Carboxylic acids and derivatives (A Level)

• Every acyl chloride reaction puts HCl on the right; every PCl₅ and SOCl₂ equation has two by-products; every curly arrow starts at a lone pair or a bond.
• HCl on the right of every acyl chloride equation. Two oxygens share the carboxylate’s charge. Addition, then elimination, then lose H⁺.

• 33.1.1 I can recall — recall the reaction by which benzoic acid can be produced: (a) reaction of an alkylbenzene with hot alkaline KMnO₄ and then dilute acid, exemplified by methylbenzene
• 33.1.2 I can describe — describe the reaction of carboxylic acids with PCl₃ and heat, PCl₅ or SOCl₂ to form acyl chlorides
• 33.1.3 I can — recognise that some carboxylic acids can be further oxidised: (a) the oxidation of methanoic acid, HCOOH, with Fehling's reagent or Tollens' reagent or acidified KMnO₄ or acidified K₂Cr₂O₇ to form carbon dioxide and water (b) the oxidation of ethanedioic acid, HOOCCOOH, with warm acidified KMnO₄ to form carbon dioxide
• 33.1.4 I can describe — describe and explain the relative acidities of carboxylic acids, phenols and alcohols
• 33.1.5 I can describe — describe and explain the relative acidities of chlorine-substituted carboxylic acids
• 33.2.1 I can recall — recall the reaction by which esters can be produced: (a) reaction of alcohols with acyl chlorides using the formation of ethyl ethanoate and phenyl benzoate as examples
• 33.3.1 I can recall — recall the reactions (reagents and conditions) by which acyl chlorides can be produced: (a) reaction of carboxylic acids with PCl₃ and heat, PCl₅ or SOCl₂
• 33.3.2 I can describe — describe the following reactions of acyl chlorides: (a) hydrolysis on addition of water at room temperature to produce the carboxylic acid and HCl (b) reaction with an alcohol at room temperature to produce an ester and HCl (c) reaction with phenol at room temperature to produce an ester and HCl (d) reaction with ammonia at room temperature to produce an amide and HCl (e) reaction with a primary or secondary amine at room temperature to produce an amide and HCl
• 33.3.3 I can describe — describe the addition-elimination mechanism of acyl chlorides in reactions in 33.3.2(a)-(e)
• 33.3.4 I can explain — explain the relative ease of hydrolysis of acyl chlorides, alkyl chlorides and halogenoarenes (aryl chlorides)

Units and significant figures are marked. The syllabus states that failure to quote units, the inclusion of units in quantities defined as ratios, and answers given to an inappropriate number of significant figures are all liable to be penalised. Give a calculated answer to the same number of significant figures as the least precise data, or one more; keep full precision in the working and round only at the end. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as procedure, recording and evaluation rather than as theory.

