Organic synthesis

• Aqueous substitutes, ethanolic eliminates. Distil stops, reflux goes on. NaBH₄ for C=O, LiAlH₄ for C=O and –COOH, H₂/Ni for C=C. At AS, a carbon is added only through CN.
• Aqueous substitutes, ethanolic eliminates. Distil stops, reflux goes on. Count the carbons first; CN adds the one.

• 21.1.1 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
• 21.1.2 I can — devise multi-step synthetic routes for preparing organic molecules using the reactions in the syllabus
• 21.1.3 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

And the C=C? Neither NaBH₄ nor LiAlH₄ reduces a C=C; H₂(g) with a Pt/Ni catalyst and heat does. So in a molecule with both a C=C and a C=O, NaBH₄ changes only the C=O, and H₂/Ni is the syllabus's reagent for the C=C.

• “Bromoethane + NaOH gives ethanol.” Correct Only with NaOH(aq) and heat, which substitutes. NaOH in ethanol and heat eliminates HBr and gives ethene. Leaving out the solvent leaves out the product.
• “Ethanol is oxidised to ethanal by heating under reflux with acidified dichromate(VI).” Correct Distil for the aldehyde, so that it leaves the flask as it forms. Under reflux it runs back into the oxidising agent and goes on to ethanoic acid. A secondary alcohol gives a ketone either way, because a ketone is not oxidised further. The apparatus for each is drawn in Topics 16 and 17.
• “Reduce the carboxylic acid with NaBH₄” or “H₂/Ni reduces the ketone.” Correct NaBH₄ reduces aldehydes and ketones only. LiAlH₄ reduces those and carboxylic acids. Neither touches a C=C; H₂(g) with a Pt/Ni catalyst and heat is the syllabus's reagent for C=C.
• “Add a carbon by heating the alcohol with KCN.” Correct Cyanide substitutes a halogenoalkane (KCN in ethanol, heat), so convert the alcohol first; HCN (with KCN as catalyst, and heat) adds to a carbonyl. The carbon is added when the nitrile is made, not when it is hydrolysed.
• “A yellow precipitate with alkaline iodine means a ketone.” Correct It means CH₃CO– or CH₃CH(OH)–. Ethanol, propan-2-ol, ethanal and propanone all give it. 2,4-DNPH decides whether a C=O is present.
• “Propene + steam gives propan-1-ol.” Correct Markovnikov addition gives propan-2-ol as the major product. Propan-1-ol is the minor product, which makes it a by-product, not nothing.
• “Bromoethane + NaOH gives ethanol.” Repair With aqueous NaOH and heat. NaOH in ethanol with heat gives ethene.
• “Ethanol → ethanal by heating under reflux with acidified dichromate(VI).” Repair Distil for the aldehyde; reflux gives the acid.
• “Propene + steam → propan-1-ol.” Repair Markovnikov addition gives propan-2-ol as the major product; propan-1-ol is the minor one.
• “Add a carbon by reacting the alcohol with KCN.” Repair KCN in ethanol substitutes a halogenoalkane; convert the alcohol first (HX(g), PCl₅, SOCl₂ …). HCN with KCN catalyst adds to a carbonyl compound.
• “Reduce the acid to the alcohol with NaBH₄.” Repair Only LiAlH₄ reduces a carboxylic acid.
• “H₂/Ni reduces the ketone to the alcohol.” Repair H₂(g), Pt/Ni catalyst and heat is the syllabus's reagent for C=C. A C=O is reduced with NaBH₄ or LiAlH₄.
• A route that uses a reaction from Topics 29–35. Repair AS routes use the reactions of Topics 14–19 only. If you cannot reach the target from the reaction map, check the carbon count and where the group sits again before reaching for anything else.
• “The by-product of the substitution is the alcohol.” Repair The alcohol is the product. Identify the real by-products: NaBr from the substitution, and the alkene from the elimination that competes with it.
• “The carbon is added when the nitrile is hydrolysed.” Repair The carbon is added when the nitrile is made (KCN in ethanol, or HCN with KCN catalyst); hydrolysis keeps the count.
• A positive tri-iodomethane test recorded as “a ketone”. Repair It shows CH₃CO– or CH₃CH(OH)–; 2,4-DNPH separates the two.

• 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 22, return to the Paper 5-style item in Practical skills and add the instrumental evidence to the answer. Topic 36 repeats this chapter at A Level with the reactions of Topics 29–35 added; before starting it, redo the route studio here from memory. 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 21 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 Paper 1 item on this topic turns on one distinction: which reagent and condition gives the named product, which of four molecules gives a stated set of test results, how many steps a conversion needs, or which by-product a step makes. A Paper 2 question gives a starting material and a target and asks you to suggest a route with reagents and conditions, to identify the type of each step, to deduce a structure from test results, or to predict the products of a molecule with two groups.
• Topic 21 has no calculation of its own. Its numbers are carbon counts, which decide whether a nitrile or hydroxynitrile step is needed, and the overall yield of a route, which is the product of the step yields. A route question can carry a mass or percentage-yield calculation from Topic 2; the relative atomic masses come from the Data section.
• Topic 21 is the theory behind the Paper 3 organic observational question, in which unknowns are identified with the syllabus's functional-group tests and every observation is recorded in a grid drawn in advance. A Paper 5-style question can ask you to plan that sequence of tests or to evaluate whether a proposed test really shows that a synthesis worked.
• 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.