Monday, 19 November 2012

Chemical of the Week: Dicyanoacetylene

Welcome to the first Chemical of the Week! This week, we shall have a brief look at compound composed entirely of carbon and nitrogen: dicyanoacetylene.

Dicyanoacetylene, or carbon subnitride, is a fairly interesting compound. Structurally, it is completely linear, thanks to its alternating triple bonds. Ciganek and Krespan (1968) report that dicyanoacetylene was first synthesised by the scientists Moureu and Bongrand in 1909 and was achieved by dehydrating the bisamide of acetylenedicarboxylic acid, also known as 2-butynediamide. Graupner and Saunders (2008) report that dicyanoacetylene can be produced by heating 2-butynediamide with phosphorous pentoxide under vacuum.

At the time, this compound has been noted for its high reactivity, an aspect which is not surprising when one considers the presence of two electron-withdrawing groups (EWG's) at the terminal ends of this molecule (specifically, two cyano groups). Due to the presence of these electron-withdrawing groups and their attachment to an acetylene backbone, dicyanoacetylene is a potent dienophile, and of particular use in the Diels-Alder reaction. Indeed, it is so reactive that it is capable of reacting with dienes that are considered, due to low reactivity, poor candidates for the Diels-Alder reaction.

Ah, the things chemists find fascinating... 

Aside from the work of Moureu and Bongrand, there have been numerous methods proposed for the synthesis of dicyanoacetylene, with five proposed by Ciganek and Krespan back in 1968. Two reactions of note were found capable of producing dicyanoacetylene: the heating of nitrogen gas at 2500°C over graphite; and the gas-phase pyrolysis of a chemical with a rather cool structure (below), 4,5-dicyano-1,3-dithiol-2-one.

Told you it was a cool structure!

Though I'm sure I could happily go on a tangent regarding 4,5-dicyano-1,3-dithiol-2-one, dicyanoacetylene has a few more fascinating bits of information up its sleeve. Due to the presence of three triple covalent bonds in its structure, dicyanoacetylene is thermodynamically unstable, and under the right conditions will explode to form carbon powder and nitrogen gas. As such, when exploring the heat of combustion of dicyanoacetylene, Armstrong and Marantz (1960) analysed the results of earlier experiments and proposed that dicyanoacetylene will burn in an oxygen atmosphere and produce a flame with a temperature in excess of 5000K (4726.85°C).

Let me repeat that.

In theory, dicyanoacetylene will burn in an oxygen atmosphere and produce a flame with a temperature greater than 4700°C.

In reality dicyanoacetylene doesn't disappoint. In an oxygen atmosphere, it burns with an intense blue-white flame with a temperature of 5260K (4986°C)!. That is a truly incredible temperature.

Even more remarkable was that the work of Kirshenbaum and Grosse (1956), who used the experimentally determined heat of combustion of dicyanoacetylene in oxygen, in combination with the enthalpy data for carbon monoxide and molecular nitrogen, to discover that burning dicyanoacetylene in an atmosphere of ozone will increase the heat of combustion. Indeed, burning dicyanoacetylene in an atmosphere of ozone (at standard atmospheric pressure) will produce a flame with a temperature of 5516K (5242.85°C).

At this point I am tempted to make a joke about dicyanoacetylene being hot stuff, but I fear I would be burned in the process...

Continuing on, this fascinating molecule has also been found in the atmosphere of Titan, the largest moon of Saturn. Yung (1987), proposed new chemical schemes for the formation of both cyanogen and dicyanoacetylene in order to explain data obtained by the infrared spectra obtained by the Voyager 1 spacecraft. Detection of dicyanoacetylene, and other compounds, is of importance as it gives important information regarding the atmospheric chemistry of Titan.

Titan: Known hideout for dicyanoacetylene.
(Photo courtesy of NASA)
Of particular note is that the abundance of dicyanoacetylene in Titan's atmosphere varies based on the season. According to Samuelson et. al. (1997), in the spring of the northern hemisphere, the stratosphere cools, allowing dicyanoacetylene vapour to condense out of the atmosphere into lower, cooler regions that will be protected by the shadow of the cloud layers above. In the upper stratosphere, the progression of the season means increasing levels of sunlight, which breaks down dicyanoacetylene in a photolytic process. All in all, these variations in the concentration of dicyanoacetylene in Titan's atmosphere gives us a remarkable means of studying the weather on Titan.

So there you have it. A fascinating molecule that burns at an incredible temperature, and is also found in the far-flung reaches of our very own solar system.

Until next time,

1 comment:

  1. Very interesting article.

    When I first saw that incredibly high temperature of the adiabatic flame temperature of Dicyanoacetylene I was amazed. I tried to reproduce that result using my modified CEA code from NASA and I could only reproduce that result if only CO and N2 were considered in the products of a strictly Stoichiometry reaction, exactly as A. D. KIRSHENBAUM
    A. V GROSS did.

    Generally, many other species should be included, such as CO2, CN, NO etc. Calculating the adiabatic temperature in this much more realistic condition for the products composition, the result was a much lower maximum temperature of ~4077 K obtained when the oxid fuel weight ratio was ~1.

    Indeed a Dicyanoacetylene is hot, but not that hot as claimed by above mentioned reference. Their temperature estimate was at least 1000 K overestimated.