chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, ethenium, protonated ethylene or ethyl cation is a positive ion with the formula . It can be viewed as a molecule of

ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...

() with one added proton (), or a molecule of

ethane Ethane ( , ) is a naturally occurring Organic compound, organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is List of purification methods ...

() minus one hydride ion (). It is a

carbocation Carbocation is a general term for ions with a positively charged carbon atom. In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further ...

; 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 , , , and with ethane at ambient temperature and pressures below 1 mmHg. (Other donors such as and form ethanium preferably to ethenium.) At room temperature and in a rarefied methane atmosphere, ethanium slowly dissociates to ethenium and . 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''- and is believed to be

exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...

Structure

The structure of ethenium's

ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the 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

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

References

{{Reflist, refs= Margaret French and Paul Kebarle (1975), "Pyrolysis of {{chem, 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 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 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 Carbocations Physical chemistry