6.2.1: Amines

Question 1

Amine functional group

Answer 1

R–NH₂

Question 2

How are amines formed?

Answer 2

By replacing 1 or more of the Hs on NH₃ with an R group (usually an alkyl group)
⟶ Primary amine = 1 H replaced
⟶ Secondary amine = 2 Hs replaced
⟶ Tertiary amine = 3 Hs replaced

Question 3

Primary and secondary amines can hydrogen bond to

Answer 3

each other and water

Question 4

Tertiary amines can hydrogen bond to

Answer 4

water and other hydroxylic solvents (not each other)

Question 5

The “basic” nature of an amine depends on what?

Answer 5

The groups attached to it.

Question 6

Which R groups make an amine more basic? Why?

Answer 6

Aliphatic group (e.g. methylamine)
⟶ Electron releasing substituents increase basicity as (due to the positive inductive effect) electron density is increased and the lone pair is more effective.
⟶ Leads to increased stability of the ammonium-type cation formed

Question 7

Which R groups make an amine less basic? Why?

Answer 7

Aromatic group (e.g. benzene ring)
⟶ Electron withdrawing substituents decrease basicity as the electron density on the N is lowered and the lone pair is less effective.
⟶ Ammonium-type cation, if formed, is less stable and more likely to release the H+.

Question 8

Amines are usually _______ bases than ammonia

Answer 8

Stronger - but depends on the groups attached and their inductive or withdrawing effects.

Question 9

Two types of amines and their synthesis

Answer 9

• Aliphatic
⟶ Haloalkane + ethanolic ammonia

• Aromatic
⟶ Heat, reflux nitrobenzene with Sn and conc. HCl

Question 10

Synthesis of aliphatic amines (mechanism)

Answer 10

Nucleophilic substitution

Question 11

Synthesis of aliphatic amines (reagents)

Answer 11

Excess ethanolic ammonia, haloalkane

Question 12

Synthesis of aliphatic amine (conditions)

Answer 12

Heat in a sealed tube in excess alcoholic (ethanolic) solution under pressure

Question 13

What is the purpose of the ethanol in synthesis of aliphatic amines?

Answer 13

Ethanol makes NH₃ a better nucleophile.

Question 14

Synthesis of aliphatic amines (equation)

Answer 14

CH₃CH₂Br + NH₃ ⟶ CH₃CH₂NH₂ + HBr

Following this, HBr reacts with excess ammonia to produce NH₄Br (s)

Question 15

Synthesis of aliphatic amines: why must excess haloalkane be avoided?

Answer 15

• Because the product amine can still act as a nucleophile

* Can react with excess haloalkane leading to the formation of higher order (2⁰, 3⁰, quaternary ammonium salt) amines

Question 16

Synthesis of aromatic amines (reagents)

Answer 16

• Sn
• Conc HCl
• NaOH (to liberate the product from its ammonium salt)

Question 17

Synthesis of aromatic amines (conditions)

Answer 17

Heat under reflux

Question 18

Nitrobenzene is _______ to form the aromatic amine phenylamine (a.k.a. aminobenzene or aniline)

Answer 18

reduced

Question 19

Synthesis of aromatic amines (equation)

Answer 19

C₆H₅NO₂ + 6[H] –Sn–> C₆H₅NH₂ + 2H₂O

Question 20

Why are amines basic?

Answer 20

• Lone pair of electrons on N atom accepts an H⁺

* Amines are neutralised by acids to form ammonium salts