Nitrogen compounds (A Level)

• Free lone pair: base. Lone pair shared with a ring: weak base. Lone pair shared with C=O: barely a base. Diazotise below 10 °C, couple cold, in NaOH(aq).
• Where is the lone pair? Count the carbons. Below 10 °C, then cold in NaOH(aq). Below the isoelectric point: positive, to the negative electrode.

• 34.1.1 I can recall — recall the reactions (reagents and conditions) by which primary and secondary amines are produced: (a) reaction of halogenoalkanes with NH₃ in ethanol heated under pressure (b) reaction of halogenoalkanes with primary amines in ethanol, heated in a sealed tube/under pressure (c) the reduction of amides with LiAlH₄ (d) the reduction of nitriles with LiAlH₄ or H₂/Ni
• 34.1.2 I can describe — describe the condensation reaction of ammonia or an amine with an acyl chloride at room temperature to give an amide
• 34.1.3 I can describe — describe and explain the basicity of aqueous solutions of amines
• 34.2.1 I can describe — describe the preparation of phenylamine via the nitration of benzene to form nitrobenzene followed by reduction with hot Sn/concentrated HCl followed by NaOH(aq)
• 34.2.2 I can describe — describe: (a) the reaction of phenylamine with Br₂(aq) at room temperature (b) the reaction of phenylamine with HNO₂ or NaNO₂ and dilute acid below 10 °C to produce the diazonium salt; further warming of the diazonium salt with H₂O to give phenol
• 34.2.3 I can describe — describe and explain the relative basicities of aqueous ammonia, ethylamine and phenylamine
• 34.2.4 I can recall — recall the following about azo compounds: (a) describe the coupling of benzenediazonium chloride with phenol in NaOH(aq) to form an azo compound (b) identify the azo group (c) state that azo compounds are often used as dyes (d) that other azo dyes can be formed via a similar route
• 34.3.1 I can recall — recall the reactions (reagents and conditions) by which amides are produced: (a) the reaction between ammonia and an acyl chloride at room temperature (b) the reaction between a primary amine and an acyl chloride at room temperature
• 34.3.2 I can describe — describe the reactions of amides: (a) hydrolysis with aqueous alkali or aqueous acid (b) the reduction of the C=O group in amides with LiAlH₄ to form an amine
• 34.3.3 I can state — state and explain why amides are much weaker bases than amines
• 34.4.1 I can describe — describe the acid/base properties of amino acids and the formation of zwitterions, to include the isoelectric point
• 34.4.2 I can describe — describe the formation of amide (peptide) bonds between amino acids to give di- and tripeptides
• 34.4.3 I can interpret — interpret and predict the results of electrophoresis on mixtures of amino acids and dipeptides at varying pHs (the assembling of the apparatus will not be tested)

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.

