AS Chemistry: Nitrogen Compounds — Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Nitrogen Compounds revision notes

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Section A

1. Ethanenitrile, CH₃CN, is heated under reflux with dilute sodium hydroxide, and the mixture is then acidified. Name the organic product present before acidification and the organic product obtained after acidification. [2]

Section B

2. Ethylamine can be prepared from bromoethane.

(a) Give the reagent and conditions for the reaction of bromoethane with ammonia, and name the mechanism. [3] (b) Explain why a mixture of products is obtained and how the yield of the primary amine can be improved. [3]

3. Bromoethane is converted into a nitrile, then into a carboxylic acid.

(a) Give the reagent and conditions for converting bromoethane into a nitrile, and name the mechanism. [3] (b) Give the reagent and conditions for hydrolysing the nitrile to a carboxylic acid. [2] (c) State how many carbon atoms the final carboxylic acid has, compared with the original bromoethane, and explain why. [2]

4. Ethanal (CH₃CHO) reacts with hydrogen cyanide to form 2-hydroxypropanenitrile.

(a) Name the type of mechanism involved. [1] (b) Explain why the product is obtained as a mixture containing equal amounts of both enantiomers (a racemic mixture — the term is formally introduced at A Level). [3]

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A Level extension

Syllabus outcome 19.1 is limited to one production reaction (a halogenoalkane with excess ammonia to give a primary amine) and states explicitly that classification of amines as primary, secondary or tertiary will not be tested at AS. The questions below — basicity ordering, reduction of a nitrile to an amine, phenylamine manufacture and salt formation — belong to A Level Topic 34 and are included here only as forward context; see the Amines: Aliphatic and Aromatic study guide for the full A Level treatment.

5. Rank ammonia, methylamine and phenylamine in order of increasing basicity and explain the order fully, referring to inductive effects and delocalisation. [5]

6. Give the reagent for reducing ethanenitrile to ethylamine and state one advantage of this route over the halogenoalkane-plus-ammonia route. [2]

7. Phenylamine is made industrially from nitrobenzene.

(a) Give the reagents and conditions. [2] (b) State one industrial use of phenylamine. [1]

8. Write the equation for the reaction of methylamine with hydrochloric acid and name the product. [2]

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Answers

1. Before acidification: sodium ethanoate, CH₃COONa (accept the ethanoate ion; ammonia is also released) [1]. After acidification: ethanoic acid, CH₃COOH [1].

2. (a) Excess concentrated ammonia in ethanol, heated in a sealed tube [1] [1]; nucleophilic substitution [1]. (b) The primary amine formed is itself a nucleophile and can attack more bromoethane [1], giving secondary and tertiary amines and finally a quaternary ammonium salt [1]. Using a large excess of ammonia makes it more likely that a bromoethane molecule meets ammonia rather than the amine, raising the yield of the primary amine [1].

3. (a) KCN dissolved in ethanol, heated under reflux [1] [1]; nucleophilic substitution [1]. (b) Dilute hydrochloric acid, heated under reflux [1], giving the carboxylic acid plus an ammonium salt [1]. (c) The carboxylic acid has one more carbon atom than bromoethane [1], because the CN⁻ ion itself supplies the extra carbon when it substitutes for the bromine [1]. (The other nitrile route, addition of hydrogen cyanide to an aldehyde or ketone, adds its extra carbon differently — by nucleophilic addition across the C=O bond, not substitution.)

4. (a) Nucleophilic addition [1]. (b) The carbonyl carbon is planar [1], so the CN⁻ nucleophile can attack with equal probability from either face of the flat carbon [1], producing equal amounts of both enantiomers — a racemic mixture, which has no net optical rotation (the rotation/optical-activity language is A Level; at AS, “equal amounts of both enantiomers form” is the required statement) [1].

5. Increasing basicity: phenylamine < ammonia < methylamine [1]. In methylamine the alkyl group is electron-releasing (positive inductive effect) [1], which increases the electron density on the nitrogen lone pair, making it more available to accept a proton [1]. In phenylamine the lone pair is delocalised into the benzene ring [1], so it is much less available, making phenylamine a far weaker base than ammonia [1].

6. Lithium tetrahydridoaluminate(III) in dry ether, or hydrogen with a nickel catalyst [1]. Advantage: it gives only the primary amine, with no further substitution, so the product is much easier to purify [1].

7. (a) Tin and concentrated hydrochloric acid, heated under reflux [1]; then excess sodium hydroxide to liberate the free amine from its salt [1]. (b) Manufacture of dyes [1]. (Also accept: pharmaceuticals, polymers.)

8. CH₃NH₂ + HCl → CH₃NH₃⁺Cl⁻ [1]; methylammonium chloride [1].

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Where marks are usually lost

• Stopping at the carboxylate salt after alkaline hydrolysis of a nitrile — acidification is needed to obtain the free carboxylic acid.
• (A Level extension questions) Saying amines are basic because they contain nitrogen, without mentioning the lone pair.
• Omitting “excess ammonia” as the way to favour the primary amine.
• Using aqueous rather than ethanolic KCN when preparing a nitrile from a halogenoalkane — aqueous conditions favour hydrolysis to the alcohol instead of substitution to the nitrile.
• Stating that a hydroxynitrile forms as a racemic mixture without explaining why, in terms of the planar carbonyl compound being attacked from either face with equal probability — a bare assertion earns fewer marks than the explanation.
• Forgetting that both nitrile preparation routes (from a halogenoalkane, or from a carbonyl compound) add exactly one carbon atom, which is the key reason nitriles appear in multi-step synthesis questions where the target has one more carbon than the given starting material.
• (A Level extension questions) Forgetting the second step (NaOH) in the phenylamine preparation; explaining phenylamine’s weakness by the inductive effect rather than delocalisation; trying to reduce a nitrile with NaBH₄ — it is not a powerful enough reducing agent.

For condensed recall notes on this topic, see the Nitrogen Compounds revision notes; for the full explanation with worked examples, see the Nitrogen Compounds: Amines and Nitriles study guide.