Dienes in acid

It often happens that the atoms in starting material and product cannot be correlated without some extra distinction being made by isotopic labelling. The isomerization of Z-l-phenylbutadiene to the E-diene in acid looks like a simple reaction. Protonation of the Z-alkene would give a stabilized secondaiy benzylic cation that should last long enough to rotate. Loss of the proton would then give the more stable E-diene. 

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