Week 6 © Pearson Education Ltd 2009 This document may have been altered from the original Describe the preparation of aliphatic amines by substitution of halogenoalkanes. Describe the preparation of aromatic amines by reduction of nitroarenes. Describe the synthesis of an azo dye by diazotisation and coupling. State the use of these reactions in the formation of dyestuffs.

Week 6 © Pearson Education Ltd 2009 This document may have been altered from the original Reduction of nitrobenzene to make phenylamine (aniline)

Aliphatic amines Made by nucleophilic substitution o the halogen in a halogenoalkane by ammonia. Halogenoalkane is a liquid and ammonia is a gas so a co-solvent, ethanol is used. Heat is required and so since ammonia is a gas and will be less available to react at high temperature the reaction is carried out in a sealed tube to increase pressure.

Equation Preparation of Ethylamine C 2 H 5 Br + NH 3 → C 2 H 5 NH 2 +HBr What side reactions may take place? How would adding excess ammonia help to reduce the problem? Excess ammonia added: HBr + NH 3 → NH 4 Br

Role of Excess Ammonia In the presence of excess ammonia it is less likely that long chain ethylamine will be able to get to the bromoethane to react with the δ+ carbon due to steric hindrance. Ammonia is a smaller molecule and less likely to get tangled up in the chains.

Side reactions Ethyamine is a stronger base than ammonia with a higher electron density on the nitrogen loan pair. This means that the newly prepared amine is more/less likely to attack the halogenoalkane than the ammonia. The result is that di and triethylamine can be formed as unwanted products of the reaction.

Aromatic amines Aromatic amines cannot be made by the same reaction as is used to make aliphatic amines. Arenes undergo electrophilic attack and nitrogen lone pairs are repelled by the ring electrons. In halogenoarenes the halogen lone pair electrons are delocalised towards the ring.

This increases the electron density in the C- Hal bond, strengthening it. As a result direct nucleophilic substitution of the halogen by ammonia is unlikely. Instead nitrobenzene is REDUCED using metal and acid. The reducing agent is hydrogen generated by heating xs conc. hydrochloric acid under reflux with metallic tin. It removes the oxygen from the NO 2 group and replaces them with hydrogen atoms.

Week 6 © Pearson Education Ltd 2009 This document may have been altered from the original Reduction of nitrobenzene to make phenylamine (aniline)

Azo Dyes Until 1856 fabrics were generally dull in colour because the only dyes generally available were derived from natural materials like woad or indigo. Synthetic dyes were discovered by accident by William Perkin. A major raw material used in the production of synthetic dyes is phenylamine.

Azo Dyes Azo dyes made from aromatic amines are generally brightly coloured and bond well to fabrics leading to colour fastness and resistance to fading. These dyes are called azo dyes because they contain the azo functional group -N=N- which forms part of an extensive delocalised system. This delocalised system is called a chromophore – the part of a molecule which causes colour.

Synthesis of an Azo Dye This is a 2 stage reaction which starts with an aryl amine. 1. Reaction with nitrous acid. Coupling with phenol. 1. Reaction with nitrous acid. This is unstable so is formed in situ below 5 o C. NaNO 2(aq) + HCl (aq) → NaCl + HNO 2 (aq)

Azo Dyes Nitrous acid reacts with amines to produce an ionic salt in aqueous solution. R-NH 2 +HNO 2 +HCl → R +2H 2 O The ion formed is called a diazonium ion. If the amine is phenylamine the ionic compound formed is called benzene diazonium chloride. The reaction is called diazotisation.

Week 6 © Pearson Education Ltd 2009 This document may have been altered from the original Diazotisation reaction of phenylamine

Azo Dyes The aromatic ring stabilises the diazonium ion by delocalising its electrons. Even so, above 10 o C, the ion decomposes and nitrogen gas and phenol are formed. C 6 H 5 N H 2 O → C 6 H 5 OH + N 2 + H + If the amine was an aliphatic amine e.g. ethylamine the addition of nitrous acid causes immediate evolution of nitrogen gas and a mixture of other products e.g. ethanol, ethene are formed.

Reactions of Phenylamine Like phenol the ring is activated by partial delocalisation of the nitrogen lone pair round the ring. This reduces the base strength of the amine but makes the ring more susceptible to electrophilic attack. Like phenol, it reacts readily with aqueous bromine to give 2,4,6- tribromophenylamine, a white precipitate.

Coupling Reactions The positive charge on the N 2 + group of the benzenediazonium ion means that this group is a strong electrophile. It attacks other benzene rings which are activated e.g. phenol. E.g. C 6 H 5 N C 6 H 5 OH → C 6 H 5 N 2 C 6 H 5 OH +H + (4-hydroxyphenyl)azobenzene This is a coupling reaction. If a cold solution of benzenediazonium chloride is added to a cold solution of phenol in sodium hydroxide a bright orange precipitate is formed.

Azo Compounds (4hydroxyphenyl)azobenzene is an example of an azo compound. All such compounds are brightly coloured. (see chromophores earlier. Unlike diazonium compounds azo compounds are stable and so do not lose their colour.

Week 6 © Pearson Education Ltd 2009 This document may have been altered from the original Reaction between benzenediazonium chloride and phenol