Animated Solution for Chemistry - Organic Chemistry: Identify the compound that exhibits tautomerism

Welcome to the fascinating world of tautomerism, a special type of structural isomerism where two distinct molecules exist in a constant, dynamic equilibrium. Unlike standard isomers that sit quietly in their respective forms, tautomers are shape-shifters. They rapidly interconvert by the migration of an atom—most commonly a proton—accompanied by the shifting of a double bond.

The most famous and frequently tested variant of this phenomenon is keto-enol tautomerism. In this dance, a ketone (or aldehyde) transforms into an enol (an alkene with a hydroxyl group). But what dictates whether a molecule can perform this chemical gymnastics?

The secret ingredient for keto-enol tautomerism is the $\alpha$-hydrogen. For a compound to exhibit this behavior, it must possess at least one acidic hydrogen atom attached to the carbon directly adjacent to the carbonyl group ($C=O$).

When the conditions are right, this $\alpha$-hydrogen detaches from its carbon and bonds to the carbonyl oxygen. Simultaneously, the carbon-oxygen double bond shifts to become a carbon-carbon double bond. This elegant proton transfer is the heartbeat of tautomerism.

Let's evaluate the options provided in our problem:

1. 2-butene ($C{H}_3-CH=CH-C{H}_3$): This molecule is a simple alkene. It completely lacks a carbonyl group, meaning it has no mechanism to undergo keto-enol tautomerism. We can safely eliminate it.

2. Lactic acid ($C{H}_3-CH(OH)-COOH$): While lactic acid does contain a carbonyl group within its carboxylic acid moiety, carboxylic acids generally do not exhibit observable keto-enol tautomerism. The resonance stabilization within the carboxylate group is overwhelmingly dominant, making the enol form highly unfavorable.

3. 2-pentanone ($C{H}_3-CO-C{H}_2-C{H}_2-C{H}_3$): Here we have a classic ketone. If we look at the carbon atoms adjacent to the carbonyl group, we find $\alpha$-hydrogens on both sides! Because it possesses these acidic protons, 2-pentanone can easily undergo tautomerization to form its corresponding enol:

Now, let's look at phenol. This is a classic trick question in organic chemistry. When we look at phenol, we see an enol—a hydroxyl group attached directly to a carbon-carbon double bond within an aromatic ring.

Because the aromatic ring provides massive thermodynamic stability, phenol exists almost entirely ($>99.99\%$) in this enol form. However, it is still in a dynamic equilibrium with its non-aromatic keto form (cyclohexa-2,4-dienone or cyclohexa-2,5-dienone). Even though the keto form is present in vanishingly small amounts, the equilibrium exists, meaning phenol does exhibit tautomerism.

Both 2-pentanone and phenol possess the necessary structural features to undergo tautomerization. 2-pentanone shifts from a stable keto form to a less stable enol form, while phenol shifts from a highly stable enol form to a highly unstable keto form.

Therefore, the correct answers are (c) and (d).