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.[cite]10.1186/1752-153X-7-94[/cite] 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?”.
Posts Tagged ‘Tutorial material’
Mechanistic arrow pushing. A proposed addition to its rules.
Wednesday, June 12th, 2013Secrets of a university lecturer: 1981-1983.
Thursday, June 6th, 2013Many 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.
Transition states for the (base) catalysed ring opening of propene epoxide.
Wednesday, May 8th, 2013The 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.
How to predict the regioselectivity of epoxide ring opening.
Sunday, April 28th, 2013I 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.[cite]10.1021/ja01208a047[/cite]
Feist’s acid. Stereochemistry galore.
Thursday, April 4th, 2013Back 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[cite]10.1021/ct300359s[/cite], 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[cite]10.1021/ja00747a019[/cite]. I thought I might here unpick some of the wonderfully subtle stereochemical analysis that this little molecule became subjected to.
The mechanism of ester hydrolysis via alkyl oxygen cleavage under a quantum microscope
Tuesday, April 2nd, 2013A sideways look at the mechanism of ester hydrolysis.
Friday, March 29th, 2013The 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.
The mysterious (aromatic) structure of n-Butyl lithium.
Sunday, March 17th, 2013n-Butyl lithium is hexameric in the solid state[cite]10.1002/anie.199305801[/cite] and in cyclohexane solutions. Why? Here I try to find out some of its secrets.