Preparation of Amines

We have discussed all the basics about amines, their physical properties, and nomenclature in the previous chapter. Are you not interested to know how you can prepare amine? This is a very interesting concept. You will find out when you read it! This is an important subject from the exam point of view as well. Hence, it is important that you take extra care of the equations and methods. We will now start with the Preparation of Amines.

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Methods of Preparation of Amines

Hoffmann’s Bromamide Reaction

We can also prepare amines (only primary)  by Hoffmann degradation. In this method, the amine will have one carbon atom less than the amide. The reaction proceeds via formation of nitrene.

Mechanism of above reaction is as given below:

Br₂  KOH  KO^—Br HBr

Mechanism

2NaOH Br₂  NaOBr NaBr H₂O

(a) 

(b)

R — NH₂  CO₂    →     R — N = C = O (Isocyanate)

Curtius, Schmidt, and Lossen Rearrangement

These reactions are basically rearrangement reactions in which carbon migrates from carbon to nitrogen with the formation of an isocyanate. In these migration processes i.e., 1, 2 shift, migrating group is an alkyl or aryl group and leaving group may be:

• Br in Hoffmann rearrangement
• N₂ in Curtius and Schmidt rearrangement

The isocyanate formed on hydrolysis gives amine. Let us now look at these reactions in greater details.

• Curtius Reaction: Acid chloride on treatment with sodium azide give acid azides which on pyrolysis gives isocyanates. On further hydrolysis, it gives corresponding amines.

RCOCl NaN₃  RCON₃  NaCl

• Schmidt Reaction: Carboxylic acid reacts with the hydrazoic acid in the presence of concentrated H₂SO₄ to give isocyanates.

• Lossen Reaction: Hydroxylamine on treatment with acid chloride gives acyl derivatives of hydroxylamine. The acyl derivatives exist in two tautomeric form keto form called hydroxamic form and enol form called hydroximic acid. The hydroxamic form.The hydroxamic form (keto form) forms O-acyl derivatives of hydroxamic form which on heating with bases forms isocyanates and finally amines upon hydrolysis.

Preparation of Amines

1) By Reduction of Nitroethane

Ethylamine can be obtained by the reduction of nitroethane. The reaction is as follows:

2) By Reduction of Methyl Cyanide

Ethylamine can also be obtained by reducing methyl cyanide. The reaction takes place as below:

3) By Reduction of Acetamide

Ethylamine is obtained by reduction of acetamide with sodium and absolute alcohol or LiAlH₄ in ether or hydrogen in presence of nickel catalyst.

4) By Reduction of Aldoxime

Aldoxime on reduction with hydrogen and nickel catalyst or sodium and absolute alcohol or LiAlH₄ in ether yields ethylamine.

5) By the Hydrolysis of Ethyl Isocyanate

Ethyl isocyanate on heating with caustic potash solution undergoes hydrolysis forming ethylamine.

6) By the Acid Hydrolysis of Ethyl Isocyanide

Ethyl isocyanide undergoes hydrolysis with a mineral acid and forms ethylamine.

7) By Schmidt Reaction

In this reaction, the acyl azide (RCON)₂ and alkyl isocyanate (R —NCO) are formed as an intermediate.

 

Solved Example for You

Q: How can we prepare ethylamine in the laboratory?

Ans: We can prepare ethylamine in the laboratory by Hofmann’s bromide reaction. Propionamide is heated with bromine and potassium hydroxide solution.