Tautomerization vs. Resonance

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Tautomerization and resonance are both ways that a single molecule can represent two different molecules. In this lesson, we will learn how these forms are different.

Overview

Chemical structures begin to get confusing when we are looking at rapid changes made within a molecule. When this occurs, a single molecule acts as though it is more than one molecule. So, suddenly one thing is somehow two different things. It doesn't seem to make sense sometimes. There are two types of rapid changes that can occur in molecules: tautomerization and resonance.

Tautomerization and resonance can be confusing to differentiate, because in either case when they are in a solution you will find both structures or forms at any given time. In other words, a solution of acetone will have both the keto and enol tautomers. And a solution of ozone will have molecules with the double bond between either oxygen atoms. So how are these structures different?

Tautomerization is moving around bonds while resonance is only moving around electrons. Technically two resonance structures are not two different structures, it is actually more accurate to draw dotted lines because the molecule isn't usually completely one structure or the other, but shows characteristics from both structures. Tautomers are a specific type of constitutional isomers, in that the bond rearrangement occurs quickly, forming an equilibrium between the two compounds. So in a solution, you would find both rearrangements.

Tautomerization

The most popular tautomerization is the enol and keto tautomers. With this type of tautomerization, we use a ketone or a molecule with a carbon double bonded to an oxygen. Although we typically show ketones in this manner, in reality, a solution of ketones will be switching back and forth in rapid equilibrium with another form, the enol. The enol moves a hydrogen from the alpha carbon onto the oxygen, moving the carbon-oxygen double bond to a carbon-carbon double bond.


Keto and enol
Keto and enol


The keto-enol tautomerization can occur in either basic or acid conditions. The acid-catalyzed tautomerization first puts a hydrogen atom onto the oxygen, then removes the hydrogen from the alpha carbon:


In acid tautomerization the hydrogen is first added to the oxygen
Acid reaction


The base-catalyzed tautomerization first removes the hydrogen from the alpha carbon, and then puts a hydrogen onto the oxygen:


In base tautomerization the hydrogen is first removed from the alpha carbon
Base reaction


Tautomers can be recognized by asking first if actual atoms have changed positions. Next, we need to determine if the two constitutional isomers revert back and forth in rapid equilibrium. While we can memorize a few common or important tautomers (such as the keto-enol tautomer) we can only tell if it will occur in rapid equilibrium experimentally.

Resonance

Now let's take a look at resonance structures. In resonance structures, we are only moving electrons, no atoms. Let's take a look at ozone. It is simply three oxygen atoms connected. This means that we have a total of 18 valence electrons (6 on each oxygen). Oxygen has no charge if it has 2 sets of lone pairs. It has a positive charge with only 1 set of lone pairs and a negative charge with 3 sets of lone pairs.

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