In chemistry, ethenium, protonated ethylene or ethyl cation is a positive ion with the formula . It can be viewed as a molecule of ethylene with one added proton, or a molecule of ethane minus one hydride ion . It is a carbocation; more specifically, a nonclassical carbocation.
Ethenium has been observed in rarefied gases subjected to radiation. Another preparation method is to react certain proton donors such as , , , and with ethane at ambient temperature and pressures below 1 mmHg. (Other donors such as and form ethanium preferably to ethenium.)
At room temperature and in a rarefied methane atmosphere, ethanium slowly dissociates to ethenium and . The reaction is much faster at 90 °C.
Contrary to some earlier reports, ethenium was found to be largely unreactive towards neutral methane at ambient temperature and low pressure (on the order of 1 mmHg), even though the reaction yielding sec- and is believed to be exothermic.
The structure of ethenium's ground state was in dispute for many years, but it was eventually agreed to be a non-classical structure, with the two carbon atoms and one of the hydrogen atoms forming a three-center two-electron bond. Calculations have shown that higher homologues, like the propyl and n-butyl cations also have bridged structures. Generally speaking, bridging appears to be a common means by which 1° alkyl carbocations achieve additional stabilization. Consequently, true 1° carbocations (with a classical structure) may be rare or nonexistent.