Tikalon Blog by Dev Gualtieri

Elusive Ethylenedione

August 17, 2015

Ball-and-stick models of molecules are useful aids to understanding the different arrangement possibilities of atoms. Linus Pauling (1901-1994) was a major proponent of such model building, which was put to exquisite use by James Watson (b. 1928) and Francis Crick (1916-2004) in discovering the molecular structure of DNA. The central ideas in this model-building are that atoms and their ions have particular sizes, and the distance between them when they bond is likewise predictable.

Linus Pauling and the cover of an abridged copy of his Chemical Bond book

Linus Pauling with some ball-and-stick models, and the cover of an abridged copy of his Chemical Bond book.[1] (A scan of the cover of my copy of the book, and a Pauling photo from the Library of Congress, via Wikimedia Commons.)

It's easy to place sticks between ball models of atoms when they have a single bond, since it's obvious where to place the stick. Atomic carbon has four bonding orbitals equally distributed in space along the axes of a tetrahedron, and it's easy to model methane (CH₄) in which there's a single bond to a hydrogen atom in each of these directions.

When we move to a molecule like ethylene (CH2=CH2) with a double bond between carbon atoms, deciding how the bonds connect is more difficult (see figure). I wrote about multiple carbon-carbon bond in a previous article (Carbyne, August 28, 2013).

Diagram of an ethylene molecule, which belongs to the D2h point group.

(Via Wikimedia Commons.)

The carbon-carbon bonds can "bend" this way to form ethylene through orbital hybridization in which the usual atomic orbitals mix to create valence bonds of different shapes. Orbital hybridization is one of the many ideas of chemical bonding developed by Linus Pauling.

Once we've demonstrated that we can form a carbon-carbon double bond in ethylene, how hard could it be to to replace the two singly-bonded hydrogens at each carbon with a double bond to something like oxygen? This would create the molecule, ethylenedione, also called ethylene dione and ethene 1,2-dione, C₂O₂ or O=C=C=O, as shown in the figure. The synthesis of this molecule has been elusive, and there has never been evidence for its existence, even as a transient compound, in organic-oxygen mixtures. It's suspected that this compound is a short-lived intermediate in some chemical reactions, and it might have a role in atmospheric chemistry.

Two views of C₂O₂, a ball-and-stick model, left, and a space-filling model, right. (left image and right image, via Wikimedia Commons.)

Ethylenedione has a long history. It was first conjectured to exist in 1913, and it was claimed to be the active ingredient of the patent medicine, Glyoxylide, in the 1940s.[3] The US Food and Drug Administration found that this drug was merely distilled water with no beneficial effects, and sales of the drug were discontinued.[3] Now scientists at the University of Arizona have created and observed ethylenedione as a short-lived species using anion photoelectron spectroscopy.[2-3]

Says Andrei Sanov, a professor in the University of Arizona Department of Chemistry and Biochemistry, who conducted this research with doctoral student, Andrew Dixon,

"We are not talking about some complex compound here... This is a small molecule with only four atoms and an 'obvious' structure. Shouldn't modern science be able to tackle it?"[3]

Prof. Andrei Sanov of the University of Arizona at blackboard

Prof. Andrei Sanov of the University of Arizona sketches the details of ethylenedione on a blackboard.

(University of Arizona photo by John de Dios.)

In their synthesis of ethylenedione, the research team decided to use glyoxal (C₂H₂O₂) as a precursor. This chemical has been avoided for such synthesis, since it has a high water content in its reagent form. Dixon discovered a molecular sieve capable of stripping away the water. Furthermore, instead of working with neutral species, they started with negatively-charged ions.[3] Then, using laser pulses to strip away the electron from the anion, they formed ethylenedione.[3]

The molecule exists for just about half a nanosecond. Says Sanov, "This seems very short by human standards, but is in fact a long lifetime in the molecular realm."[3] After this brief time, the ethylenedione, which is effectively a diradical, splits into two carbon monoxide molecules (CO).[3]

As Sanov explains, radicals are "molecules with unpaired electrons that are 'underemployed' and looking for action. This means that they are eager to react, because the making and breaking of chemical bonds is controlled by electrons. A radical is a molecule that has one such 'underemployed' electron. A diradical has two."[3]

Ethylenedione might be important to atmospheric chemistry. Says Dixon,

"Given that glyoxal, its precursor, is a known pollutant and byproduct of combustion processes, whether man-made or natural, and given that OCCO seems to be a trivial molecule to create in our methodology, it is possible that it too could result from such processes, which, if true, could make it an unknown player in the atmosphere."[3]

This research was funded by National Science Foundation.

Many research laboratories have a painted concrete block motif.

This is the spectroscopy lab of Andrei Sanov at the University of Arizona.

(University of Arizona photo by John de Dios.)

Elusive Ethylenedione

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