Electrocyclic Ring Opening of The Cyclobutene Radical Cation

The electrocyclic ring opening of cyclobutene to give 1, 3-butadiene is a well-understood chemical isomerization. The process occurs with a lower than anticipated activation energy presumably because it is an “allowed” reaction. In order to ascertain whether the electrocyclic ring opening occurs for the gas-phase cyclobutene radical cation, we have undertaken a comparative study of the chemical properties of ionized cyclobutene and 1, 3-butadiene. Both ions exhibit nearly identical chemical reactivities with a variety of neutral molecules including methyl vinyl ether, furan, ethene, propene, isomeric butenes, and isomeric pentenes. The explanation proposed to account for these results is that the cyclobutene ion rapidly undergoes (within less than 2 ms) an electrocyclic isomerization to give the 1, 3-butadiene ion. The maximum activation energy for this process is estimated to be less than 7 kcal/mol. The chemical properties of the molecular ions of 1, 2-butadiene, 1-butyne, and 2-butyne were determined using the same reactant neutral molecules, and they differ considerably from the 1, 3-butadiene and cyclobutene ions. The results are interpreted to exclude other isomeric [C₄H₆⁺.] species in the electrocyclic reaction.