Ethenium

Ethenium (C
2H+
5)

In chemistry, ethenium or protonated ethylene 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 carbonium ion.

Preparation

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]

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

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.

References

1. Margaret French and Paul Kebarle (1975), "Pyrolysis of C
   2H+
   7 and other ion-molecule reactions in methane containing traces of ethane". Canadian Journal of Chemistry, volume 53, pages 2268-2274. doi:10.1139/v75-318
2. G. I. Mackay, H. I. Schiff, D. K. Bohme (1981), "A room-temperature study of the kinetics and energetics for the protonation of ethane" Canadian Journal of Chemistry, volume 59, issue 12,pages 1771-1778. doi:10.1139/v81-265
3. F. H. Field , M. S. B. Munson (1965), "Reactions of gaseous ions. XIV. Mass spectrometric studies of methane at pressures to 2 Torr". Journal of the American Chemical Society, volume 87, issue 15, pages 3289–3294 doi:10.1021/ja01093a001