This chemistry grades 11-12 lesson on esterification notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on esterification and check yourself with the quiz.
Quick answer: Carboxylic acids react with alcohols, in the presence of concentrated sulfuric acid, to form esters and water in a reversible equilibrium. Moreover, esters hydrolyse back to acids and alcohols, completely with alkali. In addition, acyl chlorides, RCOCl, react far faster with water, alcohols, ammonia and amines by nucleophilic addition-elimination.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.3.9
Esterification notes: the key ideas
What are carboxylic acids and how do they react?
Carboxylic acids contain the –COOH group and take the ending -oic acid in their names, for example ethanoic acid, CH₃COOH. Also, they are weak acids: they dissociate only partly in water, CH₃COOH ⇌ CH₃COO⁻ + H⁺. Also, they still show typical acid reactions, producing salts called carboxylates.
| Reagent | Equation | Observation |
|---|---|---|
| Magnesium | 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂ | fizzing, hydrogen gas |
| Sodium carbonate | 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ | effervescence, CO₂ turns limewater milky |
| Sodium hydroxide | CH₃COOH + NaOH → CH₃COONa + H₂O | no visible change, warms slightly |
Acid molecules hydrogen-bond strongly, often in pairs, so they have high boiling points and the small ones dissolve in water.
How are esters made?
An ester forms when you heat a carboxylic acid with an alcohol and a few drops of concentrated sulfuric acid, which acts as a catalyst. Moreover, the OH of the acid and the H of the alcohol leave as water. The reaction is reversible, so the mixture contains all four substances at equilibrium.
An ester contains the group –COO– with an alkyl group on each side, written RCOOR’. In addition, the name is the alkyl group from the alcohol followed by the stem of the acid with the ending -oate: ethanol and ethanoic acid give ethyl ethanoate.
Esterification: a worked example on Kc
1.00 mol of ethanoic acid and 1.00 mol of ethanol reach equilibrium with 0.667 mol of ester. Find Kc.
Acid = alcohol = 1.00 − 0.667 = 0.333 mol; water = 0.667 mol. Volume cancels, so Kc = (0.667 × 0.667) / (0.333 × 0.333) = 4.0.
Predict the amount of ester when you mix 1.00 mol of acid with 2.00 mol of ethanol (Kc = 4.0).
Let x = amount of ester. Then x² / [(1 − x)(2 − x)] = 4, so 3x² − 12x + 8 = 0 and x = 0.845 mol (the other root exceeds 1). Using more alcohol raises the yield to 0.845 mol, compared with 0.667 mol.
To push the equilibrium to the right, use an excess of one reagent or remove water or the ester as it forms. A catalyst speeds up the approach to equilibrium but does not change Kc.
How are esters broken down?
Water splits an ester back into acid and alcohol. Also, with dilute acid under reflux the reaction is the reverse of esterification and is incomplete. With aqueous sodium hydroxide the base converts the carboxylic acid to its salt as soon as it forms, so the reaction goes to completion: CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH.
The same reaction makes soap. A fat is a triglyceride, an ester of propane-1,2,3-triol (glycerol) with three fatty acids, and heating it with NaOH gives glycerol and three sodium carboxylate molecules, which are the soap. We make biodiesel by reacting plant oils with methanol, which swaps glycerol for methanol to form methyl esters.
Why are acyl chlorides so reactive?
You make an acyl chloride, RCOCl, by warming a carboxylic acid with thionyl chloride: CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl. Both by-products are gases, so the product is easy to separate. The carbonyl carbon has a large δ+ charge because both oxygen and chlorine pull electrons away, and Cl⁻ is a good leaving group. Acyl chlorides therefore react with water, alcohols, ammonia and amines at room temperature, giving off misty HCl.
Esterification and the acyl chloride mechanism
The mechanism is nucleophilic addition-elimination: the nucleophile adds to the C=O carbon to give a tetrahedral intermediate, then the C=O re-forms and chloride leaves.
Do not write esterification with a one-way arrow: it is an equilibrium (⇌). Also do not put the acid part first in the name: CH₃COOC₂H₅ is ethyl ethanoate, not “ethanoate ethyl”. Also, acyl chloride reactions are not reversible and need no catalyst.
Esterification: a worked example with an acyl chloride
7.85 g of ethanoyl chloride (M = 78.5 g/mol) reacts with excess ethanol. Moreover, the yield is 85 %. Then, find the mass of ethyl ethanoate (M = 88.0 g/mol).
n = 7.85 / 78.5 = 0.100 mol. Theoretical mass = 0.100 × 88.0 = 8.80 g. Actual = 0.85 × 8.80 = 7.48 g.
To name an ester, treat it like a surname and first name: the alkyl “surname” comes from the alcohol and is on the oxygen side; the “first name” is the acid stem and is on the C=O side. For hydrolysis, break the C–O single bond next to the C=O.
- Esterification: RCOOH + R’OH ⇌ RCOOR’ + H₂O, conc. H₂SO₄ catalyst, reflux; Kc about 4 for ethyl ethanoate
- Ester name = alkyl from alcohol + acid stem + -oate (ethyl ethanoate)
- Alkaline hydrolysis is irreversible and gives a carboxylate salt plus an alcohol
- RCOOH + SOCl₂ → RCOCl + SO₂ + HCl
- Acyl chlorides react with H₂O, ROH, NH₃ and amines at room temperature by addition-elimination
Making and reacting esters and acyl chlorides: frequently asked questions
Why do we do esterification under reflux?
Heating speeds up the slow reaction, and reflux condenses vapours so that volatile reactants and the ester do not escape. The mixture can then reach equilibrium.
Why does alkaline hydrolysis go to completion?
The hydroxide converts the carboxylic acid into its carboxylate ion as it forms. That ion is negative and does not react with the alcohol, so the reverse reaction cannot occur.
Why use an acyl chloride instead of a carboxylic acid to make an ester?
The reaction is fast at room temperature and irreversible, so the yield is high. In addition, the drawback is that acyl chlorides are corrosive and give off HCl.
What is the role of concentrated sulfuric acid in esterification?
It acts as a catalyst, and it also absorbs some of the water formed, which moves the equilibrium slightly towards the ester.
How do we make soap from fat?
A fat is a triglyceride. Also, heating it with sodium hydroxide hydrolyses the three ester links, giving glycerol and three sodium carboxylate molecules, which are the soap.
