Carboxylic Acids and Derivatives (A Level)

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Cambridge International AS & A Level Chemistry (9701)

Carboxylic Acids and Derivatives (A Level)

The big idea: This builds on the AS chapter Carboxylic Acids and Derivatives. Swapping the –OH of a carboxylic acid for –Cl gives an acyl chloride, one of the most reactive compounds you meet. It turns water, alcohols, phenols, ammonia and amines into acids, esters and amides at room temperature, all by the same addition–elimination mechanism.

Summary — What This Chapter Covers

Benzoic acid from alkylbenzenes; acyl chlorides from carboxylic acids with PCl₃, PCl₅ or SOCl₂.
Methanoic acid and ethanedioic acid can be oxidised further, to CO₂.
Acid strength: carboxylic acids > phenols > alcohols; chlorine substituents make carboxylic acids stronger.
Esters, including phenyl esters, from acyl chlorides.
Acyl chlorides react with water, alcohols, phenol, ammonia and amines by addition–elimination.
Ease of hydrolysis: acyl chloride > alkyl chloride > aryl chloride.

1. Carboxylic Acids

33.1

Making benzoic acid

Heat an alkylbenzene such as methylbenzene with hot alkaline KMnO₄, then add dilute acid: C₆H₅CH₃ + 3[O] → C₆H₅COOH + H₂O.

Making acyl chlorides

Three chlorinating reagents 3CH₃COOH + PCl₃ → 3CH₃COCl + H₃PO₃ (heat)
CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl (room temperature)
CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl

With SOCl₂ both by-products are gases, so the acyl chloride is easy to separate. With PCl₅, steamy fumes of HCl are seen.

Further oxidation of two acids

Methanoic acid, HCOOH, still contains an H–C=O (aldehyde-like) group. It is oxidised to CO₂ and H₂O by Fehling's reagent (red-brown precipitate of Cu₂O), Tollens' reagent (silver mirror), acidified KMnO₄ (purple to colourless) or acidified K₂Cr₂O₇ (orange to green): HCOOH + [O] → CO₂ + H₂O.
Ethanedioic acid, HOOCCOOH, is oxidised by warm acidified KMnO₄ to CO₂: HOOCCOOH + [O] → 2CO₂ + H₂O. The manganate(VII) is decolourised.

Relative acidities: carboxylic acids, phenols, alcohols

Compound	Approx. pK_a	Why
Ethanoic acid	4.8	In the carboxylate ion the negative charge is delocalised over two electronegative O atoms (both C–O bonds become equal). This stabilises the anion most.
Phenol	10.0	Charge on O is partly delocalised into the ring, but onto carbon atoms, which are less electronegative than O, so the stabilisation is smaller.
Ethanol	about 16	No delocalisation; the electron-donating alkyl group intensifies the charge on O.

Test	Carboxylic acid	Phenol	Alcohol
Na(s)	H₂	H₂	H₂
NaOH(aq)	reacts	reacts	no reaction
Na₂CO₃(aq)	CO₂ effervescence	no CO₂	no CO₂

Chlorine-substituted carboxylic acids

Cl is electronegative and electron-withdrawing. It pulls electron density away from the COO⁻ group, which spreads out (delocalises) the negative charge and stabilises the anion; it also weakens the O–H bond. So the acid is stronger.

More Cl atoms → stronger acid.
Cl closer to the COOH group → stronger acid (the effect fades with distance).

Acid	pK_a
CH₃COOH	4.76
CH₂ClCOOH	2.86
CHCl₂COOH	1.29
CCl₃COOH	0.65

Worked Example

Put these in order of increasing acid strength and explain: 2-chlorobutanoic acid, 4-chlorobutanoic acid, butanoic acid, 2,2-dichlorobutanoic acid.

2. Esters

33.2

3. Acyl Chlorides

33.3

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Also in the full note

Esters from acyl chlorides
Preparation and reactions
The addition–elimination mechanism

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Carboxylic Acids and Derivatives (A Level)