Amines

Question 1

How do primary aliphatic amines act as Bronsted-Lowry bases?

Answer 1

• Primary aliphatic amines act as Bronsted-Lowry bases because the lone pair of electrons on the nitrogen is readily available for forming a dative covalent bond with a H+ and so accepting a proton. They are weak bases as only a low concentration of hydroxide ions is produced

Question 2

Why are primary aliphatic amines stronger bases than ammonia?

Answer 2

• Primary aliphatic amines are stronger bases than ammonia as the alkyl groups are electron releasing and push electrons towards the nitrogen atom and so making the lone pair of electrons on the nitrogen more readily available to accept protons.

Question 3

Why are secondary amines stronger bases than primary amines?

Answer 3

• Secondary amines are stronger bases than primary amines because they have more alkyl groups that are substituted onto the N atom in place of H atoms. Therefore more electron density is pushed onto the N atom (as the inductive effect of alkyl groups is greater than that of H atoms), so making the lone pair of electrons on the nitrogen more readily available.

Question 4

Write an equation to show how primary aliphatic amines act as Bronsted-Lowry bases

Answer 4

Question 5

Why are tertiary amines weaker bases than secondary amines?

Answer 5

• The tertiary amines and corresponding ammonium salts are less soluble in water and this makes them less strong bases than the secondary amines.

Question 6

Describe the base strength of aromatic amines

Answer 6

• Primary aromatic amines such as phenylamine do not form basic solutions because the lone pair of electrons on the nitrogen delocalise with the ring of electrons in the benzene ring. This means the lone pair of electrons on the N is less able to accept protons.

Question 7

What is the overall order of base strength?

Answer 7

Question 8

What do all amines react with acids to become?

Answer 8

• All amines will react with acids to become ammonium salts

Question 9

Write an equation for the formation of methylammoniumchloride

Answer 9

• The ionic salts formed in this reaction means that the compounds are soluble in the acid eg, phenylamine is not very soluble in water but phenylammonium chloride is soluble

Question 10

How can an ammonium salt be converted back into an amine?

Answer 10

• Addition of NaOH to an ammonium salt will convert it back to the amine

Question 11

What will the ionic salts be if the water is evaporated

Answer 11

• These ionic salts will be solid crystals, if the water is evaporated, because of the strong ionic interactions

Question 12

• How can you make a basic buffer from an amine?

Answer 12

• Basic buffers can be made from combining a weak base with a salt of that weak base eg:
• Ammonia and ammonium chloride
• Methylamine and methylammonium chloride
• Ethylamine and ethylammonium chloride

Question 13

• The process of forming a primary amine in a one step reaction of halogenoalkanes with ammonia?

Answer 13

• Primary amines can be formed by the nucleophilic substitution reaction between halogenoalkanes and ammonia in a one step reaction
• However, as the lone pair of electrons is still available on the N in the amine formed, the primary amine can react in the same nucleophilic way in a successive series of reactions forming secondary, tertiary amines and quaternary ammonium salts
• This is therefore not a good method for making primary amines because of the further reactions
• It would mean that the desired product would have to be separated from the other products

Question 14

• How can you limit further substitution?

Answer 14

• Using excess ammonia can limit the further subsequent reactions and will maximise the amount of primary amine formed
• Excess NH3 means that the haloalkane is more likely to react with NH3 than with the amine formed
• Using an excess of the halogenoalkane will promote the formation of the quaternary salt

Question 15

• What are amines?

Answer 15

• Amines are derivatives of ammonia with one or more of the H atoms replaced by an alkyl or aryl group

Question 16

• Write an equation for the reaction between dimethylamine and ethanoic acid

Answer 16

Question 17

• Write an equation for the reaction between ethylamine and sulfuric acid

Answer 17

Question 18

• Write an equation for the reaction between triphenylamine and ethanedioic acid

Answer 18

Question 19

• How can you prepare amines from nitriles?

Answer 19

• Convert halogenoalkane to nitrile by using KCN in aqueous ethanol (heat under reflux)
• CH3CH2Br + CN- -> CH3CH2CN + Br-
• Reduce nitrile to amine by using LiAlH4 in dry ether or by reducing with H2 using a Ni catalyst
• CH3CH2CN + 4[H] -> CH3CH2CH2NH2

Question 20

• What is a disadvantage of preparing amines from nitriles?

Answer 20

• It is a two step reaction that may therefore have a low yield
• KCN is toxic

Question 21

• How can you reduce nitroarenes to aromatic amines?

Answer 21

• Reagent: Sn and HCl or Fe and HCl
• Conditions: Heating under reflux
• Mechanism: Reduction
• This reduction reaction can also be done with catalytic hydrogenation (H2 using a Ni catalyst)

Question 22

• What are amides?

Answer 22

• Amides are derivatives of carboxylic acids and have the functional group of -CONH2

Question 23

Answer 23

• Propanamide

Question 24

Answer 24

• N-phenylpropanamide

Question 25

Answer 25

• N-ethyl-N-methylpentanamide

Question 26

Answer 26

• Method: Measure pH WITH A METER • Methylamine would have a higher pH / ammonia would have a lower pH

Question 27

Answer 27