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Elucidation of Reaction Mechanisms - Crossover Experiments and Kinetics

Crossover Experiments

Crossover experiments are particularly useful in studying the reaction mechanisms of many molecular rearrangements. The question, as far as these reactions are concerned, is often whether or not the reorganization of the carbon skeleton is the result of an intramolecular displacement ("real" rearrangement), or an intermolecular exchange of atoms or a group of atoms. Crossover experiments may help find answers to such questions. In crossover experiments a mixture of starting products that differ from each other only in one characteristic group is applied. The structure of the products is what then tells us whether or not an intermolecular exchange of atoms or a group of atoms has occured.

The principles of crossover experiments are illustrated below:

  • Benzidine rearrangement If a mixture of starting products containing 2,2'-dimethoxyhydrazobenzene and 2,2'-diethoxyhydrazobenzene is applied to the benzidine rearrangement, a product mixture that consists of two symmetrically substituted benzidines is obtained. Thus, the reaction obviously takes an intramolecular course, otherwise unsymmetrically substituted benzidines must also be formed.
Benzidine rearrangement.
  • Claisen rearrangement The Claisen rearrangement of allyl phenyl ether yields 2-allylphenol. If a starting product mixture, consisting of allyl phenyl ether and 2-methylallyl 4-methylphenyl ether, is applied to this reaction, an intermolecular and an intramolecular mechanism ought to yield different product mixtures:
Claisen rearrangement in case of an intramolecular mechanism.
Claisen rearrangement in case of an intermolecular mechanism.

The intramolecular mechanism of the Claisen rearrangement would only yield 2-allylphenol and 4-methyl-2-(2-methylallyl)-phenol. In contrast, an intermolecular mechanism would additionally result in 2-(2-methylallyl)-phenol and 2-allyl-4-methylphenol. The Claisen rearrangement must obviously be an intramolecular reaction, since no crossover products are obtained.

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