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

January 31, 2013

Secrets of a university tutor: unravelling a mechanism using spectroscopy.

Filed under: Uncategorised — Tags: , , , — Henry Rzepa @ 11:41 am

It is always rewarding when one comes across a problem in chemistry that can be solved using a continuous stream of rules and logical inferences from them. The example below[1] is one I have been using as a tutor in organic chemistry for a few years now, and I share it here. It takes around 50 minutes to unravel with students.



  1. K. Harano, M. Eto, K. Ono, K. Misaka, and T. Hisano, "Sequential pericyclic reactions of unsaturated xanthates. One-pot synthesis of hydrobenzo[c]thiophenes", Journal of the Chemical Society, Perkin Transactions 1, pp. 299, 1993. http://dx.doi.org/10.1039/P19930000299

December 27, 2010

Do marauding electrons go in packs?

Filed under: Interesting chemistry — Tags: , , — Henry Rzepa @ 9:24 pm

Is there a preferred pack size for electrons on the move? Or put less flamboyantly, is there an optimum, and a maximum number of arrows (electron pairs) that one might push in revealing the mechanism of a concerted reaction? A sort of village-instinct for electrons. Consider the following (known, DOI: 10.1016/S0040-4039(00)98289-3) reaction


December 16, 2010

Following one’s nose: a quadruple bond to carbon. Surely I must be joking!

Filed under: Hypervalency,Interesting chemistry — Tags: , , , , — Henry Rzepa @ 11:16 pm

Do you fancy a story going from simplicity to complexity, if not absurdity, in three easy steps? Read on! The following problem appears in one of our (past) examination questions in introductory organic chemistry. From relatively mundane beginnings, one can rapidly find oneself in very unexpected territory.


December 1, 2010

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

Filed under: Interesting chemistry — Tags: , , , , — Henry Rzepa @ 4:48 pm

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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