The previous post explored why E2 elimination reactions occur with an antiperiplanar geometry for the transition state. Here I have tweaked the initial reactant to make the overall reaction exothermic rather than endothermic as it was before. The change is startling. The exothermicity is of course due to the aromatisation of the ring. The IRC is however quite different from before. IRC for E2 elimination.

The 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.

In the previous post, I went over how a reaction can be stripped down to basic components. That exercise was essentially a flat one in two dimensions, establishing only what connections between atoms are made or broken. Here we look at the three dimensional arrangements. It all boils down to identifying what the stereochemistry of the bonds marked with a wavy line are. To make it simple, let us start with the molecule shown on the left.

Its a bit like a jigsaw puzzle in reverse, finding out to disassemble a chemical reaction into the pieces it is made from, and learning the rules that such reaction jigsaws follow. The following takes about 45-50 minutes to follow through with a group of students.

Chemistry 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.

The 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.

I thought I would launch the 2012 edition of this blog by writing about shared space . If you have not come across it before, it is (to quote Wikipedia), “an urban design concept aimed at integrated use of public spaces.” The BBC here in the UK ran a feature on it recently, and prominent in examples of shared space in the UK was Exhibition Road. I note this here on the blog since it is about 100m from my office.

I 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.

A 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.

The 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. 0.0 5.3 kcal/mol The forbidden dance The allowed dance. As is sometimes the case in real life, the forbidden option has the lower activation barrier!

In 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.