Many university chemistry departments, and mine is no exception, like to invite applicants to our courses to show them around. Part of the activities on the day is an “interview” in which the candidate is given a chance to shine. Over the years, I have evolved questions about chemistry which can form the basis of discussion, and I thought I would share one such question here. It starts by my drawing on the blackboard (yes, I really still use one!) the following molecular structure.
Archive for the ‘Interesting chemistry’ Category
Secrets of a university admissions interviewer
Sunday, September 19th, 2010Solid carbon dioxide: hexacoordinate carbon?
Friday, September 17th, 2010Carbon dioxide is much in the news, not least because its atmospheric concentration is on the increase. How to sequester it and save the planet is a hot topic. Here I ponder its solid state structure, as a hint to its possible reactivity, and hence perhaps for clues as to how it might be captured. The structure was determined (DOI 10.1103/PhysRevB.65.104103) as shown below.
The oldest reaction mechanism: updated!
Tuesday, September 14th, 2010Unravelling reaction mechanisms is thought to be a 20th century phenomenon, coincident more or less with the development of electronic theories of chemistry. Hence electronic arrow pushing as a term. But here I argue that the true origin of this immensely powerful technique in chemistry goes back to the 19th century. In 1890, Henry Armstrong proposed what amounts to close to the modern mechanism for the process we now know as aromatic electrophilic substitution [1]. Beyond doubt, he invented what is now known as the Wheland Intermediate (about 50 years before Wheland wrote about it, and hence I argue here it should really be called the Armstrong/Wheland intermediate). This is illustrated (in modern style) along the top row of the diagram.
The mechanism of aromatic electrophilic substitution
References
- "Proceedings of the Chemical Society, Vol. 6, No. 85", Proceedings of the Chemical Society (London), vol. 6, pp. 95, 1890. http://dx.doi.org/10.1039/PL8900600095
Bio-renewable green polymers: Stereoinduction in poly(lactic acid)
Saturday, July 24th, 2010Lactide is a small molecule made from lactic acid, which is itself available in large quantities by harvesting plants rather than drilling for oil. Lactide can be turned into polymers with remarkable properties, which in turn degrade down easily back to lactic acid. A perfect bio-renewable material!
The weirdest bond of all? Laplacian isosurfaces for [1.1.1]Propellane.
Wednesday, July 21st, 2010In this post, I will take a look at what must be the most extraordinary small molecule ever made (especially given that it is merely a hydrocarbon). Its peculiarity is the region indicated by the dashed line below. Is it a bond? If so, what kind, given that it would exist sandwiched between two inverted carbon atoms?
Non-covalent interactions (NCI): revisiting Pirkle
Thursday, July 15th, 2010NCI (non-covalent interactions) is the name of a fascinating new technique for identifying exactly these. Published recently by Johnson, Keinan, Mori-Snchez, Contreras-Garca, Cohen and Yang, it came to my attention at a conference to celebrate the 20th birthday of ELF when Julia Contreras-Garcia talked about the procedure. It is one of those methods which may seem as if it merely teases out the obvious about a molecule, but it is surprising how difficult seeing the obvious can be sometimes. I have blogged about this previously, in discussing the so-called Pirkle reagent. On that occasion, I used the QTAIM technique to identify so-called critical points in the electron density. NCI goes one stage further in identifying surfaces of interaction rather than just single points, the idea being that this focuses attention on regions in molecules which are primarily responsible for binding, stereoselection and other aspects of molecular selectivity.
Tunable bonds looked at in a different way
Sunday, July 11th, 2010The title of this post merges those of the two previous ones. The tunable C-Cl bond brought about in the molecule tris(amino)chloromethane by anomeric effects will be probed using the Laplacian of the electronic density.
Looking at bonds in a different way: the Laplacian.
Tuesday, July 6th, 2010The Cheshire cat in Alice’s Adventures in Wonderland comes and goes at will, and engages Alice with baffling philosophical points. Chemical bonds are a bit like that too. In the previous post, we saw how (some) bonds can be tuned to be strong or weak simply by how a lone pair of electrons elsewhere in the molecule is oriented with respect to the bond. Here I explore another way of looking at bonds. To start, we must introduce a quantity known as ∇2ρ(r), henceforth termed the Laplacian of the electron density ρ(r).
Tunable bonds
Saturday, July 3rd, 2010Car transmissions come in two types, ones with fixed ratio gears, and ones which are continuously variable. When it comes to chemical bonds, we tend to think of them as being very much of the first type. Bonds come in fixed ratios; single, aromatic, double, triple, etc. OK, they do vary, but the variations are assumed as small perturbations on the basic form. Take for example the molecule shown below. The bonds as shown are all clearly single (the wedge and hashed bond are merely stereochemical notations). No-one would really think of drawing this molecule in any other way, and this idea of the transferability of bonds between molecules (all double bonds react in specific ways which are different from single bonds, and they also have characteristic spectroscopic properties, etc) is what allows molecules to be classified.