When an allyl cation undergoes a cycloaddition under photochemical conditions with ethylene (bonds forming at the ends of the π-systems),the cycloaddition is described as

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The cycloaddition of an allyl cation with ethylene under photochemical conditions, where bonds form at the ends of the π-systems, is described as [2+2] because two π bonds are involved from each reactant. It is suprafacial because the bonding occurs on the same face of the π systems involved in the reaction.

When an Allyl Cation Undergoes a Cycloaddition Under Photochemical Conditions