Nitrogen compounds

• Ethanol, not water. Two NH₃, one NH₄Br. The nitrile carbon is a new carbon, and hydrolysis keeps it.
• Ethanol, not water. Two NH₃, one NH₄Br. Cyanide adds a carbon; hydrolysis keeps it; alkali needs acidification.

• 19.1.1 I can recall — recall the reactions by which amines can be produced: (a) reaction of a halogenoalkane with NH₃ in ethanol heated under pressure (Classification of amines will not be tested at AS Level.)
• 19.2.1 I can recall — recall the reactions by which nitriles can be produced: (a) reaction of a halogenoalkane with KCN in ethanol and heat
• 19.2.2 I can recall — recall the reactions by which hydroxynitriles can be produced: (a) the reaction of aldehydes and ketones with HCN, KCN as catalyst, and heat
• 19.2.3 I can describe — describe the hydrolysis of nitriles with dilute acid or dilute alkali followed by acidification to produce a carboxylic acid

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.

• “Heat the halogenoalkane with aqueous KCN” or “… with aqueous ammonia.” Correct Ethanol, not water, for KCN and for NH₃. In water the halogenoalkane is also hydrolysed, by OH⁻ and by water itself, and the alcohol forms. The solvent is part of the reagent: “KCN in ethanol, heat” and “NH₃ in ethanol, heated under pressure”.
• “CH₃CH₂Br + NH₃ → CH₃CH₂NH₂ + HBr.” Correct HBr cannot survive in excess ammonia: a second NH₃ takes the proton. CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br.
• “CH₃CH₂CN is ethanenitrile, because it is made from bromoethane.” Correct Count the carbon of the C≡N group: three carbons, propanenitrile. The nitrile carbon is carbon 1 of the name.
• “Hydrolysing the nitrile with NaOH(aq) gives the carboxylic acid.” Correct Alkali gives the carboxylate salt and ammonia. The acid is obtained only after acidification with a dilute strong acid, which is a separate step with its own equation.
• “Hydrolysis adds the extra carbon.” Correct The carbon was added when the nitrile was made. Hydrolysis turns C≡N into COOH on the same carbon and leaves the count unchanged.
• “NaOH, heat” with no solvent, as a reagent for a halogenoalkane. Correct NaOH(aq) gives the alcohol by substitution; NaOH in ethanol gives the alkene by elimination. Without the solvent the answer does not say which reaction it means.
• “Bromoethane + ammonia gives ethylamine and HBr.” Repair The HBr is taken up by a second ammonia molecule: the other product is NH₄Br, and two NH₃ appear in the equation.
• “Use aqueous ammonia.” Repair Ammonia in ethanol, heated under pressure. Water would compete as a nucleophile and give the alcohol by hydrolysis.
• “Excess ammonia is used to speed the reaction up.” Repair It is used so that the primary amine is the main product, because the amine formed is itself a nucleophile and can attack more halogenoalkane.
• “The tube is sealed to keep the halogenoalkane in.” Repair It is sealed because ammonia is a gas: under pressure it stays dissolved at the temperature of the reaction.
• “CH₃CH₂CN is ethanenitrile.” Repair Count the nitrile carbon: three carbons, propanenitrile.
• “KCN(aq), heat.” Repair KCN in ethanol, heat. In water the halogenoalkane is also hydrolysed to the alcohol.
• A curly arrow drawn from the nitrogen of CN⁻ to the carbon. Repair The lone pair used is on the carbon of :CN⁻, which is the atom that bonds. The arrow starts at that lone pair.
• “HCN adds to a halogenoalkane.” Repair HCN with KCN catalyst adds to aldehydes and ketones; a halogenoalkane is substituted by KCN in ethanol.
• “Propanone and HCN give 2-hydroxypropanenitrile.” Repair That is the product from ethanal. Propanone gives (CH₃)₂C(OH)CN, 2-hydroxy-2-methylpropanenitrile: the second methyl group is a branch on carbon 2.
• “Hydrolysing a nitrile with NaOH(aq) gives the acid.” Repair It gives the carboxylate salt and ammonia; the acid needs acidification afterwards.
• “Nitrile hydrolysis gives an amine.” Repair Hydrolysis gives a carboxylic acid; the nitrogen leaves as NH₄⁺ (with acid) or NH₃ (with alkali).
• “Hydrolysis of the nitrile adds a carbon.” Repair The carbon was added when the nitrile was made; hydrolysis keeps the count.

• 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.
• Where the first arrow starts. It starts at the lone pair on the carbon of CN⁻, not at the nitrogen, not at the letter C and not at the minus sign, and it ends at the δ+ carbon. The second starts at the middle of the C–Br bond and ends at Br. Draw the lone pair and the dipole first; they are part of the mechanism.
• Choose the route from the target. A target carboxylic acid with an OH on carbon 2 comes from an aldehyde or ketone by route 2. A target acid with no OH next to the carboxyl group comes from a halogenoalkane by route 1. Either way, give the reagent, the solvent and the condition for each step, and the carbon count as a check. Topic 21 builds longer routes out of exactly these steps.
• Interleave with the chapters that use this one. Topic 21 builds synthetic routes from the nitrile step: re-answer the chain-extension drill there. Topic 22 identifies nitriles and amines by infrared: re-answer which absorption shows that step 1 of a route has worked. Topic 34, at A Level, takes amines much further: re-answer the reasons for the amine conditions before you start it. 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 19 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 or one carbon of a name: which reagent in which solvent gives the nitrile rather than the alcohol, how many carbons the product of a two-step route contains, which name belongs to CH₃CH(OH)CN. A structured question asks you to give reagents and conditions, write balanced equations with the right by-product (NH₄Br, KBr, NH₄Cl), explain a condition such as the excess of ammonia, describe a mechanism with curly arrows, or suggest a two-step synthesis.
• The calculations are mole ratios through a route, one mole of starting material to one mole of product at each step, with an overall percentage yield, and the Mᵣ values built from the Aᵣ values in the Data section (C 12.0, H 1.0, N 14.0, O 16.0, Br 79.9). The electronegativities that make C–Br polar (C 2.5, Br 2.6) and the infrared ranges for C≡N and N–H used in topic 22 are Data-section values too: you are given them, not asked to recall them.
• No Paper 3 procedure uses cyanide, and the syllabus does not list it among the practical materials. What this topic supplies is heating under reflux and the handling of toxic, volatile organic reagents. A Paper 5 planning question can use nitrile hydrolysis as its context: how the extent of hydrolysis is measured by titration against a control, what is kept constant, which error is systematic, and what the risks are. The practical-skills section sets that plan out in full.
• 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.