In my previous post I speculated why bis(trifluoromethyl) ketone tends to fully form a hydrate when dissolved in water, but acetone does not. Here I turn to asking why formaldehyde is also 80% converted to methanediol in water? Could it be that again, the diol is somehow preferentially stabilised compared to the carbonyl precursor and if so, why?
Archive for the ‘Interesting chemistry’ Category
Spotting the unexpected. The hydration of formaldehyde.
Monday, March 12th, 2012Spotting the unexpected. The trifluoromeric effect in the hydration of the carbonyl group.
Friday, March 9th, 2012The equilibrium for the hydration of a ketone to form a gem-diol hydrate is known to be highly sensitive to substituents. Consider the two equilibria:
An orbital analysis of the stereochemistry of the E2 elimination reaction
Saturday, February 4th, 2012The so-called E2 elimination mechanism is another one of those mainstays of organic chemistry. It is important because it introduces the principle that anti-periplanarity of the reacting atoms is favoured over other orientations such as the syn-periplanar form; Barton used this principle to great effect in developing the theory of conformational analysis. Here I explore its origins. (more…)
The “shocking” Xe-Au bond.
Saturday, January 21st, 2012Chemistry rarely makes it to the cover of popular science magazines. Thus when this week, the New Scientist ran the headline “Forbidden chemistry. Reactions they said could never happen“, I was naturally intrigued. The examples included Woodward and Hoffmann’s “symmetry-forbidden” reactions, which have been the subject of several posts here already. But in the section on nobel gas chemistry, the same Hoffmann is reported as having been shocked to hear of a compound of xenon and gold, both of which in their time were thought of as solidly inert, and therefore even more unlikely to form a union.
Secrets of a university tutor: tetrahedral intermediates.
Sunday, January 8th, 2012The tetrahedral intermediate is one of those iconic species on which the foundation of reaction mechanism in organic chemistry is built. It refers to a (normally undetected and hence merely inferred) species formed initially when a nucleophilic reagent attacks a carbonyl compound. Its importance to understanding the activity of enzymes cannot be overstated. An example of this genre is shown below, in which a thiol reacts with an acyl cyanide to form the species ringed in green.
Violations. There are none! Part 2.
Monday, December 26th, 2011I left the story of the molecule below on the precipice of a cliff. I had shaved off the four benzo groups (blue) in the time honoured computational tradition of clearing away distractions. Unfortunately, it became clear as the story unfolded that the benzo groups had a distractingly critical role to play, and so its time to start adding them back again, but in stages.
To recapitulate, this reaction (with four benzo groups) has a half life of ~30 minutes at 353K, giving it a a free energy barrier of ~26.2 kcal/mol. According to Woodward and Hoffmann, it is a “forbidden” reaction (4n, n=2, electrons require an antarafacial component from somewhere). Shorn of the benzo groups, such a component developed in one of the forming phenyl rings (forming a Möbius benzene). But, by adding two benzo groups back to this phenyl ring (turning it into an anthracene), we will stop this process in its tracks. (more…)
Quadruple antarafacial delight.
Sunday, December 18th, 2011A feature of many a classic review article is that not only does it organise and rationalise existing literature, but it will predict new chemistry as well. I have already noted Woodward and Hoffmann’s (WH) review as achieving the former, and here I take a (sideways) look at one of their predictions. (more…)
Molecular gymnastics in 2+2 cycloadditions. Two different moves compared.
Thursday, December 15th, 2011The previous post showed how the 2+2 cycloaddition of an alkene could occur by a sort of sideways insinuation of the bonds. I have also shown how the same reaction can occur with a dramatic rotation of one of the double bonds. This post compares the two moves side by side.
Molecular gymnastics in 2+2 cycloadditions.
Wednesday, December 14th, 2011In this earlier post, I described how the stereochemistry of π2+π2 cycloadditions occurs suprafacially if induced by light, and how one antarafacial component appears if the reaction is induced by heat alone. I also noted how Woodward and Hoffmann (WH) explained that violations to their rules were avoided by mandating a change in mechanism requiring stepwise pathways with intermediates along the route. Here I illustrate how the stereochemistry of a thermal π2+π2 cycloaddition can indeed avoid an antarafacial component by performing appropriate gymnastic contortions instead of a mechanistic change (a WH violation certainly in the letter of their law, if not their spirit).
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Mechanistic morphemes. Perisolvolysis of a cyclopropyl chloride.
Tuesday, December 13th, 2011There are many treasures in Woodward and Hoffmann’s (WH) classic monograph. One such is acetolysis of the endo chloride (green), which is much much faster than that of the exo isomer (red). The explanation given in their article (p 805) confines itself to succinctly stating that only loss of the endo halogen can be concerted with a required disrotatory ring opening of the cyclopropane. Demonstrating the truth of this statement by computational modelling turns out to be an interesting challenge. (more…)