AS Chemistry: Nitrogen Compounds — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — 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. Explain why amines are basic, referring to the nitrogen lone pair. [2]

Section B

2. Ethylamine can be prepared from bromoethane or from ethanenitrile, two routes worth comparing directly.

(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. The nitrogen atom has a lone pair of electrons [1] which can accept a proton, forming a dative covalent bond [1] — the same reasoning that explains ammonia’s own basicity.

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

• 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.