There is often a disconnect between how a text-book (schematically) represents a reaction and a more quantitive “reality” revealed by quantum mechanics. Is the bromination of ethene to give 1,2-dibromoethane one such example? Text-books will show how ethene interacts with bromine to form a cyclic bromonium cation, which with the liberated bromide anion makes for an ion-pair.

The conformational analysis of cyclohexane is a mainstay of organic chemistry. Is there anything new that can be said about it? Let us start with the diagram below: This identifies the start of the process as a chair conformation of cyclohexane, with D _3d symmetry.

Metathesis reactions are a series of catalysed transformations which transpose the atoms in alkenes or alkynes.

Alkene metathesis is part of a new generation of synthetic reaction in which a double C=C bond is formed from appropriate reactants where no bond initially exists (another example is the Wittig reaction), with the involvement ^† of a 4-membered-ring metallacyclobutane ring 1 (again, very similar to the Wittig). I thought it might make a good addition to my collection of reaction mechanisms and so as the first step

This is the follow-up to the previous post exploring a typical nucleophilic addition-elimination reaction. Here is the elimination step, which as before requires proton transfers. We again adopt a cyclic mechanism to try to avoid the build up of charge separation during those proton movements. Elimination step to form an oxime. Click for animation of reaction mode.

The mechanism of forming an oxime from nucleophilic addition of a hydroxylamine to a ketone is taught early on in most courses of organic chemistry. Here I subject the first step of this reaction to form a tetrahedral intermediate to quantum mechanical scrutiny. The first decision is to decide which atom of the hydroxylamine acts as the nucleophile. Reaction 1 shows the oxygen and reaction 2 the nitrogen.

Semibullvalene is a molecule which undergoes a facile [3,3] sigmatropic shift. So facile that it appears this equilibrium can be frozen out at the transition state if suitable substituents are used. This is a six-electron process, which leads to one of those homologous questions; what happens with ten electrons? A 5,5 double Möbius sigmatropic rearrangement. Click for 3D model.

The four-electron thermal cycloaddition (in reverse a cheletropic elimination) of dichlorocarbene to ethene is a classic example of a forbidden pericyclic process taking a roundabout route to avoid directly violating the Woodward-Hoffmann rules.

I am following up on one unfinished thread in my previous post; a candidate was proposed in which the transition state for [3,3] sigmatropic rearrangement in a semibullvalene might be frozen out to become instead a stable minimum. The hypothesis was that such a species would be aromatic, a bis -homoaromatic species to be precise, itself an interesting variation on the normal theme of aromaticity.

Semibullvalene is an unsettling molecule. Whilst it has a classical structure describable by a combination of Lewis-style two electron and four electron bonds, its NMR behaviour reveals it to be highly fluxional. This means that even at low temperatures, the position of these two-electron bonds rapidly shifts in the equilibrium shown below. Nevertheless, this dynamic behaviour can be frozen out at sufficiently low temperatures.