Question

1-Phenyl-2-butene has an ultraviolet absorption at \(\lambda_{\max }=208 \mathrm{nm}\) \((\varepsilon=8000) .\) On treatment with a small amount of strong acid, isomerization occurs and a new substance with \(\lambda_{\max }=250 \mathrm{nm}(\varepsilon=15,800)\) is formed. Propose a structure for this isomer, and suggest a mechanism for its formation.

Short answer

The isomer is likely 1-phenyl-1,3-butadiene, formed via carbocation rearrangement under acidic conditions.

Step-by-step solution

Analyze the Original Structure

1-Phenyl-2-butene is an alkene where a phenyl group is attached to the second carbon of the butene chain. The original absorption at \( \lambda_{\max} = 208 \text{ nm} \) indicates that the double bond and the phenyl group are involved in conjugation.

Identify Isomerization Changes

On treatment with acid, the UV absorption shifts to \( \lambda_{\max} = 250 \text{ nm} \), suggesting increased conjugation in the system, likely due to the formation of a new double bond alignment.

Propose New Structure

A probable structure for the isomer is 1-phenyl-1,3-butadiene, where the conjugation extends across a longer section of the molecule, influencing the larger \( \lambda_{\max} \) value.

Suggest a Mechanism

Under acidic conditions, the carbocation intermediate rearranges such that it forms a more delocalized system. Protonation at the original double bond may lead to a shift, allowing for the creation of the more stable conjugated diene structure.

Conclude with Observations

The shift in \( \lambda_{\max} \) and \( \varepsilon \) implies a transition from localized to extended conjugation, which supports the proposed isomer structure and mechanism.

Key concepts

Isomerization

Isomerization refers to the process by which a molecule is transformed into another molecule with the same molecular formula but a different structural arrangement. In the case of 1-phenyl-2-butene, exposure to a strong acid facilitates this transformation. The UV-Vis absorption shift from \( \lambda_{\max} = 208 \, \text{nm} \) to \( \lambda_{\max} = 250 \, \text{nm} \) hints at changes in the conjugation patterns, signifying that isomerization has occurred. The strong acid likely catalyzes the rearrangement of the double bonds in the alkene, creating a new alignment that enhances conjugation within the molecule.
This transformation is crucial as it leads to different physical and chemical properties, which can be detected through UV-Vis spectroscopy as a change in the wavelength of maximum absorption.

Conjugation

Conjugation in organic chemistry describes the overlap of p-orbitals across adjacent atoms, which results in delocalization of electrons. This delocalization affects the stability and reactivity of molecules, and it is also detectable via spectroscopy. In 1-phenyl-2-butene, initial conjugation involves the double bond and the attached phenyl group, resulting in a UV-Vis absorption at \( \lambda_{\max} = 208 \, \text{nm} \).
Following isomerization, a more extensive conjugated system is formed, which accounts for the shift to a longer wavelength of \( \lambda_{\max} = 250 \, \text{nm} \). This is because the extended conjugation allows for lower energy transitions, explaining the higher \( \lambda_{\max} \).

• Extended conjugation leads to increased stability of the molecule.
• The presence of conjugation can influence the reactivity of the compounds.

Conjugation's impact on wavelength and absorptivity provides insight into structural changes at the molecular level.

Carbocation Rearrangement

Carbocation rearrangement is a key mechanism in organic reactions, particularly under acidic conditions. When 1-phenyl-2-butene is treated with a strong acid, protonation occurs, creating a carbocation intermediate at the site of initial double bond.
This carbocation is inherently unstable but can rearrange to form a more stable structure. In this particular scenario, the reorganization allows the carbocation to align with an extended conjugation system, facilitating the shift in UV-Vis absorption to \( \lambda_{\max} = 250 \, \text{nm} \).
The stability of carbocations often drives rearrangements, with the molecule altering its structure to stabilize the positive charge over a larger area, or by transitioning into a molecule with lower energy.

• Rearrangement can lead to more substituted and thus more stable carbocations.
• It greatly influences the final structure and properties of the molecule.

Understanding carbocation rearrangement helps in predicting and explaining the structural outcomes of reactions like isomerization.