Question

1-Phenyl-2-butene has an ultraviolet absorption ${\mathbf{\lambda }}_{\max }\mathbf{=}\mathbf{208}\mathbf{nm}$($\mathbf{\epsilon }\mathbf{=}\mathbf{8000}$). On treatment with a small amount of strong acid, isomerization occurs and a new substance with ${\mathbf{\lambda }}_{\max }\mathbf{=}\mathbf{250}\mathbf{nm}$($\mathbf{\epsilon }\mathbf{=}\mathbf{15}\mathbf{,}\mathbf{8000}$) is formed. Propose a structure for this isomer, and suggest a mechanism for its formation.

Short answer

Structure of the isomer formed

1-phenyl-2-butene 1-phenyl-1butene

The mechanism for formation of 1-phenyl-1-butene

Step-by-step solution

Ultraviolet absorption

Ultraviolet absorption spectroscopy is used in analytical chemistry for the quantitative determination of analytes or samples and for the highly conjugated organic compounds.

Structure for isomer

In 1-phenyl-2-butene, the alkene double bond is protonated to yield an intermediate carbocation, which loses a proton to give a product in which the double bond is conjugated with the aromatic ring, as shown by the increased value of${\mathrm{\lambda }}_{\max }$.

1-phenyl-2-butene 1-phenyl-1butene

The mechanism for formation of 1-phenyl-1-butene

Structure of the isomer formed