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.