9701 Chemistry · Topics 18 & 32 · AS + A Level
Carboxylic Acids and Derivatives Cheat Sheet — A Level Chemistry 9701
This topic is really one question repeated: how does the group attached to the carbonyl change how easily a nucleophile attacks it? The sheet covers the acidity of carboxylic acids and why chlorine substituents strengthen them, esterification and both acid and alkaline hydrolysis, and the reactivity order of acyl chloride, ester and amide with the reasoning behind it.
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What’s on this cheat sheet
Carboxylic Acids and Derivatives
01 · Properties
–COOH hydrogen bonds strongly, and in a non‑polar solvent two molecules pair into a dimer. Boiling points are therefore higher than those of alcohols of similar Mr.
Short chains are soluble in water; solubility falls as the hydrocarbon chain grows.
02 · Why they are acidic
RCOOH ⇌ RCOO⁻ + H⁺. The carboxylate ion is stabilised by delocalisation over both oxygens, so the equilibrium lies further right than for an alcohol.
They are still weak acids, only partially dissociated. Electron‑withdrawing groups strengthen them: CCl₃COOH ≫ CH₂ClCOOH > CH₃COOH, because chlorine pulls charge away and stabilises the anion further.
03 · Acid reactions
| Reagent | Products |
|---|---|
| NaOH | salt + water |
| Na₂CO₃ | salt + H₂O + CO₂ (fizzes) |
| Na | salt + H₂ |
| alcohol / conc. H₂SO₄ | ester + water |
| LiAlH₄ in dry ether | primary alcohol |
| PCl₅ | acyl chloride + HCl + POCl₃ |
The carbonate test is the standard way to confirm –COOH: only a carboxylic acid among the common organics fizzes.
04 · Making carboxylic acids
Oxidise a primary alcohol or an aldehyde: reflux with acidified K₂Cr₂O₇.
Hydrolyse a nitrile with dilute HCl under reflux, or an ester or amide with acid or alkali. Oxidising a methyl side chain on a benzene ring with hot alkaline KMnO₄ gives benzoic acid.
05 · Esterification
conc. H₂SO₄ catalyst, reflux
Reversible, so the yield is limited; remove the ester by distillation to drive it right.
Name the acid part second: ethyl ethanoate = ethanol + ethanoic acid.
06 · Esters — properties and uses
No O–H, so no hydrogen bonding between molecules: esters are volatile with sweet fruity smells, used as flavourings, perfumes, solvents and plasticisers.
Natural fats and oils are esters of glycerol with long‑chain fatty acids; unsaturated ones are liquid because the cis double bonds stop the chains packing.
07 · Hydrolysis of esters
ester + H₂O ⇌ acid + alcohol · reversible, incomplete
Alkaline hydrolysis — NaOH(aq), reflux
ester + NaOH → salt + alcohol · irreversible, better yield
With a fat this is saponification, and the sodium salt of the fatty acid is soap.
08 · Acyl chlorides A2
RCOCl is far the most reactive derivative: chlorine is electronegative and a good leaving group, so the carbonyl carbon is strongly δ+.
Every reaction is vigorous at room temperature and gives off steamy HCl fumes.
09 · Reactions of acyl chlorides A2
+ ROH → ester + HCl
+ NH₃ → amide + HCl
+ RNH₂ → N‑substituted amide + HCl
+ C₆H₅OH → phenyl ester + HCl
All are nucleophilic acyl substitution: the nucleophile attacks the δ+ carbon, then Cl⁻ leaves.
10 · Order of reactivity A2
acyl chloride > ester > amide
The better the leaving group and the more δ+ the carbon, the faster the attack. This is why acyl chlorides make esters and amides in one step where the acid itself needs a catalyst and an equilibrium.
11 · Worked example — identify the ester
Fizzes → carboxylic acid formed
CHI₃ positive → the alcohol is ethanol
C₄H₈O₂ − C₂H₅O = C₂H₃O → ethanoate
→ ethyl ethanoate, CH₃COOC₂H₅
12 · Equations to know
CH₃COOH + NaOH → CH₃COONa + H₂O
CH₃COOH + PCl₅ → CH₃COCl + HCl + POCl₃
CH₃COOH + 4[H] → CH₃CH₂OH + H₂O
CH₃COCl + H₂O → CH₃COOH + HCl
CH₃COCl + C₂H₅OH → CH₃COOC₂H₅ + HCl
CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl
13 · Relative acid strengths
| Acid | pKa |
|---|---|
| CCl₃COOH | 0.7 |
| CH₂ClCOOH | 2.9 |
| CH₃COOH | 4.8 |
| phenol | 10.0 |
More electronegative atoms near the –COOH pull electron density away from the anion, spreading the charge and stabilising it — so the acid is stronger and pKa smaller.
14 · Practical points
Esterification and hydrolysis are both done under reflux: the condenser returns volatile material so nothing is lost and the mixture can be heated for long enough.
Acyl chlorides are handled in dry apparatus and a fume cupboard — they hydrolyse violently in moist air, releasing HCl.
Marks lost here
— Naming the ester the wrong way round; the alkyl group from the alcohol comes first.
— Writing esterification with a single arrow; it is an equilibrium.
— Giving the free acid as the product of alkaline hydrolysis; it is the sodium salt until you acidify.
— Saying phenol fizzes with carbonate — only carboxylic acids do.
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Carboxylic Acids and Derivatives — Frequently Asked Questions
Why is chloroethanoic acid stronger than ethanoic acid?
Chlorine is electronegative and withdraws electron density along the carbon chain. This delocalises the negative charge on the carboxylate ion more widely, stabilising it and pushing the dissociation equilibrium further right. More chlorines means a stronger acid.
What is the difference between acid and alkaline hydrolysis of an ester?
Acid hydrolysis is reversible and gives the carboxylic acid plus the alcohol. Alkaline hydrolysis — saponification — goes to completion because the carboxylate salt formed cannot react back, and the acid must be liberated afterwards by adding a strong acid.
Why are acyl chlorides so much more reactive than esters and amides?
Chlorine is strongly electron-withdrawing and a good leaving group, so the carbonyl carbon is highly δ+ and readily attacked. In amides the nitrogen lone pair delocalises into the carbonyl, reducing the δ+ charge and making them the least reactive.
