Photochemical Cycloaddition ([2+2])

2 CH₂=CH₂ → Cyclobutane

Aperçu

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.

Participants

Rôle	Substance	Coefficient	État
Réactif	Ethylene C₂H₄	2	(g)

Exemple du quotidien

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

Importance industrielle

[2+2] photocycloaddition is used in organic synthesis to construct strained ring systems and in the synthesis of natural products and pharmaceuticals.

Propriétés

Type: Photochemical
Réversible: Non
Énergie: Exothermique
ΔH: -109,0 kJ/mol

Réactions associées

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.