5 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

T he 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:

Reductions of Amines with Nitric(III) Acid

• All amines react with nitric(III) acid

• As nitric(III) acid is a weak and unstable acid, it is prepared by treating NaNO₂ with an aqueous solution of a strong acid

NaNO₂(aq) + HCl(aq) ®¾ HNO₂(aq) + NaCl(aq)

2NaNO₂(aq) + H₂SO₄(aq) ®¾ 2HNO₂(aq) + Na₂SO₄(aq)

Reductions of Primary Amines with Nitric(III) Acid

Reaction of Primary Aliphatic Amines with Nitric(III) Acid

• 1° aliphatic amines react with HNO₂ to yield aliphatic diazonium salts

• Diazonium salts, even at low temperatures, are highly unstable, they decompose spontaneously by losing nitrogen to form carbocations

Example:

Reaction of Primary Aromatic Amines with Nitric(III) Acid

• 1° aromatic amines react with HNO₂ to yield benzenediazonium salts

• Benzenediazonium salts are unstable, but they do not decompose when the temperature of the reaction mixture is kept below 5°C

• The greater stability of benzenediazonium salts is due to the delocalization of the positive charge of the diazonium group

+

(–N º N) over the benzene ring

Reductions of Secondary Amines with Nitric(III) Acid

• 2° aliphatic and aromatic amines react with HNO₂ to yield

N-nitrosoamines

Examples:

Reductions of Tertiary Amines with Nitric(III) Acid

• 3° aliphatic amines don’t react with HNO₂

Example: