Organic synthesis (A Level)

• KCN adds a carbon, and a group stays on the carbon it started on. Everything else is choosing the right line of the table, in the right order.
• KCN adds a carbon, and a group stays on the carbon it started on. Distil for the aldehyde, reflux for the acid. Below 10 °C for the diazonium salt. Yields multiply.

• 36.1.1 I can — for an organic molecule containing several functional groups: (a) identify organic functional groups using the reactions in the syllabus (b) predict properties and reactions
• 36.1.2 I can — devise multi-step synthetic routes for preparing organic molecules using the reactions in the syllabus
• 36.1.3 I can — analyse a given synthetic route in terms of type of reaction and reagents used for each step of it, and possible by-products

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.

• “2-Bromopropane, then NaOH(aq), then oxidation under reflux, gives propanoic acid.” Correct KCN adds a carbon, and a group stays on the carbon it started on. Substitution, oxidation, reduction and hydrolysis all leave a group where it was: 2-bromopropane gives propan-2-ol, which oxidises to propanone, never to propanoic acid. The only steps that add a carbon to a chain are the nitrile steps (KCN on a halogenoalkane, HCN on a carbonyl), and each adds exactly one.
• “Phenol is an acid, so it effervesces with sodium carbonate.” Correct Phenol is too weak an acid to release CO₂ from a carbonate: no effervescence. It dissolves in NaOH(aq). Only the carboxylic acid effervesces, which is exactly why Na₂CO₃(aq) separates the two.
• “Reduce nitrobenzene with LiAlH₄.” Correct The syllabus reduction of a nitro group is Sn and concentrated HCl, heat, then NaOH(aq). LiAlH₄ appears in the syllabus for aldehydes, ketones, carboxylic acids, amides and nitriles, and nowhere for a nitro group.
• “Nitrate benzoic acid to make 4-nitrobenzoic acid.” Correct –COOH directs to the 3-position. The position of each substituent is fixed by whatever is on the ring when it arrives, so the order of steps is part of the answer.
• “Diazotise phenylamine at room temperature.” Correct NaNO₂ and dilute HCl below 10 °C. Above it the diazonium salt decomposes to phenol and nitrogen before it can couple.
• “Acidified dichromate(VI), reflux, turns a primary alcohol into an aldehyde.” Correct Distil for the aldehyde; reflux for the carboxylic acid. The same goes for NaOH: (aq), heat, gives the alcohol; in ethanol, heat, gives the alkene. The condition is part of the reagent.
• “Positive Fehling’s test, so it is an aldehyde.” Correct Methanoic acid also reduces Fehling’s and Tollens’ reagents (33.1.3). A negative 2,4-DNPH result, or effervescence with Na₂CO₃(aq), tells them apart.
• “Three steps at 80%, 70% and 60% give an overall yield of 70%.” Correct Yields multiply: 0.80 × 0.70 × 0.60 = 0.336, so 33.6%. Every extra step costs yield, which is why the shorter route is preferred.
• “Phenol effervesces with sodium carbonate because it is an acid.” Repair Phenol is too weak an acid to release CO₂ from a carbonate; only a carboxylic acid effervesces. Phenol does dissolve in NaOH(aq).
• “2-Bromopropane, then NaOH(aq), then oxidation under reflux, gives propanoic acid.” Repair The group stays on carbon 2: NaOH(aq) gives propan-2-ol, which oxidises to propanone. Propanoic acid needs –COOH on an end carbon: from bromoethane with KCN, then hydrolysis.
• “Ethanol → chloroethane → KCN → hydrolysis makes ethanoic acid.” Repair KCN adds a carbon, so that route gives propanoic acid. Ethanoic acid from ethanol is one step: acidified K₂Cr₂O₇, reflux.
• “Reduce nitrobenzene with LiAlH₄.” Repair The syllabus reduction of a nitro group is Sn and concentrated HCl, heat, then NaOH(aq). LiAlH₄ is for aldehydes, ketones, carboxylic acids, amides and nitriles.
• “Nitrate benzoic acid to make 4-nitrobenzoic acid.” Repair –COOH directs to the 3-position. The 4-isomer is made by nitrating methylbenzene and oxidising the side-chain afterwards.
• “Diazotise phenylamine at room temperature.” Repair Below 10 °C; above it the diazonium salt decomposes to phenol and nitrogen before it can couple.
• “Aldehyde from a primary alcohol: acidified dichromate(VI), reflux.” Repair Distil the aldehyde off as it forms; reflux carries the oxidation on to the carboxylic acid.
• “NaOH converts bromoethane to ethanol.” Repair NaOH(aq), heat, gives the alcohol; NaOH in ethanol, heat, gives the alkene. The solvent is part of the answer.
• “The other product of esterification is HCl.” Repair From an acid and an alcohol the other product is water; HCl is the other product when an acyl chloride is used.
• “Bromine water is decolourised by phenol, so phenol has a C=C.” Repair The ring is substituted, not added to. The sign of phenol (or phenylamine) is the white precipitate as well as the loss of colour.
• “Fehling’s is positive, so it must be an aldehyde.” Repair Methanoic acid also reduces Fehling’s and Tollens’ reagents (33.1.3). A negative 2,4-DNPH result, or effervescence with a carbonate, tells them apart.
• “The overall yield is the average of the step yields.” Repair The fractional yields multiply: 80%, 70% and 60% give 33.6%, not 70%.

• Score yourself: eight or nine correct and you are ready. Five or fewer and the gap is in the reactions, not in synthesis — this chapter can only combine what you already know.
• 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.
• Write the observation, then the inference. “Orange to green” is the observation; “a primary or secondary alcohol, or an aldehyde, is present” is the inference. An answer that gives only the inference has not reported what was seen, and an answer that says “goes green” without the starting colour has not reported a change.
• The four pairs told apart by the condition alone: distil (aldehyde, ketone) against reflux (carboxylic acid); NaOH(aq) (alcohol) against NaOH in ethanol (alkene); KCN in ethanol (nitrile) against aqueous conditions (alcohol); and NH₃ in ethanol, heated under pressure for the amine. Write the solvent and the heating method every time.
• Interleave with the chapters that use this one. Topic 36 is the whole of the organic syllabus used at once, so every organic chapter feeds it. When you reach topic 37, take one route from this chapter and say how you would show by chromatography, infrared or NMR that its product is the target. When you revise topics 30–35, add each new reaction to the right map. 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 36 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.
• There is no multiple-choice paper on A Level content. In Paper 4 a structured question gives you test results to deduce a structure from, a molecule with two groups to predict the products for, a start and a target to devise a route between, or a route to analyse. The reagent, the condition and the type of each step are separate answers, and “reflux” or “distil”, “(aq)” or “in ethanol”, “below 10 °C” are part of the reagent answer, not decoration.
• The arithmetic is small: the carbon count at every stage of a route, and, from AS topic 2 (2.4.1), a percentage yield from moles using relative atomic masses from the Data section, extended here to the overall yield of a route as the product of the step yields. Nothing else in the topic needs a number.
• Topic 36 has no quantitative procedure of its own. Paper 5 is written, with no laboratory work, so here the topic is the context for planning how to identify an organic unknown with the tests you carried out at AS in Paper 3 (Fehling’s, Tollens’, alkaline aqueous iodine and acidified manganate(VII), recorded in the syllabus’s words), and for evaluating the yield of a preparation. Both are set out in Practical skills.
• 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.