Esters (A2)
Introduced| English | Meaning |
|---|---|
| ester/ˈestə/ | An organic compound containing a carbonyl group bonded to an oxygen atom that is bonded to another carbon group. |
| acyl chloride/əˈkɪl ˈklɔːraɪd/ | An organic compound containing a carbonyl carbon bonded to a chlorine atom and another carbon group or hydrogen. |
| addition–elimination/əˈdɪʃn ɪˌlɪmɪˈneɪʃn/ | A reaction mechanism in which a group first adds to a molecule and another group is then eliminated. |
| saponification/ˌsæpəˌnɪfɪˈkeɪʃn/ | saponification |
| biodiesel/ˌbaɪəʊˈdiːzl/ | biodiesel |
Esters 酯, revisited
- The smell of a pear, the tang of aspirin, the fat in your food — all are esters.
- An ester forms from an alcohol (or a phenol) joined to an acid group.
- The fast, reliable way to make one is with an acyl chloride 酰氯, at room temperature.
Ester A2 reaction map
Compare ester formation, hydrolysis and transesterification routes.
An ester can be made at room temperature by reacting an acyl chloride with:
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Both alcohols and phenols react with acyl chlorides to give esters (+ HCl).
Making an ester from an acyl chloride happens at room temperature.
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Acyl chlorides are very reactive, so the reaction proceeds quickly at room temperature.
An ester is hydrolysed by hot aqueous alkali in a reaction called ______.
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It gives a carboxylate salt and an alcohol.
Esters often have sweet, fruity smells.
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They are used as flavourings and perfumes.
Making esters
- alcohol/phenol + acyl chloride → ester + HCl — quick, and not reversible.
- The acyl chloride is so reactive it needs no catalyst and no heat.
- It works where a carboxylic acid is too slow — for example, esterifying an unreactive phenol.
Make ethyl ethanoate. Ethanol + ethanoyl chloride:
CH₃COCl + C₂H₅OH → CH₃COOC₂H₅ + HCl
The –OH of the alcohol attacks the C=O carbon; the chlorine leaves as HCl (an addition–elimination 加成消去). Name it from the alcohol part first (ethyl), then the acid part (ethanoate).
Aspirin is an ester of salicylic acid.
Reacting phenol with benzoyl chloride gives:
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A phenol + an acyl (benzoyl) chloride forms the ester phenyl benzoate.
Hydrolysing an ester
- Hydrolysis is the reverse — water (or OH⁻) splits the ester back apart.
- Acid hydrolysis is reversible: you get the carboxylic acid + the alcohol back.
- Base hydrolysis (saponification 皂化) is one-way: you get a carboxylate salt + the alcohol.
Acid and base hydrolysis give different products. Acid → the carboxylic acid; base → the carboxylate salt (e.g. sodium ethanoate, not ethanoic acid). Base hydrolysis goes to completion because the salt cannot react back.
Hot KMnO4 oxidises a methyl side-chain to a COOH group
Esters all around us
- Small esters are volatile and sweet-smelling — used as flavourings, perfumes and solvents.
- Large esters make up fats and oils (esters of glycerol) and biodiesel 生物柴油.
- Saponification of a fat with NaOH is literally how soap is made.
You've got it
- an ester forms from an alcohol or phenol + an acyl chloride (+ HCl), at room temperature
- name it alcohol-part + acid-part: ethanol + ethanoyl chloride → ethyl ethanoate
- acid hydrolysis (reversible) → acid + alcohol; base hydrolysis (one-way) → salt + alcohol
- esters are everywhere: fruit smells, aspirin, fats/oils, biodiesel, soap