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 (C
2H+
5)

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. 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.

References
  1. Margaret French and Paul Kebarle (1975), "Pyrolysis of C
    2    H+
    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
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