Friday, May 18, 2007

My Small Contribution to Compounds with Pretty Colours


It was the summer of 1984. I was in the first year of my graduate work and I was cleaning up a project initiated by a previous graduate student. I had come along at the end of his run and picked up the SNS+ reactions with pi bonds and had started with alkynes. I had prepared a variety of compounds using the SNS+ / alkyne reaction and they had worked well. My supervisor decided that I should go to England to learn how to do cyclic voltametry in liquid sulphur dioxide. That brought me into the sphere of a fragile, dedicated chemist of the collegial old school. It was in Durham that I was trying to do anion exchange reactions to get rid of the fluorinated anions so that the cyclic voltametry would work better (in fact the CV never did "work"). One day, late in the afternoon, I would wash a solution of the cation into a solution of potassium iodide and to my wondering eyes the whole flask filled with a pale blue gas. In my small area I knew all that had come before me and I knew that I had discovered something new. I was seeing something important for the first time and it was important and new not only for me but for my whole discipline. My real research career had begun.

It is still unique in my area of chemistry.

The product of the reaction was a radical, traditionally an unstable, reactive family of compounds with only a fleeting existence. In this case the heterocycle was stabilized by extensive pi delocalization and the carbon atoms were shielded and electronically changed by attachment to electronegative, kinetically stable CF3 groups. This also meant that the melting point of 12 degrees C resulted in a liquid with a significant vapour pressure at room temperature. This resulted in the blue gas.

I remember when I did the melting point. I had laboriously prepared the cation salt and reduced with huge excess of sodium dithionite and then fractionally distilled the volatile products three times to remove solvent and bi-products (which I would discover later to be the result of facile photolysis reactions). Then when I had grams of pure dark green black liquid I froze it and slowly warmed the solid in a Dewar until I reached the melting point. What
I was not ready for was the high volatility of the solid near its melting point and soon I had huge shiny blue-black crystals growing perpendicular from the side of the flask. They were altogether some of the most beautiful things that I had ever seen. I would spend a whole year of my life preparing and characterizing this compound. The last problem to fall was the precipitation of the photolytic impurities using a method I had stumbled across in an early JACS paper written by Speed Marvel in which he noted that he had precipitated polysulphide impurities with a catalytic addition of amine. I found a whisper of triethyl amine did the trick just fine.

And that is my coloured compound story.

6 comments:

Ψ*Ψ said...

Cool! Is the picture one of yours, from long ago?

Liberal Arts Chemist said...

Yeah, back in 1985 I had pretty much photolysed the last best sample that I had in a heroic attempt to determine the vapour pressure of the liquid and solid when the word came that a picture of the blue gas was needed the next day for an important conference in Europe. In one day I pushed a new preparation through and did the reduction and fractional distillation of the bits and scraps of material that I had available. In the meantime I took a virgin Pyrex flask and attached a new rotoflow tap to the bottom and sealed off the neck to make a collection tube (that is why it has the weird shape). I knew that the very process of placing the carefully prepared liquid in the flask and exposing it to light would destroy the compound but it needed to be done. With the sample prepared, the University photographer (who was a somewhat eccentric natural light purist) insisted that on a cold day we take the flask outside. I kept the flask warm inside my shirt with the sample tube exposed so that the liquid would remain in the tube and the flask would be filled with the blue gas. Then when the photographer said he was ready I whipped the flask out, clamped it and the picture was taken. If you look closely you can see there is a reflection of the Biology Building on the surface of the flask. Thus the only picture of the blue gas was done. There would be no more.

tomas said...

This is quite astonishing. I know that monomeric nitroso compounds are gorgeously blue but I have never seen a flask filled with blue vapors of an organic radical. Please what is the structure?

On the lab safety-related bull subject - each institution is different, I suggest that you change the school, to go somewhere where EH&S department is not run by trolls.

Uncle Al said...

Bloody Hell, post the chemistry! Aside from the cheat of nitrogen-based radicals, what colored organic gases do we have? Diazomethane and butanedione. Inquiring minds want blue!

Chemgeek said...

10+ years ago, when I synthesized the world's first perfluorinated porphyrin (to my knowledge), 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrin my young, untenured advisor asked me what color it was.

I said, "purple."

He said, "no, it's 'tenure-purple'."

Sadly, I have no pictures.

nestor said...
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For a while it was all about research and then it was all about teaching and now it's all about trying to find a balance while teaching at a small liberal arts and science university.