Flashcards · 9701 · A Level Topic 33

Carboxylic acids and derivatives (A Level) flashcards

12 cards on A Level Topic 33, subtopics 33.1 to 33.3. Pick a subtopic or study them all.

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Effect of chlorine substituents on the strength of ethanoic acid: Electron-withdrawing Cl atoms pull electron density away and stabilise the carboxylate ion, so acid strength increases: CCl₃COOH > CH₂ClCOOH > CH₃COOH.
How is a carboxylic acid converted into an acyl chloride?: With SOCl₂ (or PCl₅ or PCl₃). e.g. CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl.
Why can methanoic acid be oxidised further, unlike most carboxylic acids?: HCOOH still has an H on the carbonyl carbon (aldehyde-like), so it gives positive results with Tollens', Fehling's and acidified KMnO₄ or K₂Cr₂O₇.
Oxidation of ethanedioic acid: Warm acidified KMnO₄ oxidises it to CO₂: 2MnO₄⁻ + 16H⁺ + 5C₂O₄²⁻ → 2Mn²⁺ + 10CO₂ + 8H₂O (self-indicating).
Why are carboxylic acids acidic while alcohols are not?: In the carboxylate ion the negative charge is delocalised over both oxygen atoms, stabilising it. An alkoxide ion has no such delocalisation.

How are esters of phenol made, and why not from the carboxylic acid?: React phenol with an acyl chloride: C₆H₅COCl + C₆H₅OH → C₆H₅COOC₆H₅ (phenyl benzoate) + HCl. Phenol reacts only slowly and reversibly with carboxylic acids.
Phenyl benzoate: The aryl ester C₆H₅COOC₆H₅, made from benzoyl chloride and phenol.
General reaction: acyl chloride + alcohol or phenol: Acyl chloride + alcohol → ester + HCl. Acyl chloride + phenol → aryl ester + HCl. Fast and irreversible at room temperature.

Acyl chlorides with water, alcohols, ammonia and primary amines: Water → carboxylic acid. Alcohol → ester. Ammonia → primary amide. Primary amine → N-substituted amide. All release HCl (steamy fumes).
Why are acyl chlorides much more reactive than carboxylic acids?: The carbonyl carbon is bonded to two electronegative atoms (O and Cl) and Cl donates little lone-pair density into C=O, so the carbon is strongly δ+; Cl⁻ is also a good leaving group.
Outline the addition–elimination mechanism of an acyl chloride with a nucleophile: Addition: the nucleophile lone pair attacks the δ+ carbonyl carbon; π electrons move to O, giving a tetrahedral intermediate. Elimination: C=O re-forms, Cl⁻ leaves, and loss of H⁺ gives the product + HCl.
Ethanoyl chloride with excess ammonia: CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl