Keto-Enol Tautomerism

 

Keto-Enol Tautomerism is a process where an equilibrium occurs between the keto form (ie a normal-looking ketone) and the enol form (a double bond adjacent to an alcohol) of a carbonyl, acheived through the movement of atoms and breaking of single bonds.

Spoiler alert: In most cases, the keto form is highly favored.

What is the keto form of a carbonyl? The keto form is what you are used to looking at. It is any normal-looking aldehyde or ketone, in which there is a carbonyl and hydrogen atoms alpha to the carbonyl.  See below:

What is the enol form of a carbonyl? The word “enol” can be broken down into two parts: “en” (pronounced “een”) as in part alkene, and “ol” as in part alcohol. The two parts, the alcohol and alkene, come off of the same carbon atom. See below:

What is this crazy thing called tautomerization? Tautomerization is a process where single bonds are broken and atoms are rearranged to give a different structure.  This is different from resonance structures, where you NEVER break a single bond. In Keto-Enol Tautomerism,  a carbonyl double bond is broken,  an alkene double bond is formed, and a hydrogen atom migrates from the alpha carbon to the oxygen atom, forming an alcohol.

keto-enol tautomerism

What is this equilibrium you speak of? Remember, that energy within the solution can cause structures to form which aren’t the most thermodynamically favored. This is what is happening here. There is an equilibrium between the keto and enol form, almost exclusively favoring the keto form. However, the enol form does exist in solution.  The ratio of keto to enol forms depends on several factors, which we will discuss further below.

When is the enol form more favored or the major form?  We think there are four situations where the enol form is more favored (but not necessarily the major form) you should learn.

  • Situation 1: When you get conjugation out of it.  Conjugation will mean more enol form.

  • Situation 2: When you get increased hydrogen bonding.  (and in this case, some conjugation too)  This one is sweet because the hydrogen-bonding makes a six-membered ring.

  • Situation 3: When you can get aromaticity out of it.  In this case, there is nothing but the enol form.

  • Situation 4: If you can get a more substituted alkene.  There will be more of B in the enol form than A, but still not a ton of B.  There will be more of B in the enol form in its equilibrium because tetra-substituted alkenes are more stable than di-substituted.