It is not often that an article on the topic of illusion and deception makes it into a chemical journal. Such is addressed (DOI: 10.1002/anie.201102210) in no less an eminent journal than Angew Chemie. The illusion (or deception if you will) actually goes to the heart of how we represent three-dimensional molecules in two dimensions, and the meanings that may be subverted by doing so. A it happens, it is also a recurring theme of this particular blog, which is the need to present chemistry with data for all three dimensions fully intact (hence the Click for 3D captions which often appear profusely here).
Buses (and dyotropic rearrangements) always come in threes.
June 12th, 2011The last two posts have played a game of find the electrons. We saw how the dyotropic rearrangement of ethane borrowed electrons from the C-C bond, and how 1,2,dibromoethane went ionic on us. How about this mixed system, in which a hydrogen and a BH2 swap their positions?
More is more: the dyotropic rearrangement of 1,2-dibromoethane.
June 12th, 2011In the previous post, I discussed what we could learn from ethane by forcing it into a pericyclic dyotropic rearrangement. We saw how it voraciously scavenged two electrons from the C-C bond to achieve this. What if we give it more electrons? Thus 1,2-dibromoethane undergoing the same reaction.
Less is more: the dyotropic rearrangement of ethane
June 11th, 2011In a time when large (molecules) are considered beautiful (or the corollary that beauty must be big), it is good to reflect that small molecules may teach us something as well. Take ethane. Is there anything left which has not been said about it already? Well, consider the reaction below, in which two hydrogen atoms mutually hop from one carbon to the other.
Hafnium and Niels Bohr
June 5th, 2011In 1923, Coster and von Hevesy[1] claimed discovery of the element Hafnium, atomic number 72 (latin Hafnia, meaning Copenhagen, where the authors worked) on the basis of six lines in its X-ray spectrum. The debate had long raged as to whether (undiscovered) element 72 belonged to the rare-earth group 3 of the periodic table below yttrium, or whether it should be placed in group 4 below zirconium. Establishing its chemical properties finally placed it in group 4. Why is this apparently arcane and obscure re-assignment historically significant? Because, in June 1922, in Göttingen, Niels Bohr had given a famous series of lectures now known as the Bohr Festspiele on the topic of his electron shell theory of the atom. Prior to giving these lectures he had submitted his collected thoughts in January 1922[2].
References
- D. COSTER, and G. HEVESY, "On the Missing Element of Atomic Number 72", Nature, vol. 111, pp. 79-79, 1923. https://doi.org/10.1038/111079a0
- N. Bohr, "Der Bau der Atome und die physikalischen und chemischen Eigenschaften der Elemente", Zeitschrift f�r Physik, vol. 9, pp. 1-67, 1922. https://doi.org/10.1007/bf01326955
Conformational restriction involving formyl CH…F hydrogen bonds.
May 31st, 2011The title of this post paraphrases E. J. Corey’s article in 1997 (DOI: 10.1016/S0040-4039(96)02248-4) which probed the origins of conformation restriction in aldehydes. The proposal was of (then) unusual hydrogen bonding between the O=C-H…F-B groups. Here I explore whether the NCI (non-covalent-interaction) method can be used to cast light on this famous example of how unusual interactions might mediate selectivity in organic reactions.
Déjà vu all over again. Are H…H interactions attractive or repulsive?
May 31st, 2011The Pirkle reagent is a 9-anthranyl derivative (X=OH, Y=CF3). The previous post on the topic had highlighted DIST1, the separation of the two hydrogen atoms shown below. The next question to ask is how general this feature is. Here we take a look at the distribution of lengths found in the Cambridge data base, and focus on another interesting example.
The inner secrets of an ion-pair: Isobornyl chloride rearrangements.
May 29th, 2011Observation of the slow racemization of isobornyl chloride in a polar solvent in 1923-24 by Meerwein led to the recognition that mechanistic interpretation is the key to understanding chemical reactivity. The hypothesis of ion pairs in which a chloride anion is partnered by a carbocation long ago entered the standard textbooks (see DOI 10.1021/ed800058c and 10.1021/jo100920e for background reading). But the intimate secrets of such ion-pairs are still perhaps not fully recognised. Here, to tease some of them them out, I use the NCI method, which has been the subject of several recent posts.
Blogs, Twitter, Wikis and other on-line tools: the movie!
May 27th, 2011Libraries (and librarians) are evolving rapidly. Thus a week or so ago one of our dynamic librarians here, approached some PhD students and academics to ask them how they used “Web 2.0” (thanks Jenny!). The result was edited (thanks John!) and uploaded, where you can see it below (embedded in this post, I might add, using HTML5). No doubt there is more of this genre to come. Libraries nowadays it seems, are not just about books and journals, but about the full digital experience (not to mention sustenance; ours is now one of the more popular places for students to eat!).