A - Amines Flashcards
What are amines?
Compounds based on ammonia where one or more hydrogen atoms have been replaced by alkyl or aryl groups.
What are primary, secondary and tertiary amines?
- Primary amine - where one hydrogen on ammonia is replaced by an organic group.
- Secondary amine - where two hydrogen atoms are replaced with organic groups.
- Tertiary amine - where three hydrogen atoms are replaced with organic groups.
It refers to the number of R-groups on the nitrogen atom.
How can you get a quaternary ammonium ion?
The lone pair of electrons on the nitrogen atom of a tertiary amine can bond to a fourth organic group, giving a quaternary ammonium ion.
What is an aromatic amine?
Where a hydrogen atom on ammonia is replaced with a benzene ring.
How are amines named?
Using the suffix, -amine.
In what two ways can primary aliphatic amines be prepared?
- By the reaction of ammonia with halogenoalkanes.
2. By the reduction of nitriles.
What is an aliphatic compound?
A non-aromatic compound.
Outline what happens when you heat a halogenoalkane with excess ammonia. (The reaction mechanism).
- Ammonia attacks the carbon in the halogenoalkane, and the electrons are transferred to the halogen.
- The halogen ion is released.
- This forms an alkylammonium salt.
- A second ammonia molecule donates its lone pair of electrons to a hydrogen bonded to the nitrogen on the alkylammonium salt, this hydrogen then breaks off.
- The products of this are a primary amine and an ammonium salt.
These mechanisms continue as amines are nucleophiles, so as long as there is remaining halogenoalkane, substitution will occur until a quaternary ammonium salt is produced.
Why, when you heat a halogenoalkane with ammonia, do you get a mixture of primary, secondary and tertiary amines and quaternary ammonium salts?
Because the primary amine produced in the first reaction has a lone pair of electrons so is a nucleophile. So it will react with any remaining halogenoalkane in a nucleophilic substitution reaction.
Further substitutions take place as primary, secondary and tertiary amines are all nucleophiles. This continues until a quaternary ammonium salt is produced as this doesn’t have a lone pair of electrons.
The mechanism for the following reactions are similar to that of ammonia with a halogenoalkane - two amine molecules react with a halogenoalkane in succession to form a more substituted amine (e.g primary amine forms a secondary amine) and an ammonium salt.
What gives a better yield of primary amine when heating a halogenoalkane with ammonia?
An excess of ammonia.
Why is heating a halogenoalkane with ammonia not a very efficient method of preparing an amine?
As it produces a mixture of products.
However these can be separated using fractional distillation.
Outline the method used to reduce a nitrile to a primary amine in the lab.
Why would this method not be used in industry?
- Lithium aluminium hydride (LiAlH4 - a strong reducing agent) is added to a nitrile in a non-aqueous solvent (such as dry ether), followed by dilute acid.
- The product is a primary amine.
This method is too expensive for industrial use.
Outline the method used to reduce nitriles in industry.
Nitriles are reduced using hydrogen gas, with a metal catalyst such as platinum or nickel, at high temperature and high pressure. The product is a primary amine.
This is called catalytic hydrogenation.
How can you form a nitrile from a halogenoalkane?
When halogenoalkanes react with a cyanide ion in aqueous ethanol, the cyanide ion replaces the halide ion by nucleophilic substitution to form a nitrile.
How are aromatic amines prepared?
By the reduction of nitro compounds.
