Sigmatropic hydrogen shifts

[1,7]H sigmatropic shift

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Click the structures and reaction arrows in sequence to view the 3D models and animations respectively

In thermal reactions, [1,5]H shifts occur suprafacially but [1,3]H and [1,7]H shifts must be antarafacial. Antarafacial [1,3]H shifts are impossible because a rigid 3-carbon chain is too short to allow the H atom to transfer from the top to the bottom. For [1,7]H shifts the much longer chain is flexible enough to allow the transfer.

The hydrogen atom leaves the top side of the triene and adds back in on the bottom side.