Photochemical Cycloaddition ([2+2])

2 CH2=CH2 → Cyclobutane

概述

UV light promotes one alkene to its excited state, allowing a symmetry-forbidden [2+2] cycloaddition that cannot occur thermally according to the Woodward–Hoffmann rules. The product is a cyclobutane ring. This is a textbook example of how photochemistry enables reactions impossible by thermal pathways.

参与者

角色 物质 系数 状态
反应物 Ethylene C₂H₄ 2 (g)

日常示例

UV-induced [2+2] cycloaddition between adjacent thymine bases in DNA creates cyclobutane pyrimidine dimers, the main cause of sunburn.

工业重要性

[2+2]光环加成用于合成天然产物（如β-内酰胺类抗生素）和光刻胶材料，在药物合成和半导体制造中均有应用。

属性

类型
Photochemical
可逆
否
能量
放热
ΔH
-109.0 kJ/mol

相关反应

Photochemical Smog Formation (NO₂ Photolysis) Photochromism of Spiropyran Chlorophyll Fluorescence Photodegradation of Methylene Blue by TiO₂ Photodissociation of Chlorine Gas Silver Bromide Photodecomposition Cis-Trans Isomerization of Retinal Photosynthesis Ozone Formation by UV Light Dye-Sensitized Solar Cell (Grätzel Cell)

Frequently Asked Questions

What is the equation for Photochemical Cycloaddition ([2+2])?
The balanced equation is: 2 CH₂=CH₂ → Cyclobutane.
What type of reaction is Photochemical Cycloaddition ([2+2])?
Photochemical Cycloaddition ([2+2]) is a photochemical reaction.
Is Photochemical Cycloaddition ([2+2]) exothermic or endothermic?
Photochemical Cycloaddition ([2+2]) is exothermic (releases energy). The enthalpy change (ΔH) is -109.0 kJ/mol.