Ethenium

In chemistry, ethenium, protonated ethylene or ethyl cation is a positive ion with the formula C2H+ 5. It can be viewed as a molecule of ethylene (C2H4) with one added proton (H+), or a molecule of ethane (C2H6) minus one hydride ion (H−). It is a carbocation; more specifically, a nonclassical carbocation.

Preparation

Ethenium has been observed in rarefied gases subjected to radiation. Another preparation method is to react certain proton donors such as H+ 3, HeH+, N2H+, and N2OH+ with ethane at ambient temperature and pressures below 1 mmHg. (Other donors such as CH+ 5 and HCO+ form ethanium preferably to ethenium.)

At room temperature and in a rarefied methane atmosphere, ethanium slowly dissociates to ethenium and H2. The reaction is much faster at 90 °C.

Stability and reactions

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-C3H+ 7 and H2 is believed to be exothermic.

Structure

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 primary carbocations (carbocations with one alkyl group) achieve additional stabilization. Consequently, true primary carbocations (with a classical structure) may be rare or nonexistent.