Carboxylic acids and derivatives

• Reflux gives the acid, distillation gives the aldehyde. Acid hydrolysis gives the acid and stops at equilibrium; alkaline hydrolysis gives the salt and goes to completion.
• Count the protons: a +2 metal or a carbonate takes two molecules of acid; an alkali or a hydrogencarbonate takes one.
• Reflux gives the acid. Alkali gives the salt, and goes to completion. Alkyl first. LiAlH₄ only, with 4[H].

• 18.1.1 I can recall — recall the reactions by which carboxylic acids can be produced: (a) oxidation of primary alcohols and aldehydes with acidified K2Cr2O7 or acidified KMnO4 and refluxing (b) hydrolysis of nitriles with dilute acid or dilute alkali followed by acidification (c) hydrolysis of esters with dilute acid or dilute alkali and heat followed by acidification
• 18.1.2 I can describe — describe: (a) the redox reaction with reactive metals to produce a salt and H2(g) (b) the neutralisation reaction with alkalis to produce a salt and H2O(l) (c) the acid-base reaction with carbonates to produce a salt and H2O(l) and CO2(g) (d) esterification with alcohols with concentrated H2SO4 as catalyst (e) reduction by LiAlH4 to form a primary alcohol
• 18.2.1 I can recall — recall the reaction (reagents and conditions) by which esters can be produced: (a) the condensation reaction between an alcohol and a carboxylic acid with concentrated H₂SO₄ as catalyst
• 18.2.2 I can describe — describe the hydrolysis of esters by dilute acid and by dilute alkali and heat

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.

