4.6 - Amines

Question 1

What is an amine?

Answer 1

Amines are derivatives of ammonia where one or more of the hydrogen atoms has been replaced with an organic group.

Question 2

Describe the difference between primary, secondary and tertiary amines

Answer 2

Consider a molecule of ammonia, NH3:
1. If one of the hydrogens is replaced with an organic group it is a primary amine.
2. If two of the hydrogens are replaced with an organic group it is a secondary amine.
3. If all three hydrogens are replaced with an organic group it is a tertiary amine.

Question 3

Give the structure of a quaternary ammonium ion

Answer 3

Question 4

Give two common methods of producing aliphatic amines

Answer 4

1. Nucleophilic substitution of halogenoalkanes with ammonia.
2. Reduction of nitriles.

Question 5

What conditions are required for primary amines to be formed from halogenoalkanes?

Answer 5

The halogenoalkane must be warmed with excess ethanolic ammonia inside a sealed tube.

Question 6

Draw the mechanism for the reaction between bromoethane and ammonia

Answer 6

Question 7

When a halogenoalkane reacts with ammonia why do you get a mixture of products?

Answer 7

The reaction will produce a mixture of primary, secondary and tertiary amines and quaternary ammonium salts. This is because when a primary amine is produced it acts as a nucleophile in further reactions. When it reacts in further nucleophilic substitution reactions, secondary amines are produced. These substitution reactions continue taking place until the quaternary ammonium salt is produced.

Question 8

Why can primary, secondary and tertiary amines act as nucleophiles when quaternary ammonium ions can not?

Answer 8

In primary, secondary and tertiary amines the nitrogen has a lone pair of electrons which allows it to act as a nucleophile. The nitrogen atom in quaternary ammonium ions does not have a lone pair of electrons so cannot act as a nucleophile.

Question 9

Give the chemical equation for the reaction of bromoethane with excess ammonia to form ethylamine

Answer 9

Question 10

Give the chemical equation for the reaction of methylamine with bromoethane to form methylethylamine

Answer 10

Question 11

How can the amine be released from an amine salt?

Answer 11

Treat the amine salt with an alkali, e.g. NaOH.

Question 12

What is the chemical equation for the reaction of methylammonium chloride with sodium hydroxide?

Answer 12

Question 13

How can a nitrile be reduced to an amine?

Answer 13

The reducing agent LiAlH4 is used.
LiAlH4 in a non aqueous solvent (e.g. dry ether) should first be added to the nitrile, followed by some dilute acid.

Question 14

What is the chemical equation for the reduction of ethanenitrile to ethylamine?

Answer 14

Question 15

What compounds can aromatic amines be reduced from?

Answer 15

Aromatic amines are produced from the reduction of nitro compounds - like nitrobenzene.

Question 16

What is the reducing agent used for the reduction of nitro compounds to aromatic amines?

Answer 16

Metallic tin and concentrated hydrochloric acid

Question 17

What is the chemical equation for the reduction of nitrobenzene to phenylamine?

Answer 17

Question 18

Why do amines act as bases?

Answer 18

The nitrogen atom in amines has a lone pair of electrons. This means amines can form a dative covalent bond with a hydrogen ion. Therefore amines act as bases because they accept protons.

Question 19

How does the the strength of a base depend on the availability of the lone pair of electrons?

Answer 19

The more available a lone pair of electrons is, the more likely they are to accept a proton and so the stronger a base it will be. The higher the electron density of a lone pair, the more available the lone pair is.

Question 20

Arrange the following amines in order of decreasing strength of base: Ammonia, primary aromatic amine, primary aliphatic amine

Answer 20

In order of decreasing strength of base:
1. Primaryaliphaticamine
2. Ammonia
3. Primaryaromaticamine

Question 21

Why are primary aromatic amines weaker bases than ammonia?

Answer 21

Primary aromatic amines have a benzene ring. This has a delocalised ring of electrons which draws electrons towards itself. This means the lone pair on nitrogen gets partially delocalised into the ring which decreases the electron density of nitrogen. The lone pair is therefore much less available.

