Amines

Question 1

What are amines

Answer 1

Derivatives of ammonia in which one or more of the hydrogen atoms in the ammonia molecule have been replaced by alkyl or aryl groups

Question 2

Describe the shape of ammonia and amines

Answer 2

• Ammonia is a pyramidal molecule with bond angles of approximately 107 degrees.
• This is due to the repulsion caused by the long pair on the nitrogen atom.
• Amines keep this basic shape

Question 3

Describe the polarity of an amine and how this influences boiling points

Answer 3

• Amines are polar molecules
• Primary amines can hydrogen bond to one another using their -NH2 groups (in the same way as alcohols with their -OH groups).
• However, as nitrogen is less electronegative than oxygen, the hydrogen bonds are not as strong as those in alcohols.
• This means that the boiling points of amines are lower than those of comparable alcohols.
• Shorter chain amines are gases at room temperature whilst those with slightly longer chains are volatile liquids.

Question 4

Summarise the solubility of amines

Answer 4

• Primary amines with chain lengths up to about four carbon atoms are very soluble in both water and alcohols because they form hydrogen bonds with these solvents.
• Most amines are also soluble in less polar solvents.

Question 5

Describe the property of amines that determines how it reacts

Answer 5

• Amines have a lone pair of electrons which is important to how it reacts.
• The lone pair may be used to form a bond with:
  1) A H+ ion, when the amine is acting as a base
  2) An electron-deficient carbon atom, when the amine is acting as a nucleophile.

Question 6

Describe how amines are bases

Answer 6

• Amines can accept a proton (H+ ion).
  This means that they are Brønsted-Lowry bases.

Question 7

Describe how Amines react as bases

Answer 7

• Amines react with acids to form salts.
• The products are ionic compounds that will crystallise as the water evaporates.

Question 8

How can an amine salt be converted back into its original insoluble amine

Answer 8

By the addition of a strong bases such as sodium hydroxide that removes the proton from the salt and regenerates the insoluble amine.

Question 9

What does the strength of a base depend on

Answer 9

How readily it accepts a proton

Question 10

What does the strength of an amine as a base depend on

Answer 10

• How many alkyl groups are attached to the nitrogen atom
• This is because alkyl groups have a inductive effect which increases base strength
• The more alkyl groups present, the stronger the base is.

Question 11

Explain the inductive effect of alkyl groups in amines

Answer 11

• Alkyl groups release electrons away from the alkyl groups and towards the nitrogen atom.
• This is called the inductive effect.
• The inductive effect of the alkyl group increases the electron density on the nitrogen atom and therefore makes it a better electron pair donor.

Question 12

Why are tertiary amines not stronger bases than secondary ones (similar strength)

Answer 12

They are less soluble in water

Question 13

How does an amine having an aryl (benzene) group affect is strength as a base

Answer 13

• Aryl groups withdraw electrons from the nitrogen atom because the lone pair of electrons overlaps with the delocalised system on the benzene ring.
• This makes the nitrogen as weaker electron pair donor and therefore less attractive to protons
• This means arylamines are weaker bases than other amines and also ammonia

Question 14

Describe how amines react as a nucleophile

Answer 14

The lone pair of electrons from an amine will attack positively charged carbon atoms, meaning it will act as a nucleophile.

Question 15

How are primary amines produced via nucleophilic substitution

Answer 15

• Primary aliphatic amines are produced when halogenoalkanes are reacted with ammonia
• This is mucleohilic substitution by NH2.

Question 16

Describe what reaction happens once a primary amine has been produced by nucleophilic substitution

Answer 16

• The primary amine is also a nucleophile and this will react with the halogenoalkane to produce a secondary amine.
• The secondary amine will then react as a nucleophile to give a tertiary amine which will in turn react to produce a quarternary ammonium salt.
• This means that the nucleophilic substitution of ammonia and a halogenoalkane will produce a mixture of primary, secondary and tertiary amines and a quarternary ammonium salt.

Question 17

What is the disadvantage of producing a primary amine by the nucleophilic substitution of ammonia and a halogenoalkane

Answer 17

• A mixture of primary, secondary and tertiary amines and a quarternary ammonium salt is produced.
• Therefore it is not a very efficient way of preparing an amine, though the products may be separated by fractional distillation.
• A large excess ammonia gives a better yield of primary amine.

Question 18

What is an aliphatic amine

Answer 18

An amine that only has alkyl and hydrogen- not aryl groups

Question 19

What does ammonia act as in the nucleophilic substitution with a halogenoalkane

Answer 19

In the first stage it acts as a nucleophile and in the second stage it acts as a base.

Question 20

What is the other method (not nucleophilic substitution) of producing primary aliphatic amines from halogenoalkanes

Answer 20

• The reduction of nitriles.
• Cyanide ions are used to form the nitrile by nucleophilic substitution.
• This nitrile is then reduced.

Question 21

Describe how primary amines are produced from halogenoalkanes by the reduction of nitriles

Answer 21

Step 1) Halogenoalkanes react with the cyanide ions in aqeous ethanol. The cyanide ion replaces the halide ion by nucleophilic substitution to form a nitrile
Step 2) Nitriles contain the functional group C-(triple bond)-N. They can be reduced to primary amines with a nickel hydrogen catalyst.

Question 22

What is the advantage of producing a primary amine via the reduction of nitriles compared to the nucleophilic substitution with ammonia

Answer 22

A purer product is produced because only the primary amine can be formed.

Question 23

Describe the two steps in the formation of phenylamine from benzene

Answer 23

1) Benzene is reacted with a mixture of concentrated nitric and concentrated sulfuric acid which produces nitrobenzene.
2) Nitrobenzene is reduced to phenylamine, using tin and hydrochloric acid as the reducing agent.

Question 24

Describe how tin and hydrochloric acid reduce nitrobenzene to phenylamine

Answer 24

• The tin and hydrochloric acid react to form hydrogen, which reduces the nitrobenzene by removing oxygen atoms of the NO2 group and replacing them with hydrogen atoms.
• Since the reaction is carried out in hydrochloric acid, the salt (C6H5NH3)+ Cl- is formed.
• Sodium hydroxide is added to liberate the free amine.

Question 25

How are amides formed

Answer 25

• Amines react with acid anhydrides and acyl chlorides via a nucleophilic addition-elimination reaction to form N-Substituted amides.
• The amine adds onto the acid derivative and HCl is eliminated.

Question 26

How are amines used in industry

Answer 26

To manufacture synthetic materials such as nylon, polyurethane, dyes and drugs

Question 27

How are quarternary ammonium slats used in industry

Answer 27

They are used in the manufacture of hair and fabric conditioners.

Question 28

Why are quarternary ammonium slats useful in hair and fabric conditioners

Answer 28

• They have a long hydrocarbon chain and a positively charged organic group, so they form cations.
• Both wet fabric and wet hair pick up negative charges on their surfaces.
• This means that the positive charges of the cations attract them to the wet surface and form a coating that prevents the build up of static electricity.
• This keeps the surface of fabric smooth and prevents flyaway hair in hair conditioners.
• They are called cationic surfactants because in aqueous solution the ions cluster with their charged ends in the water and their hydrocarbon tails on the surface.