CIE A Level Chemistry Nitrogen Compounds Exam Questions

About CIE A Level Chemistry Nitrogen Compounds

Nitrogen compounds in Cambridge International 9701 begins at AS with the formation of amines from halogenoalkanes and nitriles. The A Level part is substantial: you compare the basicity of ammonia, ethylamine and phenylamine, describe the preparation of phenylamine from nitrobenzene, and study its reaction with bromine water and with nitrous acid to form a diazonium salt that couples with phenol to make an azo dye. You also cover amides, their formation and hydrolysis, and amino acids as zwitterions, peptides and electrophoresis. This topic links strongly with carboxylic acids, polymerisation and synthesis. Examiners often ask for explanations of base strength and the conditions for diazotisation.

What you need to know

• Making amines Primary amines form from halogenoalkanes with excess ethanolic ammonia, heated in a sealed tube, or by reducing nitriles with LiAlH4 or H2 with a Ni catalyst.
• Basicity (A Level) Ethylamine is a stronger base than ammonia because the alkyl group donates electrons to the nitrogen lone pair. Phenylamine is weaker because the lone pair is delocalised into the benzene ring.
• Phenylamine (A Level) Made by reducing nitrobenzene with tin and concentrated HCl, heated under reflux, followed by NaOH. It reacts with bromine water to give a white precipitate of 2,4,6-tribromophenylamine.
• Diazonium salts and azo dyes (A Level) Phenylamine reacts with HNO2 (NaNO2 and HCl) below 10 °C to form benzenediazonium chloride. This couples with phenol in alkaline solution to give a coloured azo compound containing -N=N-.
• Amides (A Level) Formed from acyl chlorides and ammonia or amines. They are hydrolysed by heating with aqueous acid or alkali and reduced to amines by LiAlH4. Amides are almost neutral because the nitrogen lone pair is delocalised over the C=O.
• Amino acids and peptides (A Level) Amino acids exist as zwitterions, which explains their high melting points and solubility. They form peptides by condensation, linked through the -CONH- peptide bond.
• Electrophoresis (A Level) Amino acids and peptides are separated by charge and size in a buffered gel with an applied voltage. Their charge depends on the pH relative to the isoelectric point.

Common mistakes in Nitrogen Compounds

• Saying phenylamine is a stronger base than ammonia because of the benzene ring; the delocalised lone pair makes it weaker.
• Forgetting the temperature condition for diazotisation; it must be below 10 °C because the diazonium salt decomposes above this.
• Using ammonia that is not in excess when making primary amines, which leads to secondary and tertiary amines.
• Calling amides basic like amines; they are close to neutral.
• Drawing the peptide bond with water still attached or with the wrong atoms; it is -CO-NH- with water eliminated.

Try three Nitrogen Compounds exam-style questions