Archive for the ‘Interesting chemistry’ Category
Wednesday, March 12th, 2014
My previous post related to the aromatic electrophilic substitution of benzene using as electrophile phenyl diazonium chloride. Another prototypical reaction, and again one where benzene is too inactive for the reaction to occur easily, is the catalyst-free bromination of benzene to give bromobenzene and HBr.
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Tags:activation energy, animation, aromatic, Boris Galabov, co-author, electrophilic, lowest energy solution, o/p director of aromatic electrophilic substitution, Paul Schleyer, pence, substitution
Posted in Interesting chemistry, reaction mechanism | 8 Comments »
Sunday, February 9th, 2014
This is the time of year when I deliver two back-2-back lecture courses, and yes I do update and revise the content! I am always on the look-out for nice new examples that illustrate how concepts and patterns in chemistry can be joined up to tell a good story. My attention is currently on conformational analysis; and here is an interesting new story to tell about it.
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Posted in General, Interesting chemistry | 2 Comments »
Friday, December 6th, 2013
Text books will announce that during aromatic electrophilic substitution, aromaticity is lost by the formation of a Wheland intermediate (and regained by eliminating a proton). Is that entirely true?
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Tags:actual energy
Posted in Interesting chemistry | 25 Comments »
Saturday, November 23rd, 2013
Mercury (IV) tetrafluoride attracted much interest when it was reported in 2007[1] as the first instance of the metal being induced to act as a proper transition element (utilising d-electrons for bonding) rather than a post-transition main group metal (utilising just s-electrons) for which the HgF2 dihalide would be more normal (“Is mercury now a transition element?”[2]). Perhaps this is the modern equivalent of transmutation! Well, now we have new speculation about how to induce the same sort of behaviour for caesium; might it form CsF3 (at high pressures) rather than the CsF we would be more familiar with.[3] Here I report some further calculations inspired by this report.
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References
- X. Wang, L. Andrews, S. Riedel, and M. Kaupp, "Mercury Is a Transition Metal: The First Experimental Evidence for HgF<sub>4</sub>", Angewandte Chemie International Edition, vol. 46, pp. 8371-8375, 2007. https://doi.org/10.1002/anie.200703710
- M. Miao, "Caesium in high oxidation states and as a p-block element", Nature Chemistry, vol. 5, pp. 846-852, 2013. https://doi.org/10.1038/nchem.1754
Tags:animation, energy, free energy barrier, free energy change, metal, pence, post-transition main group metal, potential energy surface
Posted in Hypervalency, Interesting chemistry | 13 Comments »
Saturday, October 26th, 2013
The 1H NMR spectrum of an aromatic molecule such as benzene is iconic; one learns that the unusual chemical shift of the protons (~δ 7-8 ppm) is due to their deshielding by a diatropic ring current resulting from the circulation of six aromatic π-electrons following the Hückel 4n+2 rule. But rather less well-known is the spectacular inversion of these effects as induced by the paratropic circulation of 4n electrons. A 4n+2 rule can be converted to a 4n one by the addition of two electrons, and chemically this can be done by reduction with lithium metal to form a dianion. Fortunately, this experiment has been done for a molecule known as methano[10]annulene. This is a 4n+2 aromatic molecule 1 with ten π-electrons (n=2) that can be reduced with lithium metal to form an ion-pair 2 comprising lithium cations and the twelve π-electron (4n, n=3) methano[10]annulene dianion.[1]
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References
- D. Schmalz, and H. Günther, "1,6‐Methano[10]annulene Dianion, a Paratropic 12π‐Electron Dianion with a C<sub>10</sub> Perimeter", Angewandte Chemie International Edition in English, vol. 27, pp. 1692-1693, 1988. https://doi.org/10.1002/anie.198816921
Tags:chemical shifts, lithium metal, unusual chemical shift
Posted in Interesting chemistry | No Comments »