3.3.11 Amines - Chemistry Alevel

Amines - bonding

There is some hydrogen bonding between molecules, but weaker than in alcohols.

Amines - how they act in reactions

They have a lone pair of electrons on the N atom and so they can act as bases and nucleophiles.

Preparation of primary aliphatic amines - reagents & conditions -1 STEP ONLY

Halogenoalkane, excess ammonia

Preparation of primary aliphatic amines - equation -1 STEP ONLY

2NH₃ + R-Br → R-NH₂ + NH₄Br

Preparation of primary aliphatic amines - advantages & disadvantages - 1 STEP ONLY

Method is not usually used because the amines produced undergo further substitution giving a mixture of products.

Expensive and time-consuming to separate.

Preparation of primary aliphatic amines - reagents & conditions - step 1

Halogenoalkane, potassium cyanide, in aqueous ethanol.

Preparation of primary aliphatic amines - reagents & conditions - step 2

H₂ gas with a Ni catalyst

OR

LiAlH₄ in dry ether

Preparation of primary aliphatic amines - equation - step 1

R-Br + KCN → R-CN + KBr

R-CN = -anenitrile

NOTE: the chain length increases!

Preparation of primary aliphatic amines - equation - step 2

R-CN + 2H₂ → R-CH₂NH₂

OR

R-CN + 4[H] → R-CH₂NH₂

Preparation of primary aliphatic amines - type of reaction - 2 STEP PROCESS

Reduction of nitrile compounds

Preparation of primary aliphatic amines - advantages & disadvantages - 2 STEP PROCESS

(Yield higher? as) produces no other product.

KCN is toxic & poisonous, LiAlH₄ is expensive, (yield lower as) 2 step process, H₂ is flammable/explosive

Preparation of primary aromatic amines - reagents and conditions

H₂ [gas?] and Ni catalyst

OR

Tin (Sn) and concentrated HCl

Preparation of primary aromatic amines - equation

⏣-NO₂ + 3H₂ → ⏣-NH₂ + 2H₂O

OR

⏣-NO₂ + 6[H] → ⏣-NH₂ + 2H₂O

Aromatic amines - use

Used in the manufacture of dyes.

Why can't you make aromatic amines with the same method as aliphatic amines?

There is no positive charge for the negative ions to be attracted to, instead there are delocalised electrons in a ring that repels the nucleophile.

What does the ability of amines too act as bases depend on?

The ability of the lone pair of electrons on the N atoms.

Base strength - Primary, secondary and tertiary amines compared to ammonia

______ is a stronger base.

It has (more) alkyl groups attached to the nitrogen.

Therefore the lone pair of electrons on the nitrogen is more available in _______.

Due to the positive inductive effect.

Base strength - aromatic amines compared to ammonia

_______ is a weaker base.

The lone pair of electrons on the nitrogen is less available to accept a proton

due to delocalisation.

Reaction of ammonia with halogenoalkanes - mechanism type

Nucleophilic substitution

Reaction of ammonia with halogenoalkanes - overall equation

2NH₃ + R-CH₂Br → R-CH₂NH₂ + NH₄Br

NH₄Br - depends on what takes away the H and Br

Reaction of primary amines with halogenoalkanes - mechanism type

Nucleophilic substitution

Reaction of primary amines with halogenoalkanes - overall equation

2R-CH₂NH₂ + R-CH₂Br → (R-CH₂)₂NH + R-CH₃NH₃Br

R-CH₃NH₃Br - depends on what takes away the H and Br

Reaction of secondary amines with halogenoalkanes - overall equation

(R-CH₂)₂NH + R-CH₂Br → (R-CH₂)₃N + (R-CH₂)₂NH₂Br

(R-CH₂)₂NH₂Br - depends on what takes away the H and B

Reaction of secondary amines with halogenoalkanes - mechanism type

Nucleophilic substitution

Reaction of tertiary amines with halogenoalkanes - mechanism type

Nucleophilic substitution

Reaction of tertiary amines with halogenoalkanes - overall equation

(R-CH₂)₃N + R-CH₂Br → (R-CH₂)₄NBr

*N⁺Br⁻ called tetra -thyl ammonium bromide.

Ammonia/amines and halogenoalkanes - disadvantages

The hydrogens on the nitrogen atom can be successively replaced by the alkyl group from the halogenoalkane all the way through to quaternary ammonium salts. Therefore mixture produced would have to be separated.

Ammonia/amines and halogenoalkanes - main product of primary amine - conditions

A large excess of ammonia.

Ammonia/amines and halogenoalkanes - main product of quaternary ammonium salt - conditions

A large excess of halogenoalkane.

Quaternary ammonium salt - use

Cationic surfactants; fabric softeners, hair conditioners.