Q46E - Organic Chemistry (Mcmurry)

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.

Step-by-Step Solution

Answer

Structure of the isomer formed 

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

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

Step 1: 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.

Step 2: 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