• “CH₃COOH + SOCl₂ → CH₃COCl + HCl”, or an acyl chloride reaction with no HCl. Correct Every acyl chloride equation has HCl on the right; every PCl₅ and SOCl₂ equation has two by-products (POCl₃ and HCl; SO₂ and HCl). PCl₃ takes three acid molecules and gives H₃PO₃.
• “Heat under reflux with SOCl₂.” Correct The syllabus attaches heat to the PCl₃ route only. PCl₅ and SOCl₂ are given with no condition, and the five reactions of acyl chlorides are all at room temperature with no catalyst.
• “Ethylbenzene is oxidised to phenylethanoic acid.” Correct Hot alkaline KMnO₄ cuts a single alkyl side-chain of any length back to the ring carbon: the product is benzoic acid, not phenylethanoic acid.
• “Phenol is a stronger acid than ethanoic acid because its charge is spread round a whole ring.” Correct The carboxylate spreads its charge equally over two electronegative oxygen atoms; the phenoxide only partly spreads it onto ring carbons. Ethanoic acid is the stronger acid, and only it gives CO₂ with a carbonate.
• “Acyl chlorides react by nucleophilic substitution.” Correct The mechanism is named addition–elimination, and the intermediate is tetrahedral, with O⁻ on the old carbonyl carbon.
• “Chlorobenzene is hard to hydrolyse because the ring is bulky.” Correct A chlorine lone pair is delocalised into the ring, giving the C–Cl bond partial double-bond character, and the electron-rich ring repels the nucleophile.
• “Ethylbenzene is oxidised by hot alkaline KMnO₄ to phenylethanoic acid.” Repair The whole side-chain is oxidised back to the ring carbon; the product is benzoic acid, whatever the length of the single alkyl chain.
• CH₃COOH + PCl₃ → CH₃COCl + …, balanced with one acid molecule. Repair One PCl₃ chlorinates three acid molecules: 3CH₃COOH + PCl₃ → 3CH₃COCl + H₃PO₃, with heat.
• HCl written as the only by-product of the SOCl₂ route. Repair SO₂ and HCl. Both are gases, which is why SOCl₂ is preferred.
• “Carboxylic acids cannot be oxidised.” Repair Methanoic acid, which contains H–C=O, is oxidised to CO₂ and water by Fehling’s, Tollens’, acidified KMnO₄ or acidified K₂Cr₂O₇; ethanedioic acid is oxidised to CO₂ by warm acidified KMnO₄.
• “A silver mirror proves the unknown is an aldehyde.” Repair Methanoic acid also gives one. Test for acidity too: CO₂ from a carbonate points to methanoic acid.
• “Ethanedioic acid gives a positive Tollens’ test because it is oxidised by KMnO₄.” Repair It has no H–C=O, so Fehling’s and Tollens’ reagents do not oxidise it; only warm acidified manganate(VII) does.
• “Phenol is a stronger acid than ethanoic acid because the ring delocalises the charge.” Repair The carboxylate delocalises its charge equally over two oxygen atoms, which is more stabilising than partly spreading it onto ring carbons; ethanoic acid is the stronger acid.
• “Phenol reacts with sodium carbonate to give CO₂.” Repair Phenol is too weak an acid; only the carboxylic acid liberates CO₂ from a carbonate.
• “CH₂ClCOOH is a stronger acid because chlorine is a large atom.” Repair Because chlorine is electron-withdrawing (negative inductive effect), which stabilises the anion by spreading its negative charge.
• “3-chloropropanoic acid is stronger than 2-chloropropanoic acid.” Repair The inductive effect falls off with distance along σ bonds; the 2-chloro acid, with Cl next to the COOH, is stronger.
• Phenyl benzoate made from phenol and benzoic acid with concentrated H₂SO₄. Repair Phenol does not esterify with a carboxylic acid; use benzoyl chloride at room temperature.
• “Acyl chlorides react with nucleophiles by nucleophilic substitution.” Repair By addition–elimination: addition across C=O to a tetrahedral intermediate, then elimination of Cl⁻.
• A curly arrow drawn from the δ+ carbon, or from the H of water. Repair Arrows start at a lone pair or a bond: lone pair on O to C; C=O π bond to O; O⁻ lone pair back to C; C–Cl bond to Cl; O–H bond to O for the proton loss.
• The intermediate drawn with the C=O still present. Repair The intermediate is tetrahedral, with O⁻ and a positively charged nucleophile atom on the old carbonyl carbon.
• “Chlorobenzene is not hydrolysed because the ring is bulky.” Repair Because a chlorine lone pair is delocalised into the ring, giving the C–Cl bond partial double-bond character, and the electron-rich ring repels nucleophiles.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Interleave with the chapters that use this one. Topic 34 turns the amides made in section G into amines and back into acids: re-answer “how is ethanamide made from ethanoic acid?” when you get there. Topic 35 joins dioyl chlorides to diols and diamines: re-answer “what is lost when an acyl chloride reacts with an alcohol?”. Topic 36 asks for multi-step routes: re-answer “how is an ester of a phenol made?”. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 33 is A Level content, so it is examined in Papers 4 and 5. A Level content: examined in Paper 4 (A Level structured, which also requires the AS content) and, as practical context, Paper 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• Topic 33 is A Level content, so it has no multiple-choice items: it is examined through structured questions in Paper 4. Expect to give reagents and conditions word for word (hot alkaline KMnO₄ then dilute acid; PCl₃ and heat), to write equations with every by-product, to describe or draw the addition–elimination mechanism with every curly arrow, to deduce a product from a reagent, and to explain an order of acidity or of ease of hydrolysis in terms of structure.
• The numbers this topic produces are percentage yields (Aᵣ values from the Data section Periodic Table), a ratio of Ka values from two pKa values the question supplies, and a manganate(VII)–ethanedioic acid titration using the 2 : 5 ratio. pKa values are not in the Data section: a question gives them. Calculating the pH of these acids belongs to topic 25.
• The manganate(VII)–ethanedioate titration is one of the syllabus’s named procedures, and it is the theory in 33.1.3(b). A Paper 5 question may ask you to plan it (warming, excess acid, self-indicating end-point, concordant titres), process its data, give percentage errors and suggest improvements, or evaluate an ester preparation for hazards, purification and a purity check by melting point.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.