A - Amines

Question 1

What are amines?

Answer 1

Compounds based on ammonia where one or more hydrogen atoms have been replaced by alkyl or aryl groups.

Question 2

What are primary, secondary and tertiary amines?

Answer 2

1. Primary amine - where one hydrogen on ammonia is replaced by an organic group.
2. Secondary amine - where two hydrogen atoms are replaced with organic groups.
3. Tertiary amine - where three hydrogen atoms are replaced with organic groups.

It refers to the number of R-groups on the nitrogen atom.

Question 3

How can you get a quaternary ammonium ion?

Answer 3

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.

Question 4

What is an aromatic amine?

Answer 4

Where a hydrogen atom on ammonia is replaced with a benzene ring.

Question 5

How are amines named?

Answer 5

Using the suffix, -amine.

Question 6

In what two ways can primary aliphatic amines be prepared?

Answer 6

1. By the reaction of ammonia with halogenoalkanes.

2. By the reduction of nitriles.

Question 7

What is an aliphatic compound?

Answer 7

A non-aromatic compound.

Question 8

Outline what happens when you heat a halogenoalkane with excess ammonia. (The reaction mechanism).

Answer 8

1. Ammonia attacks the carbon in the halogenoalkane, and the electrons are transferred to the halogen.
2. The halogen ion is released.
3. This forms an alkylammonium salt.
4. 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.
5. 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.

Question 9

Why, when you heat a halogenoalkane with ammonia, do you get a mixture of primary, secondary and tertiary amines and quaternary ammonium salts?

Answer 9

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.

Question 10

What gives a better yield of primary amine when heating a halogenoalkane with ammonia?

Answer 10

An excess of ammonia.

Question 11

Why is heating a halogenoalkane with ammonia not a very efficient method of preparing an amine?

Answer 11

As it produces a mixture of products.

However these can be separated using fractional distillation.

Question 12

Outline the method used to reduce a nitrile to a primary amine in the lab.

Why would this method not be used in industry?

Answer 12

1. 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.
2. The product is a primary amine.

This method is too expensive for industrial use.

Question 13

Outline the method used to reduce nitriles in industry.

Answer 13

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.

Question 14

How can you form a nitrile from a halogenoalkane?

Answer 14

When halogenoalkanes react with a cyanide ion in aqueous ethanol, the cyanide ion replaces the halide ion by nucleophilic substitution to form a nitrile.

Question 15

How are aromatic amines prepared?

Answer 15

By the reduction of nitro compounds.

Question 16

Outline the method of reducing a nitro compound to produce an aromatic amine.

Answer 16

1. Heat a mixture of a nitro compound, tin metal and concentrated hydrochloric acid under reflux. This makes a salt.
2. Then to turn the salt into an aromatic amine, you need to add an alkali, such as sodium hydroxide solution.

Question 17

What are aromatic amines used for?

Answer 17

In the manufacture of dyes.

Question 18

Why are amines bases?

Answer 18

Because they can accept a proton.

Question 19

Why would the reduction of a nitro compound to make an aromatic amine not be carried out in the lab?

Answer 19

Because benzene is a carcinogen.

Question 20

Are amines strong or weak bases?

Answer 20

Weak bases.

Question 21

What does the strength of a base depend on?

Answer 21

How available the lone pair of electrons are on the N atom (in the case of ammonia, primary aliphatic and primary aromatic amines).

The more available the lone pair, the more likely the molecule is to accept a proton, and the stronger a base it will be.

A lone pair of electrons will be more available if it’s electron density is higher.

Question 22

What is the difference in base strength between ammonia, primary aliphatic and primary aromatic amines?

Answer 22

Weakest: Primary aromatic amine

Ammonia

Strongest: Primary aliphatic amine

Question 23

Why are primary aromatic amines weaker bases than ammonia?

Answer 23

The benzene ring on the aromatic amine draws electrons towards itself from the N atom, and the nitrogen lone pair gets partially delocalised into the benzene ring.

So the electron density on the nitrogen decreases, making the lone pair much less available.

Question 24

Why are primary aliphatic amines stronger bases than ammonia?

Answer 24

Alkyl groups push electrons onto attached groups. So the electron density on the nitrogen atom increases which makes the lone pair more available.

Question 25

How are quaternary ammonium salt complexes formed?

Answer 25

Quaternary ammonium ions have a positive charge so are attracted to any negative ions that are near. This forms a quaternary ammonium salt.

Question 26

How are quaternary ammonium salts used as cat ionic surfactants?

Answer 26

Quaternary ammonium salts with at least one long hydrocarbon chain are used as cationic surfactants. The hydrocarbon tail will bind to non polar substances such as grease, while the cationic head will dissolve in water.

So they are used in fabric cleaners and hair products.

Question 27

Why are quaternary ammonium salts used in fabric conditioners?

Answer 27

The positively charged part (ammonium ion) will bind to negatively charged surfaces such as hair and fibre. This gets rid of static which is why they’re used in fabric conditioners.