back from an Ester

Q: The ester ethyl 2-methylpropanoate has been used as an artificial pineapple flavour. Draw its structure, and identify the alcohol and the acid from which it is made.

Approach:

• Ester name = [alcohol part] [acid part-oate]: "ethyl" = from ethanol; "2-methylpropanoate" = from 2-methylpropanoic acid.
• Acid: 2-methylpropanoic acid is (CH₃)₂CHCOOH.
• Alcohol: ethanol is CH₃CH₂OH.
• Ester: (CH₃)₂CH-COO-CH₂CH₃.

The reaction is: (CH₃)₂CHCOOH + CH₃CH₂OH ⇌ (CH₃)₂CHCOOCH₂CH₃ + H₂O (conc. H₂SO₄, reflux).

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8. Worked Example — Calculating an Ester Yield

Q: A student refluxes 6.0 g of ethanoic acid (CH₃COOH, M_r 60) with excess ethanol and a few drops of concentrated H₂SO₄. At equilibrium, 5.9 g of ethyl ethanoate (CH₃COOC₂H₅, M_r 88) is isolated. Calculate the % yield.

Approach:

1. Moles of CH₃COOH = 6.0 / 60 = 0.10 mol. This is the limiting reagent (ethanol is in excess).
2. Maximum moles of ethyl ethanoate = 0.10 (1:1 stoichiometry).
3. Maximum mass = 0.10 × 88 = 8.8 g.
4. Actual mass = 5.9 g.
5. % yield = (5.9 / 8.8) × 100 = 67%.

This 67% yield is the typical equilibrium conversion for ethanoic acid + ethanol esterification at reflux. To improve the yield, use a larger excess of ethanol (Le Chatelier shifts equilibrium right) or remove water (Dean-Stark trap).

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Synoptic Links

Synoptic Links — Connects to:

• ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / carboxylic-acids (Lesson 3 — the carboxylic acid is the carbonyl half of the ester).
• ocr-alevel-chemistry-alcohols-haloalkanes / combustion-and-oxidation-of-alcohols (the alcohol is the other half).
• ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / acyl-chlorides (Lesson 5 — the faster, irreversible, room-temperature route to esters using R-COCl).
• ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / condensation-polymers (Lesson 9 — polyesters such as PET are condensation polymers of a dicarboxylic acid + a diol; each repeat-unit is an ester linkage).
• ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / hydrolysis-and-comparison-of-polymers (Lesson 10 — the saponification of biodegradable polyesters such as PLA is the same hydrolysis reaction as ester hydrolysis here).
• ocr-alevel-chemistry-carbonyls-polymers-spectroscopy / proton-nmr-combined-techniques (the ester linkage –O–CH₂– gives a characteristic ¹H NMR shift at δ 4.0–4.3 ppm — the methyl/methylene next to the carbonate oxygen).
• From AS Year 12: Le Chatelier's principle (shifting equilibrium by excess reagent or product removal); reflux as a practical technique for low-boiling reactions.