Archive for March, 2012

Confirming the Fischer convention as a structurally correct representation of absolute configuration.

Tuesday, March 13th, 2012

I wrote in an earlier post how Pauling’s Nobel prize-winning suggestion in February 1951 of a (left-handed) α-helical structure for proteins[1] was based on the wrong absolute configuration of the amino acids (hence his helix should really have been the right-handed enantiomer). This was most famously established a few months later by Bijvoet’s[2] definitive crystallographic determination of the absolute configuration of rubidium tartrate, published on August 18th, 1951 (there is no received date, but a preliminary communication of this result was made in April 1950). Well, a colleague (thanks Chris!) just wandered into my office and he drew my attention to an article by John Kirkwood[3] published in April 1952, but received July 20, 1951, carrying the assertion “The Fischer convention is confirmed as a structurally correct representation of absolute configuration“, and based on the two compounds 2,3-epoxybutane and 1,2-dichloropropane. Neither Bijvoet nor Kirkwood seem aware of the other’s work, which was based on crystallography for the first, and quantum computation for the second. Over the years, the first result has become the more famous, perhaps because Bijvoet’s result was mentioned early on by Watson and Crick in their own very famous 1953 publication of the helical structure of DNA. They do not mention Kirkwood’s result. Had they not been familiar with Bijvoet’s[2] result, their helix too might have turned out a left-handed one!

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References

  1. L. Pauling, R.B. Corey, and H.R. Branson, "The structure of proteins: Two hydrogen-bonded helical configurations of the polypeptide chain", Proceedings of the National Academy of Sciences, vol. 37, pp. 205-211, 1951. https://doi.org/10.1073/pnas.37.4.205
  2. J.M. BIJVOET, A.F. PEERDEMAN, and A.J. van BOMMEL, "Determination of the Absolute Configuration of Optically Active Compounds by Means of X-Rays", Nature, vol. 168, pp. 271-272, 1951. https://doi.org/10.1038/168271a0
  3. W.W. Wood, W. Fickett, and J.G. Kirkwood, "The Absolute Configuration of Optically Active Molecules", The Journal of Chemical Physics, vol. 20, pp. 561-568, 1952. https://doi.org/10.1063/1.1700491

Spotting the unexpected. The hydration of formaldehyde.

Monday, March 12th, 2012

In my previous post I speculated why bis(trifluoromethyl) ketone tends to fully form a hydrate when dissolved in water, but acetone does not. Here I turn to asking why formaldehyde is also 80% converted to methanediol in water? Could it be that again, the diol is somehow preferentially stabilised compared to the carbonyl precursor and if so, why?

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Spotting the unexpected. The trifluoromeric effect in the hydration of the carbonyl group.

Friday, March 9th, 2012

The equilibrium for the hydration of a ketone to form a gem-diol hydrate is known to be highly sensitive to substituents. Consider the two equilibria:

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