• “Oxidising ethanol with acidified dichromate gives ethanal.” Correct Reflux gives the acid; alkali gives the salt. Heated under reflux, the aldehyde cannot escape and is oxidised on to ethanoic acid. Ethanal is obtained only by distilling it off as it forms (topic 17).
• “Heating an ester or a nitrile with NaOH(aq) gives the carboxylic acid.” Correct In alkali the product is the carboxylate salt (sodium ethanoate, CH₃COONa). The acid appears only after the mixture is acidified with a dilute strong acid.
• “Acid hydrolysis and alkaline hydrolysis of an ester both go to completion.” Correct Dilute acid gives an equilibrium mixture of ester, water, acid and alcohol, because it is the reverse of esterification. Dilute alkali goes to completion, because the carboxylate ion it forms cannot react with the alcohol to re-form the ester.
• “NaBH₄ reduces ethanoic acid to ethanol.” Correct Only LiAlH₄ reduces a carboxylic acid, and it needs 4[H]: CH₃COOH + 4[H] → CH₃CH₂OH + H₂O. NaBH₄ reduces aldehydes and ketones but leaves –COOH untouched.
• “Ethanoic acid reacting with magnesium is a neutralisation.” Correct It is a redox reaction: magnesium goes from 0 to +2 and hydrogen from +1 to 0. Neutralisation is the reaction with an alkali; the carbonate reaction is an acid–base reaction that also releases CO₂.
• “CH₃COOCH₂CH₃ is ethanoyl ethanoate” or “HCOOCH₂CH₃ is methyl ethanoate.” Correct An ester is named alkyl first, from the alcohol, then the acid with -oate. Read the formula from the carbonyl carbon: CH₃CO– came from ethanoic acid and –OCH₂CH₃ from ethanol, so ethyl ethanoate; HCO– came from methanoic acid, so HCOOCH₂CH₃ is ethyl methanoate.
• “Oxidising ethanol with acidified dichromate(VI) gives ethanal.” Repair Under reflux the product is ethanoic acid. Ethanal is obtained only by distilling it off as it forms.
• “CH₃CH₂OH + [O] → CH₃COOH + H₂O.” Repair Count the oxygens: the alcohol to the acid needs 2[O]. CH₃CH₂OH + 2[O] → CH₃COOH + H₂O. One [O] takes an aldehyde to the acid.
• “Hydrolysing a nitrile with NaOH(aq) gives the carboxylic acid.” Repair It gives the carboxylate salt and ammonia; the acid appears only after acidification.
• “Propanenitrile hydrolyses to butanoic acid, because the CN adds a carbon.” Repair The CN carbon is already counted in the nitrile’s name. Propanenitrile, CH₃CH₂CN, has three carbons and gives propanoic acid. The chain grows when the nitrile is made from a halogenoalkane, not when it is hydrolysed.
• CH₃COOH + Mg → CH₃COOMg + H₂, with no state symbols. Repair Magnesium is +2, so two ethanoates and two acid molecules: 2CH₃COOH(aq) + Mg(s) → (CH₃COO)₂Mg(aq) + H₂(g). (aq) for the acid and salt, (s) for the metal, (g) for the gas, (l) for water.
• “Ethanoic acid + magnesium is a neutralisation.” Repair It is redox: Mg 0 → +2, H +1 → 0. Neutralisation is the reaction with an alkali.
• “25.0 cm³ of 0.100 mol dm⁻³ ethanoic acid with excess Na₂CO₃ gives 60.0 cm³ of CO₂.” Repair The ratio was taken as 1 : 1. With a carbonate, 2 mol acid give 1 mol CO₂: 2.50 × 10⁻³ / 2 × 24.0 dm³ = 30.0 cm³. 60.0 cm³ is the hydrogencarbonate answer.
• “NaBH₄ reduces ethanoic acid to ethanol.” Repair Only LiAlH₄ reduces a carboxylic acid: CH₃COOH + 4[H] → CH₃CH₂OH + H₂O.
• CH₃COOH + 2[H] → CH₃CH₂OH. Repair Unbalanced: the left has two oxygens, the right one. Four [H], and the equation must also produce a molecule of water.
• “CH₃COOCH₂CH₃ is ethanoyl ethanoate”, or “HCOOCH₂CH₃ is methyl ethanoate.” Repair Alkyl from the alcohol first, then the acid with -oate: ethyl ethanoate. In HCOOCH₂CH₃ the acid part HCO– is from methanoic acid and the alkyl part is ethyl: ethyl methanoate.
• “Esterification with concentrated H₂SO₄ goes to completion.” Repair It is reversible; the concentrated acid is a catalyst and the yield is limited by the equilibrium.
• “Acid hydrolysis of an ester gives a salt”, or “alkaline hydrolysis is reversible too.” Repair Dilute acid gives the carboxylic acid and the alcohol, reversibly. Dilute alkali gives the salt and the alcohol and goes to completion, because the carboxylate ion does not react with the alcohol.

• 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. When you reach topic 19, re-answer the nitrile hydrolysis questions; in topic 21, use the reaction table to plan a two-step route from a halogenoalkane to a carboxylic acid; in topic 22, predict the infrared absorptions of ethanoic acid and of ethyl ethanoate; in topic 33 at A Level, come back to this chapter before meeting the rest of the carboxylic acid family. 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 18 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 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.
• A multiple-choice item on this topic can turn on one word of a condition: reflux or distil, dilute acid or dilute alkali, LiAlH₄ or NaBH₄, or on which product a reaction gives (the acid or the salt). A structured question asks you to give reagents and conditions and balanced equations with state symbols or with [O] and [H], identify an ester from its acid and alcohol or the reverse, and explain why alkaline hydrolysis goes to completion.
• The calculations are stoichiometry: a volume of CO₂ from a carbonate reaction (2 mol acid to 1 mol CO₂ with a carbonate, 1 : 1 with a hydrogencarbonate), a percentage yield of an ester, and the concentration of an acid from a titration. The Data section supplies Vm = 24.0 dm³ mol⁻¹ at room conditions and the relative atomic masses; you work out every Mr yourself.
• Topic 18 is the theory behind the weak acid–strong alkali titration: burette readings to 0.05 cm³, titres concordant within 0.10 cm³, thymolphthalein or thymol blue as the indicator, and the percentage error of a titre. It also supplies the carbonate test for an acid in an observation exercise, and planning or evaluation contexts such as following the hydrolysis of an ester by titration.
• 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.