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[cite]10.1039/P19930000299[/cite] 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.
Posts Tagged ‘Tutorial material’
Secrets of a university tutor: unravelling a mechanism using spectroscopy.
Thursday, January 31st, 2013The strangely attractive conformation of C17H36.
Sunday, January 13th, 2013We tend to think of simple hydrocarbons as relatively inert and un-interesting molecules. However, a recent article[cite]10.1002/anie.201202894[/cite], which was in fact highlighted by Steve Bachrach on his blog , asks what “The Last Globally Stable Extended Alkane” might be. In other words, at what stage does a straight-chain hydrocarbon fold back upon itself, and no significant population of the linear form remain? The answer was suggested to be C17H36. I thought I might subject this conformation to an NCI (non-covalent-interaction) analysis.
A conflation of concepts: Conformation and pericyclic.
Thursday, January 10th, 2013This is an interesting result I got when studying the [1,4] sigmatropic rearrangement of heptamethylbicyclo-[3.1.0]hexenyl cations. It fits into the last lecture of a series on pericyclic mechanisms, and just before the first lecture on conformational analysis. This is how they join.
The gauche effect: seeking evidence by a survey of crystal structures.
Friday, January 4th, 2013
I previously blogged about anomeric effects involving π electrons as donors, and my post on the conformation of 1,2-difluorethane turned out one of the most popular. Here I thought I would present the results of searching the Cambridge crystal database for examples of the gauche effect. The basic search is defined below
Hydrogen bond strength as a function of ring size.
Thursday, January 3rd, 2013One frequently has to confront the question: will a hydrogen bond form between a suitable donor (lone pair or π) and an acceptor? One of the factors to be taken into consideration for hydrogen bonds which are part of a cycle is the ring size. Here I explore one way of quantifying the effect for the series below, n=1-5 (4-8 membered rings).
Sharpless epoxidation, enantioselectivity and conformational analysis.
Thursday, January 3rd, 2013I return to this reaction one more time. Trying to explain why it is enantioselective for the epoxide product poses peculiar difficulties. Most of the substituents can adopt one of several conformations, and some exploration of this conformational space is needed.
Why is the Sharpless epoxidation enantioselective? Part 1: a simple model.
Sunday, December 9th, 2012Sharpless epoxidation converts a prochiral allylic alcohol into the corresponding chiral epoxide with > 90% enantiomeric excess[cite]10.1021/jo00369a032[/cite],[cite]10.1021/jo00360a058[/cite]. Here is the first step in trying to explain how this magic is achieved.
The mechanism of the Birch reduction. Sequel to benzene reduction.
Wednesday, December 5th, 2012I noted briefly in discussing why Birch reduction of benzene gives 1,4-cyclohexadiene (diagram below) that the geometry of the end-stage pentadienyl anion was distorted in the presence of the sodium cation to favour this product. This distortion actually has some pedagogic value, and so I elaborate this here.