Ethenium

In chemistry, ethenium, protonated ethylene or ethyl cation is a positive ion with the formula C
₂H⁺
₅. It can be viewed as a molecule of ethylene (C
₂H
₄) with one added proton (H⁺
), or a molecule of ethane (C
₂H
₆) minus one hydride ion (H⁻
). It is a carbocation; more specifically, a nonclassical carbocation.

Ethenium has been observed in rarefied gases subjected to radiation.^[1] Another preparation method is to react certain proton donors such as H⁺
₃, HeH⁺
, N
₂H⁺
, and N
₂OH⁺
with ethane at ambient temperature and pressures below 1 mmHg. (Other donors such as CH⁺
₅ and HCO⁺
form ethanium preferably to ethenium.)^[2]

At room temperature and in a rarefied methane atmosphere, ethanium slowly dissociates to ethenium and H
₂. The reaction is much faster at 90 °C.^[1]

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-C
₃H⁺
₇ and H
₂ is believed to be exothermic.^[3]

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.^{[clarification needed]}