Caged Hydrocarbons Hidden For Half A Century: Could Ethanoadamantanes Prove To Be Useful Molecular Indicators?
Hydrocarbons come in many shapes and sizes. Probably the most intriguing of these is caged structures. Some of these resemble diamonds and some are more like ice crystals. Not surprisingly, it is from this resemblance that diamondoids and iceanes take their names. The cage structure gives these hydrocarbons unusual properties such as high thermal stability and resistance to microbial attack. Some of these cage structures are now being used in the nanotech industry and in biomedicine.
Several years ago, while working with the Petroleum and Environmental Geochemistry Group at Plymouth University researching what compounds were present in the process-affected waters derived from the oil sands extraction industry of Alberta Canada, diamondoid acids were discovered to be hiding in the “hump” of compounds unresolved by gas chromatography. The oil extraction process concentrates polar compounds such as carboxylic acids in the wastewater. The discovery of diamondoid acids was made possible due to the superior revolving power of two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC×GC-TOFMS). Finding the diamondoid acids had been a surprising discovery, as the microbial metabolism of many of these structures had not previously been observed or even thought possible.
While looking in the chromatographic region between where the smallest diamondoid (adamantane, one cage unit, tricyclic) acids eluted and the next group (diamantane, two cage units, pentacyclic), another group of carboxylic acids (analyzed as methyl esters) were evident. These had mass spectra similar to iceane and consistent with iceane acids. Without standards, it wasn’t possible at the time to test this hypothesis, but the seed was sown for future research.
The opportunity to test this hypothesis came a few years later during research conducted by the Western Australian Organic and Isotope Geochemistry Centre. Gemma Spaak, then a Ph.D. student, had analyzed a large suite of Australian oils and condensates in order to better understand their origins and the relationships among them. Many of these were rich in diamondoid hydrocarbons, and Gemma had identified and quantified the known compounds. If iceane acids were in the oil sands’ wastewater, they must have been metabolic breakdown products of iceane hydrocarbons.
So, at this point, we thought we had a simple test. We looked in the chromatographic region where the iceanes should elute on GC×GC-TOFMS, and there they were. We also observed them as minor chromatographic peaks in a mixture of alkylated diamantanes obtained from the gas hydrate industry. It was then an easy task to semi-quantify the peaks. However, on a thorough reading of the literature, most of which was nearly 50 years old, we realized that iceane wasn’t the most thermodynamically stable structure for this size of hydrocarbon chemical formula.
The prospect of playing “Ice Ice Baby” by Vanilla Ice as the introduction to a conference talk sadly faded away. The core structure was much more likely to be ethanoadamantane, which sounds rather boring. This compound has an adamantane cage structure, but with an extra bridge giving it the same chemical formula as iceane: C12H18. There was only one way to confirm that we had ethanoadamantane, and that was to synthesize it.
Luckily, Curtin University has an excellent chemical synthesis group, and Shifaza Mohamed volunteered to give up some of her precious time to synthesize 2,4-ethanoadamantane. We could now compare the peak in our oils with the standard. When analyzed by GC×GC-TOFMS, the standard eluted at exactly the same retention times in two dimensions and had an identical mass spectrum. In science, nothing is 100%, but this was pretty conclusive evidence for ethanoadamantane and its alkyl-substituted homologs being present in the oils. By inference, this also strongly suggested that what had been thought to be iceane acids in the oil sands’ wastewater were actually ethanoadamantane acids.
In a separate and highly innovative study, Michael Wilde, a Ph.D. student at Plymouth University, had converted the oil sands acids into their corresponding hydrocarbons. This had also suggested the presence of a whole range of alkylated ethanoadamantane acids. The original reason for characterizing the oil sands wastewater was to identify what compounds were causing the reported toxic effects. Previous investigations by the Plymouth group had shown that diamondoid acids were of relatively low toxicity, although adamantane carboxylic acids had shown potential for in vivo genetic damage in gills and hemocytes of mussels. Based on the structure of the ethanoadamantane acids, it is not expected that they are any more toxic than the equivalent adamantane acids.
When analyzing oils and source rocks, diamondoid hydrocarbons can be used as molecular indicators to provide useful information about oil maturity and can be used to correlate samples. This is especially useful when sterane and hopane biomarkers are absent (biomarkers are essentially molecular fossils derived from formally living organisms). However, the low molecular weight adamantanes are quite volatile and so can be lost at any stage between when the oil or rock is collected and when it is injected into the mass spectrometer. This obviously can affect the ratios used to generate indices so researchers prefer to use the less volatile, higher weight diamantanes, but these are often in low abundance and, therefore, difficult to measure.
In our study, reported in Organic Geochemistry, it appeared that ethanoadamantanes were commonly present wherever diamondoids were found and that there were statistically significant relationships between ratios based on ethanoadamantanes and those traditionally used. Because methyl-substituted ethanoadamantanes are less volatile than the methyl-substituted adamantanes, they may serve as additional molecular indicators.
It is early days, and further studies will be needed to see how common these compounds are worldwide and how their presence in oils relate to their source rocks. The ethanoadamantanes might also prove useful in oil spill forensics. This study has built upon research spanning half a century. It has added to the toolbox of molecular indicators available in biogeochemical research and, by inference, has identified an additional class of carboxylic acids present in some process-affected waters of the oil industry.
These findings are described in the article entitled Comparison of tri-, tetra- and pentacyclic caged hydrocarbons in Australian crude oils and condensates, recently published in the journal Organic Geochemistry.