Curtius Rearrangement

Theodor Curtius

Curtius rearrangement is a very useful reaction for synthesis of carbamate, amine and urea derivatives by isocyanate. It is a rearrangement reaction which includes thermal decomposition as well. It was given by Theodor Curtius in 1885. It is also known as Curtius reaction or Curtius degradation. In this reaction isocyanate is prepared by use of acyl azide. Acyl azides are carboxylic acid (-COOH) derivatives which have the general formula RCON3. Acyl azides are generally prepared by the reaction of alkyl or aryl acyl chlorides with sodium azides. The reaction is given below –

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Preparation of Acyl azide (where R =alkyl or aryl)

What is Curtius Rearrangement Reaction? 

Theodor Curtius was doing various experiments with acyl azides. During these experiments he discovered that on thermal decomposition of an acyl azide, it gives isocyanate with loss of nitrogen gas. The isocyanate on reaction with alcohols gives carbamate and with water and amines gives primary amine and urea derivatives respectively. The reaction is given below –

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Thus, Curtius rearrangement reaction is thermal decomposition of carboxylic azides (such as acyl azide) to give isocyanate. 

Mechanism of Curtius Rearrangement Reaction 

Before it was believed that the Curtius reaction is a two-step process - The 1st step takes place by loss of nitrogen gas forming an acyl nitrene and in the 2nd step migration of R group (R=alkyl or aryl) takes place to give isocyanate.  

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Recent researches showed that Curtius reaction is not a two-step process. They showed that actually thermal decomposition is a concerted process. It means both the steps take place together. As in the research absence of any nitrene intermediate was found. Thus, reaction mechanism can be written as follows –

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Thermodynamic calculations also support the above concerted single step mechanism.

The relative ability to migrate of migrating group (R-group) in the Curtius rearrangement is as follows –

Tertiary > Secondary ~ Aryl > Primary

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