meso-2,3-Dibromobutane (2R,3S) vs (2R,3R)-2,3-Dibromobutane - Isomer Relationship

Introduction

Both molecules are 2,3-dibromobutane with two chiral centers and the same connectivity. But one is achiral and the other is optically active. How is that possible?

Formula Comparison

Both have the formula C₄H₈Br₂ with identical connectivity: CH₃–CHBr–CHBr–CH₃. Same formula, same bonding sequence.

Connectivity Analysis

The bonding is identical in both molecules. Each has bromine on C2 and C3 with terminal methyl groups. Since connectivity matches, these are not constitutional isomers.

Spatial Relationship

In the meso form (R,S), the two bromines point the same direction when drawn in an eclipsed Newman projection — there is an internal mirror plane between C2 and C3. In the (R,R) form, both centers have the same configuration, so there is no internal mirror plane.

Chirality Check

The meso compound has R at C2 and S at C3. The internal mirror plane makes the molecule achiral — its mirror image is superimposable on itself. The (R,R) form has no such plane and is chiral (optically active). This is the key distinction.

Classification

The meso form and the (R,R) form are diastereomers. They are stereoisomers (same connectivity, different 3D arrangement) but NOT mirror images. The meso compound is achiral; the (R,R) compound is chiral.

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