Posts Tagged ‘arrow pushing’

The Graham reaction: Deciding upon a reasonable mechanism and curly arrow representation.

Monday, February 18th, 2019

Students learning organic chemistry are often asked in examinations and tutorials to devise the mechanisms (as represented by curly arrows) for the core corpus of important reactions, with the purpose of learning skills that allow them to go on to improvise mechanisms for new reactions. A common question asked by students is how should such mechanisms be presented in an exam in order to gain full credit? Alternatively, is there a single correct mechanism for any given reaction? To which the lecturer or tutor will often respond that any reasonable mechanism will receive such credit. The implication is that a mechanism is “reasonable” if it “follows the rules”. The rules are rarely declared fully, but seem to be part of the absorbed but often mysterious skill acquired in learning the subject. These rules also include those governing how the curly arrows should be drawn. Here I explore this topic using the Graham reaction.[1]

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References

  1. W.H. Graham, "The Halogenation of Amidines. I. Synthesis of 3-Halo- and Other Negatively Substituted Diazirines1", Journal of the American Chemical Society, vol. 87, pp. 4396-4397, 1965. http://dx.doi.org/10.1021/ja00947a040

Early “curly” (reaction) arrows. Those of Ingold in 1926.

Wednesday, August 22nd, 2018

In 2012, I wrote a story of the first ever reaction curly arrows, attributed to Robert Robinson in 1924. At the time there was a great rivalry between him and another UK chemist, Christopher Ingold, with the latter also asserting his claim for their use. As part of the move to White City a lot of bookshelves were cleared out from the old buildings in South Kensington, with the result that yesterday a colleague brought me a slim volume they had found entitled The Journal of the Imperial College Chemical Society (Volume 6). 

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Experimental evidence for “hidden intermediates”? Epoxidation of ethene by peracid.

Sunday, August 25th, 2013

The concept of a “hidden intermediate” in a reaction pathway has been promoted by Dieter Cremer[1] and much invoked on this blog. When I used this term in a recent article of ours[2], a referee tried to object, saying it was not in common use in chemistry. The term clearly has an image problem. A colleague recently sent me an article to read (thanks Chris!) about isotope effects in the epoxidation of ethene[3] and there I discovered a nice example of hidden intermediates which I share with you now.

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References

  1. E. Kraka, and D. Cremer, "Computational Analysis of the Mechanism of Chemical Reactions in Terms of Reaction Phases: Hidden Intermediates and Hidden Transition States", Accounts of Chemical Research, vol. 43, pp. 591-601, 2010. http://dx.doi.org/10.1021/ar900013p
  2. H.S. Rzepa, and C. Wentrup, "Mechanistic Diversity in Thermal Fragmentation Reactions: A Computational Exploration of CO and CO2 Extrusions from Five-Membered Rings", The Journal of Organic Chemistry, vol. 78, pp. 7565-7574, 2013. http://dx.doi.org/10.1021/jo401146k
  3. T. Koerner, H. Slebocka-Tilk, and R.S. Brown, "Experimental Investigation of the Primary and Secondary Deuterium Kinetic Isotope Effects for Epoxidation of Alkenes and Ethylene withm-Chloroperoxybenzoic Acid", The Journal of Organic Chemistry, vol. 64, pp. 196-201, 1999. http://dx.doi.org/10.1021/jo981652x

Combichem: an introductory example of the complexity of chemistry

Sunday, December 19th, 2010

Chemistry gets complex very rapidly. Consider the formula CH3NO as the topic for a tutorial in introductory chemistry. I challenge my group (of about 8 students) to draw as many different molecules as they can using exactly those atoms. I imply that perhaps each of them might find a different structure; this normally brings disbelieving expressions to their faces.

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Anatomy of an arrow-pushing tutorial: reducing a carboxylic acid.

Wednesday, December 1st, 2010

Arrow pushing (why never pulling?) is a technique learnt by all students of organic chemistry (inorganic chemistry seems exempt!). The rules are easily learnt (supposedly) and it can be used across a broad spectrum of mechanism. But, as one both becomes more experienced, and in time teaches the techniques oneself as a tutor, its subtle and nuanced character starts to dawn. An example of such a mechanism is illustrated below, and in this post I attempt to tease out some of these nuances.

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