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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 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 (CH
4) 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, C
2O
2 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 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 (C
2H
2O
2) 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.)
References:
- Linus Pauling, "The Chemical Bond," Cornell University Press (Ithaca, 1967), via Amazon.
- Andrew R. Dixon, Tian Xue and Andrei Sanov, "Spectroscopy of Ethylenedione," Angewandte Chemie International Edition, Early View, June 18, 2015, doi: 10.1002/anie.201503423.
- Daniel Stolte, "UA Researchers Reveal Elusive Molecule," University of Arizona Press Release, July 13, 2015.
Permanent Link to this article
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