Claisen rearrangement is a powerful organic chemical reaction in which carbon – carbon bond is formed. It is one of the important name reactions of class 12 chemistry. You should not get confused with Claisen condensation as it is different from Claisen rearrangement reaction.
Claisen rearrangement is a carbon – carbon bond forming chemical reaction which was discovered by Rainer Ludwig Claisen in 1912. As the name suggests it is a rearrangement reaction which is shown by aliphatic ethers as well as aromatic ethers.
Claisen Rearrangement (aliphatic) of Allyl Vinyl Ether – In Claisen rearrangement allyl vinyl ether is converted into γ,δ-unsaturated carbonyl compound in presence of lewis acid and heat. The reaction is given below –
Claisen Rearrangement (aromatic) of Aryl Allyl Ether or Allyl Phenyl Ether – In claisen rearrangement allyl phenyl ether is converted into its aromatic enol form by rearrangement. The reaction is given below –
Claisen rearrangement belongs to ‘sigmatropic rearrangement’ class of reactions. It is a concerted process as bonds are breaking and forming at the same time.
Claisen rearrangement reaction is an exothermic and pericyclic (cyclic transition state) reaction. As it releases energy and its intermediate transition state is cyclic. Solvent effects can be seen in Claisen rearrangement. Polar solvents accelerate the reaction up to greater extent. For example ethanol or water give a ten fold higher rate constant than sulfolane.
Phenol reacts with base such as NaOH or KOH and forms phenoxide ion by removing water molecule. Phenoxide ion reacts with allyl chloride and forms allyl phenyl ether by removing chlorine. On heating rearrangement of allyl phenyl ether takes place which forms its keto form. On tautomerization keto form changes into enol form which is more stable. Reaction mechanism is given below –
It is easier than the mechanism of aromatic Claisen rearrangement. It involves only one transitional state. On heating rearrangement of allyl vinyl ether takes place which give rise to a transitional state. As transitional state is highly unstable, it converts into stable γ,δ-unsaturated carbonyl compound. The reaction mechanism is given below –
Claisen rearrangement has various variations which have been developed to improve the synthetic value of the product. Such variations are Bellus-Claisen rearrangement, Eschenmoser – Claisen rearrangement, Ireland – Claisen rearrangement etc. We are not describing these variations in detail here as these are not the part of NCERT syllabus.
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