Archive for May, 2012
Monday, May 28th, 2012
Streptomycin is an antibiotic active against tuberculosis, and its discovery has become something of a cause célèbre. It was first isolated on October 19, 1943 by a graduate student Albert Schatz in the laboratory of Selman Waksman at Rutgers University. I want to concentrate in this post on its molecular structure. Its initial isolation was followed by an extraordinarily concentrated period of about three years devoted to identifying that structure, culminating in a review of this chemistry in 1948 by Lemieux and Wolfram.[1] This review presents the structure as shown below (left). The modern rendering on the right is based on a crystal structure done in 1978.[2]‡
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References
- R. Lemieux, and M. Wolfrom, "The Chemistry of Streptomycin", Advances in Carbohydrate Chemistry, pp. 337-384, 1948. https://doi.org/10.1016/s0096-5332(08)60034-x
- "The crystal and molecular structure of streptomycin oxime selenate tetrahydrate", Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences, vol. 359, pp. 365-388, 1978. https://doi.org/10.1098/rspa.1978.0047
Tags:Albert Schatz, candidate for its formula, Historical, laboratory of Selman Waksman, MS, muse, pence, Rutgers University, tuberculosis, X-ray
Posted in Interesting chemistry | 1 Comment »
Friday, May 25th, 2012
If you have not previously visited, take a look at Nick Greeves’ ChemTube3D , an ever-expanding gallery of reactions and their mechanisms. The 3D is because all molecules are offered with X, Y and z coordinates. You also get arrow pushing‡ in 3D. Here, I argue that we should adopt Einstein, and go to the space-time continuum! By this, I mean one must also include the order in which things happen. To my knowledge, no compendium of (organic) reaction mechanisms incorporates this 4th dimension. My prelude to this post nicely illustrated this latter aspect. Here I continue with an exploration of the mechanism of forming an acyl chloride from a carboxylic acid using thionyl chloride. The mechanism shown at ChemTube3D is as below and will now be tested for its reasonableness using quantum mechanics.
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Tags:acetic acid, Reaction Mechanism, Tutorial material
Posted in Uncategorised | 3 Comments »
Saturday, May 19th, 2012
Many reaction mechanisms involve a combination of bond formation/cleavage between two non-hydrogen atoms and those involving reorganisation of proximate hydrogens. The Baeyer-Villiger discussed previously illustrated a complex dance between the two types. Here I take a look at another such mechanism, the methylation of a carboxylic acid by diazomethane.
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Tags:e-books, Tutorial material
Posted in General | 1 Comment »
Friday, May 18th, 2012
Text books (is this a misnomer, much like “papers” are in journals?) in a higher-educational chemistry environment, I feel, are at a cross-roads. What happens next?
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Tags:author, Bob Hanson, energy, GBP, iPads, PDF, skilful author, Skolnik, Steve Job, Steve Jobs, tablet devices, textbook author, Tutorial material, USD
Posted in General | 3 Comments »
Monday, May 7th, 2012
The mechanism of the reaction of alkenes known as ozonolysis was first set out in its modern form by Criegee. The crucial steps, (a), (b) and (d), are all pericyclic cycloaddition/eliminations. The last step (e) is known as reductive ozonolysis, and this step is often treated as an afterthought, part of the work-up of the reaction if you like (it is not illustrated in Criegee’s review for example). Here, I will attempt to show that it is actually a very interesting mechanistic step.
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Tags:200th post, Historical, pericyclic, S bridge
Posted in Interesting chemistry | 4 Comments »
Monday, May 7th, 2012
The Baeyer-Villiger rearrangement was named after its discoverers, who in 1899 described the transformation of menthone into the corresponding lactone using Caro’s acid (peroxysulfuric acid). The mechanism is described in all text books of organic chemistry as involving an alkyl migration. Here I take a look at the scheme described by Alvarez-Idaboy, Reyes and Mora-Diez[1], and which may well not yet have made it to all the text books!
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References
- J.R. Alvarez-Idaboy, L. Reyes, and N. Mora-Diez, "The mechanism of the Baeyer–Villiger rearrangement: quantum chemistry and TST study supported by experimental kinetic data", Organic & Biomolecular Chemistry, vol. 5, pp. 3682, 2007. https://doi.org/10.1039/b712608e
Tags:final product, free energy, Historical, stereoelectronic, Tutorial material, X5
Posted in reaction mechanism | 6 Comments »