Is science sometimes in danger of getting tunnel vision? Recently published ebook author, Ian Miller, looks at other possible theories arising from data that we think we understand. Can looking problems in a different light give scientists a different perspective?

The SN2 Reaction, And The Easter Bunny.

In the July edition of Chemistry World there was an item (p 20) on the SN2 reaction involving ballistic experiments on the reaction between hydroxide (with or without additional water molecules) and methyl iodide in helium. The results appeared to be that hydroxide plus methyl iodide by themselves simply led to ballistic outcomes. If one molecule of water could be incorporated, the iodide left on the trajectory consistent with the classic textbook SN2 result. If two molecules of water were incorporated, a longer-lived complex was formed that had a lifetime sufficiently long that the iodide could come out at any direction. That is unexceptional. However, the item ends with a statement: “the SN2 mechanism that undergrads are told is a fairy tale, up there with Santa Claus and the Easter bunny”. The commentator was surprised that any of the results supported the textbook version. So it appears that in volume 1 of my Elements of Theory, I joined the Easter bunny. Oh dear!
What can my defence possibly be? First,  I discussed the SN2 mechanism in an example of where science had not fired properly, in this case the so-called non-classical 2-norbornyl cation. In the 2-norbornyl system, leaving groups that are endo react second order to give exo products, which is the textbook SN2 reaction. However, exo leaving groups react significantly faster and give exo products, which shows that reaction is not simple SN2. There are clear reasons why the SN2 mechanism is unlikely to apply to these exo substituted molecules and the chapter pointed out that despite the extreme amount of work carried out on the 2-norbornyl system, the reason for the acceleration of the exo substituents remained unexplained. (The book has over seventy problems at the end; one was to find an explanation for the so-called non-classical ion, so for those who want an intellectual exercise, why not try your luck? As a clue, my answer relies on each side being partly correct, and each side correctly falsifying the other side in some respects. Given two Nobel prize-winners failed to reach a  conclusion over ten years, I rate this as one of the more difficult problems that I set.)
So, at the risk of being hammered again as something worse than an Easter bunny, I wish to point out that the information on the given experiments are quite consistent with the textbooks as I know them. First, there is no evidence whatsoever that there was no structural inversion (difficult to show with methyl iodide). The concept that there may be a small energy minimum on the reaction coordinate is expected if a transition state is stabilized so that it lies between two energy maxima, and if such a longer-lived intermediate can exist, the Uncertainty Principle requires that it has rotational uncertainty, let alone classical rotational motion from the collision. Finally, classical Debye-Huckel theory predicts stabilization of ionic intermediates from adjacent water molecules. There was nothing I copuld see in these experiments that is not in accord with the textbooks.
Actually, I agree with the commentator that the textbook discussion of the SN2 mechanism is an oversimplification, however my criticisms lie outside the scope of these experiments. Perhaps the subject of another blog.
Posted by Ian Miller on Jul 26, 2012 12:59 AM Europe/London

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