• “Diazotise phenylamine with NaNO₂ and HCl, then add phenol.” Correct Diazotisation is below 10 °C, with NaNO₂ and dilute acid; the coupling is done cold, with the phenol dissolved in NaOH(aq). Above 10 °C the diazonium salt gives phenol and N₂ instead of a dye. The temperature and the alkali are both part of the answer.
• “Butan-2-amine is a secondary amine.” Correct Primary, secondary and tertiary count the carbon atoms bonded to nitrogen, not where the nitrogen sits on the chain. Butan-2-amine has one carbon on N: it is primary. This is the opposite of the alcohol convention.
• “To make propylamine from 1-bromopropane, use KCN and then reduce.” Correct The nitrile route adds a carbon and gives butylamine. Count the carbons first: same chain, NH₃ in ethanol under pressure; one carbon longer, KCN then LiAlH₄ or H₂/Ni.
• “Sn and concentrated HCl reduce nitrobenzene to phenylamine.” Correct In that acid the amine is formed as phenylammonium chloride. The preparation has three parts, and NaOH(aq) is the third: it releases the free amine.
• “Phenylamine is a weaker base because the benzene ring withdraws electrons by the inductive effect.” Correct The nitrogen lone pair is delocalised into the ring’s π system, so it is less available to accept a proton. Name delocalisation for phenylamine and the electron-donating (positive inductive) effect for ethylamine.
• “Acid hydrolysis of an amide gives the carboxylic acid and ammonia.” Correct In acid the ammonia is protonated: the products are the carboxylic acid and the ammonium salt. In alkali they are the carboxylate salt and ammonia.
• “At its isoelectric point an amino acid is uncharged, H₂N–CH(R)–COOH.” Correct It is the zwitterion, ⁺H₃N–CH(R)–COO⁻: two charges, no net charge. Below the isoelectric point it is a cation and moves to the negative electrode; above it, an anion moving to the positive electrode.
• “Butan-2-amine is a secondary amine.” Repair Primary: one carbon is bonded to the nitrogen. Primary, secondary and tertiary count the carbons on N, not the position on the chain.
• “KCN, then LiAlH₄, to make propylamine from 1-bromopropane.” Repair The nitrile route adds a carbon and gives butylamine. Propylamine needs excess NH₃ in ethanol, heated under pressure.
• “Excess ammonia is used to speed the reaction up.” Repair It stops the primary amine reacting further to give secondary and tertiary amines and the quaternary ammonium salt.
• “RNH₂ + H₂O → RNH₃⁺ + OH⁻” Repair Amines are weak bases: RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻. The reversible arrow is part of the answer.
• “Trimethylamine reacts with ethanoyl chloride to give an amide.” Repair A tertiary amine has no N–H to lose as HCl, so no amide forms.
• “Sn/HCl reduces nitrobenzene to phenylamine.” (and nothing more) Repair In the acid the product is phenylammonium chloride; NaOH(aq) is needed to liberate phenylamine: C₆H₅NH₃⁺Cl⁻ + NaOH → C₆H₅NH₂ + NaCl + H₂O.
• “Phenylamine and bromine water need an AlBr₃ catalyst.” Repair No catalyst: the activated ring reacts at room temperature, giving a white precipitate of 2,4,6-tribromophenylamine and decolourising the bromine water.
• “Diazotisation: NaNO₂ at 25 °C.” Repair Below 10 °C, with dilute acid. Above 10 °C the diazonium ion decomposes to phenol and nitrogen.
• “Phenylamine is a weaker base because the ring is electron-withdrawing by the inductive effect.” Repair Because the nitrogen lone pair is delocalised into the ring’s π system and so is less available to accept a proton.
• “Couple the diazonium salt with phenol in acid.” Repair With phenol in NaOH(aq), cold: the phenoxide is the activated ring and the alkali neutralises the HCl.
• “The azo group is –N–N–” or “–N≡N–”. Repair –N=N–: a double bond between two nitrogens, with an aromatic ring on each side. N≡N belongs to the diazonium ion, which carries the + charge.
• “Amides are bases because nitrogen has a lone pair.” Repair The lone pair is delocalised onto the carbonyl oxygen and is not available; amides are much weaker bases than amines.
• “Acid hydrolysis of ethanamide gives ethanoic acid and ammonia.” Repair In acid the ammonia is protonated: ethanoic acid and the ammonium salt. Alkaline hydrolysis gives the carboxylate salt and ammonia.
• “LiAlH₄ reduces ethanamide to ethanol.” Repair The amide’s C=O becomes CH₂ and the nitrogen is kept: the product is ethylamine. It is the carboxylic acid that LiAlH₄ reduces to an alcohol.
• “At the isoelectric point the amino acid is uncharged, H₂N–CHR–COOH.” Repair It is the zwitterion, ⁺H₃N–CHR–COO⁻: two charges, no net charge.
• “Below its isoelectric point an amino acid moves to the positive electrode.” Repair Below it the amino acid is a cation and moves to the negative electrode.
• “Glycine and alanine form one dipeptide.” Repair Two: Gly–Ala and Ala–Gly, depending on which amino acid supplies the –COOH.

• 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 35 turns the peptide bond and the amide link into polymers: re-answer “draw the dipeptide and name the bond” there. Topic 36 plans syntheses: re-answer “make an amine one carbon longer”. Topic 37 reads N–H protons in NMR: re-answer “which protons exchange with D₂O?”. 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 34 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 34 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 exactly as the syllabus words them (NH₃ in ethanol, heated under pressure; hot Sn and concentrated HCl, then NaOH(aq); NaNO₂ and dilute acid below 10 °C), to write equations with every by-product, to plan a multi-step route from benzene or from a halogenoalkane, to draw a zwitterion, a dipeptide or an azo compound, and to explain an order of basicity in terms of the nitrogen lone pair.
• The numbers this topic produces are percentage yields over one or more steps and the Mᵣ of a peptide (Aᵣ values from the Data section Periodic Table, with one water lost per peptide bond), and the concentration of an amine from a titration with standard hydrochloric acid. Isoelectric points are not in the Data section: a question supplies them. Basicity is never calculated in this topic.
• Titrating an aqueous amine with hydrochloric acid is a weak base–strong acid titration from the syllabus’s list, and topic 34 is its theory: a Paper 5 question may ask you to plan it, choose the indicator, process concordant titres and give percentage errors. An electrophoresis strip is a natural data item: read the direction and distance of each spot and deduce charge, isoelectric point and relative size.
• 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.