The key difference between Hofmann and Curtius rearrangement is that Hofmann rearrangement describes the conversion of a primary amide into a primary amine whereas Curtius rearrangement describes the conversion of an acyl azide into an isocyanate.
A rearrangement reaction is a chemical conversion reaction where one chemical compound converts into a different compound that is more stable than the original compound. Hofmann and Curtius rearrangement are such two types of organic chemical reactions that have many applications in chemical synthesis processes. The Hofmann reaction was developed and named after the scientist August Wilhelm Von Hofmann while the Curtius rearrangement reaction was named after Theodor Curtius.
CONTENTS
1. Overview and Key Difference
2. What is Hofmann Rearrangement
3. What is Curtius Rearrangement
4. Side by Side Comparison – Hofmann vs Curtius Rearrangement in Tabular Form
5. Summary
What is Hofmann Rearrangement?
Hofmann rearrangement is a type of chemical reaction which describes the conversion of a primary amide into a primary amine with one fewer carbon atom. This reaction was named after August Wilhelm Von Hofmann. Sometimes, we call it the Hofmann degradation as well. The general formula for Hofmann rearrangement reaction is as follows:
Figure 01: Hofmann Rearrangement
Hofmann rearrangement reaction starts with the reaction of bromine with sodium hydroxide, which forms sodium hypobromite (an in situ reaction) which can transform the primary amide into an intermediate isocyanate molecule. Then this intermediate isocyanate compound undergoes hydrolysis to give a primary amine. During this rearrangement, a carbon dioxide molecule is released. Therefore, the final product lacks one carbon atom when compared to the number of carbon atoms in the original molecule.
What is Curtius Rearrangement?
Curtius rearrangement is a type of chemical reaction where an acyl azide converts into an isocyanate. This reaction was developed by Theodor Curtius in 1885. It is a thermal decomposition reaction. This reaction involves the loss of a nitrogen gas molecule. After this conversion, the isocyanate molecule undergoes an attack by a variety of nucleophiles – e.g. water, alcohol, and amines. This nucleophilic attack yields a primary amine, carbamate, or urea derivatives. The general formula is as follows:
Figure 02: Curtius Rearrangement
An acyl azide forms from the reaction of acid chlorides or anhydrides with sodium azide or trimethylsilyl azide. Typically, Curtius rearrangement is considered as a two-step process. The loss of nitrogen gas forms an acyl nitriene. And, this reaction is followed by the migration of the R-group to give the isocyante. However, according to recent research studies, thermal decomposition of acyl azide is a concerted process where both above-mentioned steps occur together. This happens due to the absence of any nitrene insertion or addition byproducts observed or isolated in the reaction.
What is the Difference Between Hofmann and Curtius Rearrangement?
A rearrangement reaction is a chemical reaction that converts the carbon body of a compound into a different structure that is an isomer of the original compound. The key difference between Hofmann and Curtius rearrangement is that Hofmann rearrangement describes the conversion of a primary amide into a primary amine whereas Curtius rearrangement describes the conversion of an acyl azide into an isocyanate.
Below infographic tabulates the differences between Hofmann and Curtius rearrangement.
Summary – Hofmann vs Curtius Rearrangement
Hofmann and Curtius rearrangement are two types of chemical rearrangement reactions. The key difference between Hofmann and Curtius rearrangement is that Hofmann rearrangement describes the conversion of a primary amide into a primary amine whereas Curtius rearrangement describes the conversion of an acyl azide into an isocyanate.
Reference:
1. “Hofmann Rearrangement.” Chemistry LibreTexts, Libretexts, 14 July 2020, Available here.
2. “Hofmann Rearrangement.” Wikipedia, Wikimedia Foundation, 24 June 2020, Available here.
Image Courtesy:
1. “Hofmann Rearrangement Scheme” By ~K assumed. Own work assumed (based on copyright claims) (Public Domain) via Commons Wikimedia
2. “Curtius rearrangement rxn” By lh.779 – Own work (CC BY-SA 4.0) via Commons Wikimedia