Boiling Point and Melting Point

1° or 2° amines can form strong hydrogen bonds with each other

Þ the b.p. is higher than those of alkanes but lower than those of alcohols of comparable molecular masses

∵ O – H bond is more polar than N – H bond as oxygen is more electronegative Þ stronger hydrogen bonds are formed between alcohol molecules

3° amines have lower b.p. and m.p. than 1° and 2°

all hydrogen in 3° amines are replaced by organic groups Þ no hydrogen bond can be formed

Density

Molecular mass of amines ­ Þ densities ­

Solubility

• The solubility of amines in water:
1° amines > 2° amines > 3° amines

∵ The intermolecular hydrogen bonds formed between 2° or 3° amines and water molecules decrease

• Amines are soluble in common organic solvents, e.g. hexane, ethanol and propanone

Preparation of Amines

Hydrogen reduces a nitro compounds to a 1° amine in the presence of nickel catalyst

Example:

Reduction of Nitriles

Reaction with Hydrogen

Hydrogen reduces a nitrile to a 1° amine in the presence of nickel catalyst

Example:

Reaction with Lithium Tetrahydridoaluminate

Nitriles are reduced to 1° amines by LiAlH4 in dry ether

Example:

Reduction of Amides

Reaction with Lithium Tetrahydridoaluminate

Amides can be reduced by LiAlH₄ to give 1°, 2° or 3° amines

• The product is 1° amine if R’ and R’’ = H

• The product is 2° amine if R’ or R’’ = H

Examples:

Hofmann Degradation

• Amides with no substituent on the nitrogen react with Br₂ in NaOH (or Cl₂ in NaOH) to give amines

• The amines formed have one carbon less than the parent amides

Reaction of Ammonia with Haloalkanes

1°, 2°, 3° and quaternary ammonium salts can be prepared by heating haloalkanes with ammonia under pressure

Examples:

• This method always give a mixture of products

• Successive alkylations always occur because aliphatic amines are more reactive than ammonia

Preparation of Phenylamine

Reduction of Nitrobenzene (Zinin’s reactin)

The reduction of the nitro group can be carried out by

• Catalytic hydrogenation

Reaction of Ammonia with Halobenzenes

1° phenylammines can be prepared by heating halobenzenes with ammonia under pressure

2° and 3° phenylammines can be prepared by heating halobenzenes with phenylamine under pressure

 

Reactions of Amines

Basicity of Amines

• Amines are far weaker bases than hydroxide ions but are stronger bases than water

• The lone pair electrons on nitrogen atom can accept a proton from a water molecule Þ amines are basic

Amines dissolve in mineral acids such as HCl to form salts

e.g.

Methylamine will partially dissociate into ions

The basic strength of amines is shown by the value of base dissociation constant (K_b)

 

pK_b is used for convenience as K_b is small for weak bases
pK_b = –log K_b, the smaller the pK_b value, the stronger is the base

Name	Formula	pKb
Ammonia	NH3	4.75
Primary aliphatic amines: Methylamine Ethylamine Propylamine Butylamine	CH3NH2 CH3CH2NH2 CH3(CH2)2NH2 CH3(CH2)3NH2	3.36 3.27 3.16 3.39
Aromatic amines: Phenylamine	C6H5NH2	9.38

 

The basicity of amines is governed by the availability of the lone pair electrons on the nitrogen atom

• Primary aliphatic amines are the most basic

electron-releasing alkyl groups enhance the availability of the lone pair electrons on the nitrogen atom while ammonia does not

• Aromatic amines are weaker bases than ammonia

the lone pair electrons on the nitrogen atom are delocalized over the benzene ring making it less available to a proton

Acylation of amines

• 1° and 2° amines react readily in cold with ethanoyl chloride or anhydride to form N -substituted amides

Examples:

• 1° and 2° amines react with benzoyl chloride in the presence of alkalis to form N -substituted amides

Examples: