CCEA A Level Chemistry 5.7 Amines Exam Questions

About CCEA A Level Chemistry 5.7 Amines

Section 5.7 of Unit A2 2 starts the organic nitrogen chemistry. You need to name and classify amines, explain their boiling points and solubility using hydrogen bonding, and prepare primary aliphatic amines by reducing nitriles with lithal or by reacting ammonia with halogenoalkanes. CCEA also requires the preparation of phenylamine from nitrobenzene with tin and concentrated hydrochloric acid, salt formation with acids, and the relative basicity of ammonia, aliphatic amines and phenylamine. You must describe the reaction with ethanoyl chloride, used to identify amines, and the formation of benzenediazonium chloride and its coupling with phenol and phenylamine, linking colour of dyes and indicators to delocalisation.

What you need to know

• Classification and properties Primary amines have one alkyl or aryl group on N (RNH2), secondary two and tertiary three. Primary and secondary amines can hydrogen bond, so they have higher boiling points than alkanes of similar Mr, and short chain amines dissolve in water.
• Preparation Reduce a nitrile with LiAlH4: CH3CN + 4[H] gives CH3CH2NH2. React a halogenoalkane with excess ethanolic ammonia under pressure (further substitution gives a mixture). Reduce nitrobenzene with tin and concentrated HCl, then add NaOH to release phenylamine.
• Basicity Amines accept a proton using the lone pair on N. Aliphatic amines are stronger bases than ammonia because alkyl groups release electrons, making the lone pair more available. Phenylamine is weaker because the lone pair is delocalised into the ring.
• Salts Amines react with acids to form salts, such as CH3NH3+ Cl-; adding alkali releases the free amine.
• Ethanoylation Primary and secondary amines react with ethanoyl chloride to form N-substituted amides, solids whose melting points can identify the amine.
• Diazonium salts and dyes Phenylamine with sodium nitrite and HCl below 10 C forms benzenediazonium chloride. Coupling with phenol in alkaline solution gives a yellow-orange azo compound; coupling with phenylamine also gives an azo dye. Extended delocalisation through the -N=N- link makes azo compounds coloured.

Common mistakes in 5.7 Amines

• Saying phenylamine is a stronger base than ammonia. Delocalisation of the lone pair into the ring makes it weaker.
• Letting diazotisation get warm. Above about 10 C the diazonium salt decomposes, releasing nitrogen and forming phenol.
• Writing H2 instead of [H] for reductions. Use [H], for example C6H5NO2 + 6[H] gives C6H5NH2 + 2H2O.
• Forgetting the final alkali step in making phenylamine. The acidic mixture contains the phenylammonium salt, so add NaOH to release the amine.
• Saying tertiary amines react with ethanoyl chloride to form amides. Tertiary amines have no N-H, so they do not form amides.

Try three 5.7 Amines exam-style questions