Tautomerism

by James

A few days ago I mentioned how three reactions of alkynes were kind of “weird” – hydration, oxymercuration, and hydroboration.

Today, let’s dig into that a bit.

In each of these three reactions, we break a carbon-carbon Pi bond and form a carbon-oxygen single bond and a carbon hydrogen single bond.


 

In the case of hydration (H3O+) and oxymercuration (HgSO4, H2O) the addition is “Markovnikoff”.

In the case of hydroboration (1. BH3, 2. H2O2/NaOH) the reaction is “anti-Markovnikoff”.

So far so good.

Notice how we have an alkene attached to an alcohol? We call these species “enols”.

However, there’s an extra wrinkle to the chemistry of these compounds that is just touched on in Org 1. We go into it in a lot more detail in Org 2.

Enols don’t tend to be very stable. They exist in an equilibrium with a constitutional isomer (remember – same formula, different connectivity). If you move a proton from the O to the C, and break C-C (pi) and form C-O (pi), you get a different functional group – a carbonyl.

We call this the “keto” form.

And the transformation of the enol into the keto form goes by the name “tautomerism”.

 


Tautomerism is a spontaneous process. There’s little that can be done to stop it. What this means is that as soon as the enol is formed, it will be transformed into its more stable keto form.

What that means for each of the three species above, is that they will be converted into ketones (the first two) and an aldehyde (the last one).

So in the cases we went through here, oxymercuration and hydration are a way to make ketones from alkynes. And hydroboration is a way to make aldehydes from alkynes.

You probably  haven’t learned many ways of making aldehydes/ketones yet (besides ozonolysis) so this should come in handy for synthesis problems if it’s given to you.

Thanks for reading!  James

PS Note how we always start with a terminal alkyne in these examples? One end of the alkyne is bonded to C and one is bonded to H? If we don’t use a terminal alkyne in this process, both ends of the alkyne are “equally substituted”. So we can’t have Markovnikoff selectivity, we’ll end up getting a mixture of products.

P.P.S. Reaction Guide – Oxymercuration of alkynes

P.P.P.S. Reaction Guide – Hydroboration of Alkynes