Question 22

Why are primary aliphatic amines stronger bases than ammonia?

Answer 22

Primary aliphatic amines have an alkyl group which ‘pushes’ electrons towards the nitrogen atom. This increases the electron density of nitrogen, making the lone pair more available.

Question 23

Name the mechanism for the reaction between amines and ethanoyl chloride

Answer 23

Nucleophilic addition-elimination

Question 24

Draw the mechanism for the reaction between ethanoyl chloride and methylamine

Answer 24

Question 25

What is the IUPAC name of the organic product formed when ethanoyl chloride reacts with methylamine?

Answer 25

N-methylethanamide

Question 26

What can be used to test for primary amines?

Answer 26

Cold nitric(III) acid (nitrous acid)

Question 27

What do primary aliphatic and aromatic amines produce when they react with nitric(III) acid at room temperature?

Answer 27

Aliphatic amines produce alcohols. Aromatic amines produce phenols.

Question 28

What are the observations when a primary aliphatic amine reacts with cold nitric(III) acid?

Answer 28

There is a burst of nitrogen - which is a colourless and odourless gas.

Question 29

What is produced when primary aromatic compounds react with nitric(III) acid at temperatures below 10oC?

Answer 29

Benzenediazonium compounds

Question 30

What are diazonium ions?

Answer 30

Diazonium ions are ions which contain an N2+ group.

Question 31

Give the displayed structure of the benzenediazonium ion

Answer 31

Question 32

What are coupling reactions?

Answer 32

Coupling reactions are organic reactions which involve the joining together of two chemical species. An alkaline solution is required for coupling to occur.

Question 33

Describe the coupling reaction of benzenediazonium chloride with phenol

Answer 33

- First, a solution of sodium phenoxide is produced by dissolving phenol in sodium hydroxide. - Next, the solution is cooled and added to benzenediazonium chloride. A yellow-orange solution/precipitate is formed. The product is known as an azo compound.

Question 34

What are azo compounds?

Answer 34

Compounds in which two benzene rings are joined by a nitrogen bridge.

Question 35

Give the displayed chemical equation for the reaction of a phenoxide ion with a benzenediazonium ion

Answer 35

Question 36

Describe the coupling reaction of benzenediazonium chloride with naphthalen-2-ol

Answer 36

First, naphthalen-2-ol is dissolved in sodium hydroxide. The solution is then cooled and added to benzenediazonium chloride. A bright red precipitate forms - this is an azo compound.

Question 37

Give the displayed formula of the azo compound formed when naphthalen-2-ol reacts with benzenediazonium chloride

Answer 37

Question 38

Describe the coupling reaction of benzenediazonium chloride with phenylamine

Answer 38

Phenylamine is added to a cold solution of benzenediazonium chloride. A yellow solid azo compound is produced.

Question 39

What is a chromophore?

Answer 39

A chromophore is a group or atom responsible for the colour of a compound.

Question 40

Why can azo compounds be used as dyes?

Answer 40

The nitrogen group in azo dyes has indicator properties. Depending on the conditions, it interacts differently with light due to different wavelengths of light being absorbed or reflected. This means azo compounds form brightly coloured compounds and so are good for use as dyes.

Question 41

How do coloured compounds arise?

Answer 41

Coloured compounds arise due to the absorbance and reflection of light by the compound. When white light shines on a substance, some of the wavelengths are absorbed but the remaining wavelengths are reflected and transmitted. These reflected wavelengths correspond to a specific colour which is then observed.

Question 42

How can methyl orange be used to distinguish between acids and bases?

Answer 42

- Methyl orange is an azo compound which is red in acids and yellow in bases: - In acidic conditions, the nitrogen group gains a hydrogen causing it to interact differently with light, making it appear red. In basic conditions, a hydrogen ion is lost so the nitrogen group reflects yellow wavelengths, making it appear yellow.