Posts Tagged ‘Tutorial material’
Wednesday, June 12th, 2013
A little while ago, I set out some interpretations of how to push curly arrows. I also appreciate that some theoretically oriented colleagues regard the technique as neither useful nor in the least rigorous, whereas towards the other extreme many synthetically minded chemists view the ability to push a reasonable set of arrows for a proposed mechanism as of itself constituting evidence in its favour.[1] Like any language for expressing ideas, the tool needs a grammar (rules) and a vocabulary, and perhaps also an ability to carry ambiguity. These thoughts surfaced again via a question asked of me by a student: “is the mechanism for the hydrogens in protonated benzene whizzing around the ring a [1,2] or a [1,6] pericyclic sigmatropic shift?”.
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References
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M.J. Gomes, L.F. Pinto, P.M. Glória, H.S. Rzepa, S. Prabhakar, and A.M. Lobo, "N-heteroatom substitution effect in 3-aza-cope rearrangements", Chemistry Central Journal, vol. 7, 2013. http://dx.doi.org/10.1186/1752-153X-7-94
Tags:first arrow-head, Reaction Mechanism, Tutorial material
Posted in Curly arrows | 8 Comments »
Thursday, June 6th, 2013
Many moons ago, when I was a young(ish) lecturer, and much closer in time to my laboratory roots of organic synthesis, I made some chemistry videos. One of these has resurfaced, somewhat (to me at least) unexpectedly. Nowadays of course, such demonstrations are all carried out using virtual simulations (Flash animations etc) as the equipment itself becomes less common.
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Tags:Flash, lecturer, Tutorial material
Posted in Uncategorized | 1 Comment »
Wednesday, May 29th, 2013
This is a follow-up to comment posted by Ryan, who asked about isocyanide’s role (in the form of the anion of tosyl isocyanide, or TosMIC): “In Van Leusen, it (the isocyanide) acts as an electrophile”. The Wikipedia article (recently updated by myself) shows nucleophilic attack by an oxy-anion on the carbon of the C≡N group, with the isocyanide group acting as the acceptor of these electrons (in other words, the electrophile). In the form shown below, one negatively charged atom appears to be attacking another also carrying a negative charge. Surely this breaks the rules that like charges repel? So we shall investigate to see if this really happens.
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Tags:low energy, low free-energy barrier, Reaction Mechanism, Tutorial material
Posted in Uncategorized | 2 Comments »
Wednesday, May 8th, 2013
The previous post described how the acid catalysed ring opening of propene epoxide by an alcohol (methanol) is preceded by pre-protonation of the epoxide oxygen to form a “hidden intermediate” on the concerted intrinsic reaction pathway to ring opening. Here I take a look at the mechanism where a strong base is present, modelled by tetramethyl ammonium methoxide (R4N+.–OMe), for the two isomers R=Me; R’=Me, R”=H and R’=H, R”=Me.
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Tags:Reaction Mechanism, Tutorial material
Posted in Uncategorized | No Comments »
Sunday, April 28th, 2013
I recently got an email from a student asking about the best way of rationalising epoxide ring opening using some form of molecule orbitals. This reminded me of the famous experiment involving propene epoxide.[1]
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References
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H.C. Chitwood, and B.T. Freure, "The Reaction of Propylene Oxide with Alcohols", Journal of the American Chemical Society, vol. 68, pp. 680-683, 1946. http://dx.doi.org/10.1021/ja01208a047
Tags:10.1021, energy, lowest energy, predominant product, Reaction Mechanism, Tutorial material
Posted in Interesting chemistry | No Comments »
Thursday, April 4th, 2013
Back in the days (1893) when few compounds were known, new ones could end up being named after the discoverer. Thus Feist is known for the compound bearing his name; the 2,3 carboxylic acid of methylenecyclopropane (1, with Me replaced by CO2H). Compound 1 itself nowadays is used to calibrate chiroptical calculations[1], which is what brought it to my attention. But about four decades ago, and now largely forgotten, both 1 and the dicarboxylic acid were famous for the following rearrangement that gives a mixture of 2 and 3[2]. I thought I might here unpick some of the wonderfully subtle stereochemical analysis that this little molecule became subjected to.
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References
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E.D. Hedegård, F. Jensen, and J. Kongsted, "Basis Set Recommendations for DFT Calculations of Gas-Phase Optical Rotation at Different Wavelengths", Journal of Chemical Theory and Computation, vol. 8, pp. 4425-4433, 2012. http://dx.doi.org/10.1021/ct300359s
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J.J. Gajewski, "Hydrocarbon thermal degenerate rearrangements. IV. Stereochemistry of the methylenecyclopropane self-interconversion. Chiral and achiral intermediates", Journal of the American Chemical Society, vol. 93, pp. 4450-4458, 1971. http://dx.doi.org/10.1021/ja00747a019
Tags:chemical synthesis, chemical transformations, lower energy triplet state, Reaction Mechanism, rearrangement products, Tutorial material
Posted in Interesting chemistry | 2 Comments »
Thursday, April 4th, 2013
Back in the days (1893) when few compounds were known, new ones could end up being named after the discoverer. Thus Feist is known for the compound bearing his name; the 2,3 carboxylic acid of methylenecyclopropane (1, with Me replaced by CO2H). Compound 1 itself nowadays is used to calibrate chiroptical calculations[1], which is what brought it to my attention. But about four decades ago, and now largely forgotten, both 1 and the dicarboxylic acid were famous for the following rearrangement that gives a mixture of 2 and 3[2]. I thought I might here unpick some of the wonderfully subtle stereochemical analysis that this little molecule became subjected to.
(more…)
References
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E.D. Hedegård, F. Jensen, and J. Kongsted, "Basis Set Recommendations for DFT Calculations of Gas-Phase Optical Rotation at Different Wavelengths", Journal of Chemical Theory and Computation, vol. 8, pp. 4425-4433, 2012. http://dx.doi.org/10.1021/ct300359s
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J.J. Gajewski, "Hydrocarbon thermal degenerate rearrangements. IV. Stereochemistry of the methylenecyclopropane self-interconversion. Chiral and achiral intermediates", Journal of the American Chemical Society, vol. 93, pp. 4450-4458, 1971. http://dx.doi.org/10.1021/ja00747a019
Tags:chemical synthesis, chemical transformations, lower energy triplet state, Reaction Mechanism, rearrangement products, Tutorial material
Posted in Interesting chemistry | 2 Comments »
Tuesday, April 2nd, 2013
My previous dissection of the mechanism for ester hydrolysis dealt with the acyl-oxygen cleavage route (red bond). There is a much rarer[1] alternative: alkyl-oxygen cleavage (green bond) which I now place under the microscope.
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References
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C.A. Bunton, and J.L. Wood, "Tracer studies on ester hydrolysis. Part II. The acid hydrolysis of tert.-butyl acetate", Journal of the Chemical Society (Resumed), pp. 1522, 1955. http://dx.doi.org/10.1039/jr9550001522
Tags:acetic acid, analogous energy, energy, lower energy route, Reaction Mechanism, Tutorial material
Posted in Uncategorized | 3 Comments »
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 | 17 Comments »