Rearrangement, Molecular

a restructuring of the atomic skeleton of molecules or a change in the position of functional groups. An example of molecular rearrangement is the conversion of cyclobutene upon heating to the more stable butadiene:

In olefins, a shift of a terminal double bond toward the middle of the carbon chain often takes place upon action of acids:

The major reaction that takes place when pinacol (I) is heated with dilute mineral acids or acid salts is the elimination of water to yield 2,3-dimethylbutadiene-l,3 (II); however, a small amount of pinacolone (III), a ketone with a different carbon skeleton than that of pinacol, is also formed. Pinacolone is exclusively formed when pinacol is treated with concentrated H₂SO₄, or ZnCl₂:

In the reaction of allyl halides with nucleophilic reagents, the major reaction is a substitution in which the residue of the nucleophile adds to the carbon atom that had previously been bound in the allyl halide to a halogen. Another process also takes place, in which the residue of the nucleophile adds to the allylic carbon atoms and the double bond is shifted. This secondary reaction is an example of substitution with allylic rearrangement:

The study of molecular rearrangement is relevant to the elucidation of mechanisms of chemical reactions and to carry out planned organic syntheses. Many molecular rearrangements are used in such industrially important reactions as the insomerization of petroleum hydrocarbons to obtain high-octane motor fuel, the conversion of cyclohexanone oxime to caprolactam, and the syntheses of intermediate products and dyes. (SeeARBUZOV REACTION, BECKMANN REARRANGEMENT, BENZIDINE REARRANGEMENT, DEM’IANOV REARRANGEMENT, CAMPHENE REARRANGEMENTS, and .)