MCAT Chemical and Physical Foundations Practice Exam 2025 – Complete Study Guide

The correct choice highlights the role of a hydrogen atom in the process of tautomerization, which is characterized by the transfer of a hydrogen atom between two adjacent atoms, typically resulting in the interconversion of a keto form and an enol form.

In tautomerization, the common shift involves a hydrogen atom moving from one atom (often a carbon or nitrogen atom) to an adjacent atom, such as an oxygen atom. This results in structural rearrangement, leading to distinct tautomers that can exhibit different chemical and physical properties. For instance, in the case of carbonyl compounds, the hydrogen atom typically migrates to the adjacent oxygen atom, turning the carbonyl (keto) form into its corresponding alcohol (enol) form.

While other atoms such as nitrogen and oxygen are present in many tautomeric systems, the fundamental mechanism involves the movement of a hydrogen atom, distinguishing it from other types of reactions where the focus might be on the shifting of entire groups or different atoms. Therefore, recognizing the pivotal role of hydrogen in this context is crucial for understanding the nature of tautomeric equilibria in organic chemistry.