The 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: For propanone, it lies almost entirely on the left, whereas for the hexafluoro derivative it is almost entirely on the right. The standard answer to this is that electron-withdrawing substituents destabilize the carbonyl compound more than the hydrate.
Sometimes, as a break from describing chemistry, I take to describing the (chemical/scientific) creations behind the (WordPress) blog system. It is fascinating how there do seem increasing signs of convergence between the blog post and the journal article. Perhaps prompted by transclusion of tools such as Jmol and LaTex into Wikis and blogs, I list the following interesting developments in both genres.
One thing almost always leads to another in chemistry. In the last post, I described how an antiperiplanar migration could compete with an antiperiplanar elimination. This leads to the hydroboration-oxidation mechanism, the discovery of which resulted in Herbert C. Brown (at least in part) being awarded the Nobel prize in 1979. This reaction represents a fairly steep learning curve for new students of organic chemistry.
The anti-periplanar principle permeates organic reactivity. Here I pick up on an example of the antiperiplanar E2 elimination (below, blue) by comparing it to a competing reaction involving a [1,2] antiperiplanar migration (red). The relative rates of these two processes will depend on several factors such as the ability of Cl to donate electrons (red) vs the basicity of the chloride anion (blue) and of course solvent polarity.
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.
