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A few times, Clar was not right September 22, 2008

Posted by fetzthechemist in PAHs.

Erich Clar still carries a high reputation within the fields of PAH research. Rightfully so, for he worked in the area for over fifty years. He broke ground on syntheses and purification techniques. Remember that chromatography then was not much different than what Tswett had done, powered adsorbents that had to often be crushed, sieved, and activated all in-house; in glass columns with solvents like mineral spirits that were complex mixtures.

In the latter years of his career, Clar continued to spread the breadth of his work, including developing the reaction that bears his name. This is the condensation of the cyclic three-ring ketone perinaphthanone with powdered zinc, sodium chroride, and zinc chloride in a 400 D melt. It gives the seven-ring PAH dibenzo[cd,lm[perylene. Clar also used other cyclic ketones that contained four or five rings.

I found in the course of my work that certain things Clar found in these reactions were not quite right.

Benzanthrone is a four-ring cyclic ketone. Clar found five nine-ring isomers from it. But there is a sixth which is probably either the most or second-most abundant product. Two of the ketones react asymmetrically to form it. Why Clar found the others with lower abundance is a mystery. Even adsorption chromatography on acidic alumina, similar to what he used, gives a fraction rich in it. It behaves quite differently than three of the five he found and is much greater in abundance than the other two together that might have been isolated together.

In that reaction, he isolated two PAHs of ten rings, where two bridging rings are formed instead of only one. He reported UV spectra for both and these match those separated today. But one he claimed to be red. It is not, it is a yellow-orange. A simple error due to impurity? Not really, because Professor Clar knew the wavelength relationship of PAH colors and absorbance wavelengths. That is key in his annellation theory. THe UV should have told him there was a red impurity.

One error was one of the limit of his analytical capabilities. When naphthanthrone, a five-ring cyclic ketone, is reacted, five isomers should be formed. He found only four. When the reaction was repeated, but separated by HPLC, there are five isomers. The error was that his fourth fraction contained two isomers. Unfortunately, the wavelengths for one are perfectly buried under the other, more abundant isomer. Its series of highest-wavelength bands are spread with similar spacings as the other isomer, at around 25 nm apart. The highest band is also about 25 nm higher than that in the other isomer – a perfect overlap of the lesser isomer’s bands under the other’s, except for the last one of the higher-wavelegth isomer. The one clue he might have noticed was that the fluorescence emission of the mixture gives a more bluish green color than the bright green expected of the one isomer he reported.

The moral, as far as research, is that even the best of old research can be redone with new approaches and be interesting and discover new things.



1. Rhenium - October 27, 2008

I was trying to remember the rule for six membered PAH’s, and I was thinking “was it Half’s rule, Claf’s rule? Google isn’t finding it”.
So I remembered Fetzer the Chemist was a PAH guy, and low and behold, Clar’s Rule.

Thank you.


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