Archive for the ‘Curly arrows’ Category

A curly-arrow pushing manual

Wednesday, December 4th, 2013
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I have several times used arrow pushing on these blogs. But since the rules for this convention appear to be largely informal, and there appears to be no definitive statement of them, I thought I would try to produce this for our students. This effort is here shared on my blog. It is what I refer to as the standard version; an advanced version is in preparation. Such formality might come as a surprise to some; arrow-pushing is often regarded as far too approximate to succumb to any definition, although it is of course often examined.

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

Sunday, August 25th, 2013
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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

Mechanistic arrow pushing. A proposed addition to its rules.

Wednesday, June 12th, 2013
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A little while ago, I set out some interpretations of how to push curly arrows. I also appreciate that some theoretically oriented colleagues regard the technique as neither useful nor in the least rigorous, whereas towards the other extreme many synthetically minded chemists view the ability to push a reasonable set of arrows for a proposed mechanism as of itself constituting evidence in its favour.[1] Like any language for expressing ideas, the tool needs a grammar (rules) and a vocabulary, and perhaps also an ability to carry ambiguity. These thoughts surfaced again via a question asked of me by a student: “is the mechanism for the hydrogens in protonated benzene whizzing around the ring a [1,2] or a [1,6] pericyclic sigmatropic shift?”. 

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References

  1. M.J. Gomes, L.F. Pinto, P.M. Glória, H.S. Rzepa, S. Prabhakar, and A.M. Lobo, "N-heteroatom substitution effect in 3-aza-cope rearrangements", Chemistry Central Journal, vol. 7, pp. 94, 2013. http://dx.doi.org/10.1186/1752-153X-7-94

How is the bromination of alkenes best represented?

Sunday, October 14th, 2012
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I occasionally delve into the past I try to understand how we got to our present understanding of chemistry. Thus curly arrow mechanistic notation can be traced back to around 1924, with style that bifurcated into two common types used nowadays (on which I have commented and about which further historical light at the end of this post). Here I try to combine these themes with some analysis of wavefunctions for a particularly troublesome reaction to represent, the dibromination of an alkene, which I represented in the previous post as shown below.

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Curly arrow pushing: another reality check.

Sunday, August 5th, 2012
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Two years ago, I discussed how curly arrow pushing is taught, presenting four different ways of showing the arrows. One of the comments posted to that blog suggested that all of the schemes shown below were deficient in one aspect.

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The first curly arrows. The dénouement.

Monday, July 23rd, 2012
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Recollect, Robinson was trying to explain why the nitroso group appears to be an o/p director of aromatic electrophilic substitution. Using σ/π orthogonality, I suggested that the (first ever) curly arrows as he drew them could not be the complete story, and that a transition state analysis would be needed. Here it is. 

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The first ever curly arrows. And now for something completely different.

Saturday, July 21st, 2012
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The discussion appended to the post on curly arrows is continued here. Recollect the curly arrow diagram (in modern style) derived from Robinson’s original suggestion:

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The first curly arrows…lead to this?

Friday, July 20th, 2012
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Little did I imagine, when I discovered the original example of using curly arrows to express mechanism, that the molecule described there might be rather too anarchic to use in my introductory tutorials on organic chemistry. Why? It simply breaks the (it has to be said to some extent informal) rules! Consider the dimerisation of nitrosomethane (in fact a well-known equilibrium).

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The first ever curly arrows.

Friday, July 20th, 2012
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I was first taught curly arrow pushing in 1968, and have myself taught it to many a generation of student since. But the other day, I learnt something new. Nick Greeves was kind enough to send me this link to the origin of curly arrow pushing in organic chemistry, where the following diagram is shown and Alan Dronsfield sent me two articles he co-wrote on the topic (T. M. Brown, A. T. Dronsfield and P. J. T Morris, Education in Chemistry, 2001, 38, 102-104, 107 and 2003, 40, 129-134); thanks to both of them.

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Shared space (in science).

Friday, January 6th, 2012
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I thought I would launch the 2012 edition of this blog by writing about shared space. If you have not come across it before, it is (to quote Wikipedia), “an urban design concept aimed at integrated use of public spaces.” The BBC here in the UK ran a feature on it recently, and prominent in examples of shared space in the UK was Exhibition Road. I note this here on the blog since it is about 100m from my office.

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