Why was a deuterated compound used in the last reaction on the preceding page?

In a sigmatropic rearrangement, a sigma bond in the reactant breaks, a new sigma bond forms and the p electrons rearrange.

The sigma bond that breaks is a bond to an allylic carbon; It can be of the following types:

• sigma bond between carbon and hydrogen.
• Sigma bond between two carbons.
• Sigma bond between carbon-oxygen/Nitrogen/sulfur.

In the last reaction [1,5] Sigmatropic migrations of hydrogen reaction involve three pairs of electrons that take place by a suprafacial pathway under thermal conditions shown as follows:

In the above reaction, if the reactant used is nondeuterated, the product formed would be identical to the reactant; therefore, the rearrangement (1,5-hydrogen shifts)that happens in the reaction is not detectable.

Thus, a deuterated compound is used in the last reaction to detect the rearrangement type in the sigmatropic rearrangement reaction.