Posts Tagged ‘free energy’
Monday, May 20th, 2013
Sometimes the originators of seminal theories in chemistry write a personal and anecdotal account of their work. Niels Bohr[1] was one such
and four decades later Robert Woodward wrote “The conservation of orbital symmetry” (Chem. Soc. Special Publications (Aromaticity), 1967, 21, 217-249; it is not online and so no doi can be given). Much interesting chemistry is described there, but (like Bohr in his article), Woodward lists no citations at the end, merely giving attributions by name. Thus the following chemistry (p 236 of this article) is attributed to a Professor Fonken, and goes as follows (excluding the structure in red):
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References
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N. Bohr, "Der Bau der Atome und die physikalischen und chemischen Eigenschaften der Elemente", Zeitschrift f�r Physik, vol. 9, pp. 1-67, 1922. http://dx.doi.org/10.1007/BF01326955
Tags: electrocyclic, energy, final product, free energy, Gerhard Fonken, Historical, Niels Bohr, pericyclic, professor, Reaction Mechanism, Robert Woodward, Woodward
Posted in Uncategorized | 3 Comments »
Wednesday, May 15th, 2013
In the preceding post, I introduced Dewar’s π-complex theory for alkene-metal compounds, outlining the molecular orbital analysis he presented, in which the filled π-MO of the alkene donates into a Ag+ empty metal orbital and back-donation occurs from a filled metal orbital into the alkene π* MO. Here I play a little “what if” game with this scenario to see what one can learn from doing so.
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Tags: African Union, alkene-metal compounds, empty metal orbital, energy, filled metal orbital, free energy, Historical, lower energy form, metal
Posted in Hypervalency, Interesting chemistry | No Comments »
Monday, May 6th, 2013
In a previous post on the topic, I remarked how the regiospecific ethanolysis of propene epoxide[1] could be quickly and simply rationalised by inspecting the localized NBO orbital calculated for either the neutral or the protonated epoxide. This is an application of Hammond’s postulate[[2] in extrapolating the properties of a reactant to its reaction transition state. This approach implies that for acid-catalysed hydrolysis, the fully protonated epoxide is a good model for the subsequent transition state. But is this true? Can this postulate be tested? Here goes.
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References
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H.C. Chitwood, and B.T. Freure, "", Journal of the American Chemical Society, vol. 68, pp. 680-683, 1946. http://dx.doi.org/10.1021/ja01208a047
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G.S. Hammond, "", Journal of the American Chemical Society, vol. 77, pp. 334-338, 1955. http://dx.doi.org/10.1021/ja01607a027
Tags: CF 3 CO, free energy, good model for the subsequent transition state, lower free energy barrier, Reaction Mechanism
Posted in Interesting chemistry | 1 Comment »
Friday, March 29th, 2013
The mechanism of ester hydrolysis is a staple of examination questions in organic chemistry. To get a good grade, one might have to reproduce something like the below. Here, I subject that answer to a reality check.
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Tags: ALSO, co-operative, energy, energy well, ester hydrolysis, free energy, Reaction Mechanism, shallow energy, solvation energy, Tutorial material
Posted in Uncategorized | 16 Comments »
Sunday, March 10th, 2013
I mentioned in the last post that one can try to predict the outcome of electrophilic aromatic substitution by approximating the properties of the transition state from those of either the reactant or the (presumed Wheland) intermediate by invoking Hammond’s postulate[1]. A third option is readily available nowadays; calculate the transition state directly. Here are the results of exploring this third variation.
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References
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G.S. Hammond, "", Journal of the American Chemical Society, vol. 77, pp. 334-338, 1955. http://dx.doi.org/10.1021/ja01607a027
Tags: free energy, Reaction Mechanism, Tutorial material
Posted in Uncategorized | 2 Comments »
Monday, December 24th, 2012
tpap[1], as it is affectionately known, is a ruthenium-based oxidant of primary alcohols to aldehydes discovered by Griffith and Ley. Whereas ruthenium tetroxide (RuO4) is a voracious oxidant[2], its radical anion countered by a tetra-propylammonium cation is considered a more moderate animal[3]. In this post, I want to try to use quantum mechanically derived energies as a pathfinder for exploring what might be going on (or a reality-check if you like).
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References
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S.V. Ley, J. Norman, W.P. Griffith, and S.P. Marsden, "
Tetrapropylammonium Perruthenate, Pr
4
N
+
RuO
4
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, TPAP: A Catalytic Oxidant for Organic Synthesis
", Synthesis, vol. 1994, pp. 639-666, 1994. http://dx.doi.org/10.1055/s-1994-25538
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D.G. Lee, U.A. Spitzer, J. Cleland, and M.E. Olson, "The oxidation of cyclobutanol by ruthenium tetroxide and sodium ruthenate", Canadian Journal of Chemistry, vol. 54, pp. 2124-2126, 1976. http://dx.doi.org/10.1139/v76-304
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D.G. Lee, Z. Wang, and W.D. Chandler, "Autocatalysis during the reduction of tetra-n-propylammonium perruthenate by 2-propanol", The Journal of Organic Chemistry, vol. 57, pp. 3276-3277, 1992. http://dx.doi.org/10.1021/jo00038a009
Tags: energy, free energy, low energy elimination, metal, react freq, Reaction Mechanism, RuO4+ ethanol, triplet state energy, Tutorial material
Posted in Interesting chemistry | No Comments »
Thursday, December 20th, 2012
I have written earlier about dihydrocostunolide, and how in 1963 Corey missed spotting the electronic origins of a key step in its synthesis.[1]. A nice juxtaposition to this failed opportunity relates to Woodward’s project at around the same time to synthesize vitamin B12. The step in the synthesis that caused him to ponder is shown below.
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References
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E.J. Corey, and A.G. Hortmann, "", Journal of the American Chemical Society, vol. 87, pp. 5736-5742, 1965. http://dx.doi.org/10.1021/ja00952a037
Tags: dispersion energy, free energy, Historical, pericyclic
Posted in Interesting chemistry | No Comments »
Sunday, December 9th, 2012
Sharpless epoxidation converts a prochiral allylic alcohol into the corresponding chiral epoxide with > 90% enantiomeric excess[1],[2]. Here is the first step in trying to explain how this magic is achieved.
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References
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J.M. Klunder, S.Y. Ko, and K.B. Sharpless, "Asymmetric epoxidation of allyl alcohol: efficient routes to homochiral .beta.-adrenergic blocking agents", The Journal of Organic Chemistry, vol. 51, pp. 3710-3712, 1986. http://dx.doi.org/10.1021/jo00369a032
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R.M. Hanson, and K.B. Sharpless, "Procedure for the catalytic asymmetric epoxidation of allylic alcohols in the presence of molecular sieves", The Journal of Organic Chemistry, vol. 51, pp. 1922-1925, 1986. http://dx.doi.org/10.1021/jo00360a058
Tags: animation, asymmetric epoxidation, Enantioselective, free energy, lower energy conformations, Reaction Mechanism, Tutorial material
Posted in Uncategorized | 1 Comment »
