The separation of amines by Hoffmann’s method can be carried out via the treatment of the mixture of amines with diethyl oxalate. The whole separation can be carried out by the following diagram:
Separation of Primary, secondary, and tertiary amines by Hoffmann’s method
Diethyl oxalate reacts with the mixture of amines as:
1o amine forms crystalline solid, dialkyloxamide
2o amine forms an oily liquid, dialkyloxamic ester
No reaction with 3o amine as it lacks replaceable hydrogen atom
Filtration of mixture
Filtration is carried out on a reaction mixture containing dialkyl oxamide, dialkyloxamic ester, tertiary amine, and ethyl alcohol to obtain solid dialkyl oxamide, which is then heated with aqueous KOH to liberate primary (1o) amine.
The remaining mixture containing 3o amine, dialkyloxamic ester, and ethanol is now fractionally distilled. Because tertiary amine is more volatile than alcohol, it is distilled first. Finally, to recover secondary 2o amine, the dialkyloxamic ester that remains as the residue is treated with aqueous KOH. These are distilled to produce two fractions of amine and alcohol.
In this way, primary, secondary, and tertiary amines can be separated via Hoffmann’s method.
Separation of 1o, 2o, and 3o amines Video
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