Odd-Even Crystallography News
by David Bradley
Just when chemists thought they knew everything there is to know about the basics, something new has turned up. It seems that there are more organic compounds with an even number of carbon atoms than odd, which is very odd especially as the discoverers have absolutely no idea why.
Gautam Desiraju of the University of Hyderabad was planning a lecture on the subject of polymorphic materials - ones with more than one structural form - and asked a colleague, Jagarlapudi Sarma of the Indian Institute of Chemical Technology, also in Hyderabad, to classify those in the Cambridge Crystallographic Database based on carbon count. Within an hour they had spotted a weird pattern - there were more compounds with even numbers of carbon atoms than odd.
Statistically this should not happen - why would there be any more likelihood of even over odd? The expected pattern would be 50:50 just as with tossing a coin. Desiraju at first suspected it was an artefact of the small (a mere 150000!) number of compounds in the CCDC database. More than 12 million organic compounds have been recorded by the Chemical Abstracts Service [It now stands at more than 18 million as of April 1999 - DB] and they thought there could not possibly be a bias of this sort once a significant number of compounds were tallied.
Within the hour they were in contact with Swiss chemist Jack Dunitz for whom Sarma had worked, with what they thought would be an absurd suggestion. They asked Dunitz to count the carbons in almost 7 million compounds held in the Beilstein database to which he had direct access.
Dunitz who is an organic chemist at the Swiss Federal Institute of Technology, ETH Zurich was intrigued and he and colleague Engelbert Zass set about trawling and tallying this massive database. Amazingly, they came up with the same result as Desiraju: that there are more compounds with even carbon counts than odd.
At this point Desiraju roped in another colleague Ashwini Nagia - a biosynthesis expert and they began looking for an explanation, they fought with all kinds of theories but they all failed to bear the weight of scrutiny. Desiraju even explained the problem to his twelve year old son, Govind, who had just learned about atoms and molecules at school, but he too was at a loss for an explanation.
The chemists sent a letter to Nature in the hope that in publicising the discovery someone might come up with an idea. That was the end of 1996 - nothing has turned up so far...we will keep you posted.
This item first appeared in Issue 3 of Elemental Discoveries during Autumn 1996