Henry Rzepa's Blog Chemistry with a twist

February 18, 2019

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

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]



  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

June 18, 2014

Anchoring chemistry.

I was reminded of this article by Michelle Francl[1], where she poses the question “What anchor values would most benefit students as they seek to hone their chemical intuition?” She gives as common examples: room temperature is 298.17K (actually 300K, but perhaps her climate is warmer than that of the UK!), the length of a carbon-carbon single bond, the atomic masses of the more common elements.



  1. M. Francl, "Take a number", Nature Chemistry, vol. 5, pp. 725-726, 2013. http://dx.doi.org/10.1038/nchem.1733

May 18, 2014

Benzene. As you have never seen it represented before!

Filed under: Historical — Tags: , , — Henry Rzepa @ 4:38 pm

Continuing my european visits, here are two photos from Bonn. First, a word about how the representation of benzene evolved, attributed to Kekulé.


December 19, 2010

Combichem: an introductory example of the complexity of chemistry

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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