Physics:Ethenium

Not to be confused with Ethanium.

Ethenium (C₂H+5)

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 1° alkyl carbocations achieve additional stabilization. Consequently, true 1° carbocations (with a classical structure) may be rare or nonexistent.