Q.15 The molecules A and B are a pair of ____________.
(A) enantiomers
(B) diastereomers
(C) conformational isomers
(D) constitutional isomers

The molecules A and B are a pair of conformational isomers (more specifically, atropisomers caused by restricted rotation about the aryl–aryl bond).

Introduction

In stereochemistry questions, students are often asked whether two complex aromatic molecules are enantiomers, diastereomers, conformational isomers, or constitutional isomers. The keyphrase molecules A and B are a pair of conformational isomers captures the core learning goal: to recognize that these structures differ only by a hindered rotation around a single bond, giving rise to atropisomerism rather than a change in connectivity or mirror‑image relationship.​

Why A and B are conformational isomers

• The skeleton and connectivity of A and B are identical: same heterocyclic framework, same nitro group, same ester linkage, and same substituents attached to the same atoms. This rules out any constitutional isomerism because no bonds are broken or rearranged between the two structures.​

• The difference lies in the spatial orientation of the fused aromatic systems around a single aryl–aryl (or aryl–heteroaryl) bond. Such rotation is sterically hindered by ortho‑substituents, generating stable atropisomers that are classic examples of conformational isomers with restricted rotation.​

• These conformers are not related as non‑superimposable mirror images; instead, they are related by rotation about the axis, so they are not enantiomers. They also do not differ at some, but not all, stereocenters; thus they are not diastereomers.

Therefore, the correct option is (C) conformational isomers.

Explanation of each option

Enantiomers (Option A)

• Definition: Enantiomers are stereoisomers that are non‑superimposable mirror images of each other, usually arising from one or more chiral centers or a chiral axis.​

• For a pair to be enantiomers, every stereogenic element must be inverted, and each molecule must be the mirror image of the other. In molecules A and B, the relative 3D arrangement does not correspond to a mirror‑image relationship; they differ by rotation around a bond, not by complete inversion of all stereocenters. Hence A and B are not enantiomers.​

Diastereomers (Option B)

• Definition: Diastereomers are stereoisomers that are not mirror images of each other and differ at one or more—but not all—stereocenters.​

• Diastereomerism typically arises in molecules with multiple stereocenters (or axes/planes of chirality) where some, but not all, stereochemical elements are inverted. In A and B, the connectivity and all stereogenic elements are unchanged; only a conformational arrangement about a single bond differs. This corresponds to conformational isomerism (atropisomerism), not diastereomerism.​

Conformational isomers (Option C – Correct)

• Definition: Conformational isomers (conformers) are stereoisomers that differ by rotation about single (σ) bonds without breaking covalent bonds. When rotation is strongly hindered, the resulting stable conformers are termed atropisomers.​

• In molecules A and B, bulky ortho‑substituted aromatic rings prevent free rotation. This generates two distinct, isolable conformations around the axis, corresponding exactly to the relationship shown: molecules A and B are a pair of conformational isomers (atropisomers) with the same connectivity and formula but different spatial orientation about a hindered bond.​

Constitutional isomers (Option D)

• Definition: Constitutional (structural) isomers have the same molecular formula but differ in the connectivity (bonding sequence) of atoms.​

• Examples include changing a functional group position, moving a substituent to another atom, or altering the chain branching. In A and B, every atom is connected to the same neighbors; only the 3D arrangement differs. Since no bonds are rearranged, they cannot be constitutional isomers.​

Key takeaways for exams

• Always check connectivity first: if it changes, think constitutional isomers; if it remains the same, think stereoisomers.​

• If stereoisomers are mirror images, they are enantiomers; if not mirror images but differ at stereocenters, they are diastereomers.​

• If the only difference is rotation about a bond—especially a sterically hindered aryl–aryl axis as in A and B—the relationship is conformational isomers (atropisomers), which is the correct classification in this question.​