Archive for August, 2012
Digital repositories. An update to the update.
Monday, August 13th, 2012Dynamic effects in nucleophilic substitution at trigonal carbon (with Na+) revisited.
Monday, August 13th, 2012This reaction looks simple but is deceptively complex. To recapitulate: tolyl thiolate (X=Na) reacts with the dichlorobutenone to give two substitution products in a 81:19 ratio, a result that Singleton and Bogle argue arises from a statistical distribution of dynamic trajectories bifurcating out of a single transition state favouring 2 over 3. On the grounds (presumably) that the presence of both the cation X (=Na+) and H-bonded solvent (ethanol) are uninfluential, neither species was explicitly included in the transition state model used to derive the dynamics. I speculated whether in fact the spatial distribution of counterions and solvent (set up by explicit hydrogen bonds and O…Na+ interactions) might in fact be perturbed from un-influential randomness by co-ordination to the carbonyl group present in the system. I also raised the issue of what the origin of the electronic effects leading to the major product might be.
The stereochemical origins of the Wittig reaction.
Tuesday, August 7th, 2012This is another of those textbook reactions, involving reaction of a carbonyl compound with a phosphonium ylid to form an alkene and a phosphine oxide. The reaction continues to be frequently used, in part because it can be highly stereospecific.
The Curtius rearrangement. One step or two?
Monday, August 6th, 2012The Curtius reaction is represented in most chemistry texts and notes as following path (a) below. It is one of a general class of thermally induced rearrangement which might be described as elimination/migration (in a sense similar to this ring contraction migration/elimination), in this case implicating a nitrene intermediate if the two steps occur consecutively. Wikipedia is normally very much on the ball with this sort of thing, and a comment about the reaction mechanism there notes that current evidence prefers route (b), avoiding nitrene intermediacy (and hence formally removing this from examples of nitrene reactions).
Curly arrow pushing: another reality check.
Sunday, August 5th, 2012Two 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.