Question

Only three stereoisomers are possible for 2,3 -dibromo-2, 3-dichlorobutane. Draw them, indicating which pair are enantiomers (optical isomers). Why does the other isomer not have an enantiomer?

Short answer

Two are enantiomers, and the third is a meso compound without an enantiomer due to its symmetry.

Step-by-step solution

Understand the Compound Structure

2,3-dibromo-2,3-dichlorobutane is a molecule with a four-carbon chain. The number 2,3 in the name indicates that both bromine (Br) and chlorine (Cl) atoms are attached to the second and third carbon atoms, respectively.

Identify the Chiral Centers

The carbons at positions 2 and 3 each have four different groups attached: a hydrogen, a chlorine, a bromine, and the rest of the butane chain. Thus, both are chiral centers.

Draw the Enantiomers

Draw the two enantiomers by interchanging any two groups bound to either chiral center to achieve two non-superimposable mirror images. For example, switch the Br and Cl on carbon 2 in one geometry and match the substitution with carbon 3.

Identify the Meso Compound

Draw the meso compound, which has an internal plane of symmetry making it achiral, despite having two chiral centers. It involves arranging the substituents such that the molecule can be divided into symmetrical halves, such as keeping identical substituents opposite each other on the chiral centers.

Compare the Structures

The two enantiomers will be mirror images that cannot be superimposed, contrasting with the meso compound that is symmetric. Analyze the structures: enantiomers are characterized by lacking a plane of symmetry, while the meso compound has one.

Key concepts

Chiral Centers in Organic Molecules

Chiral centers play a crucial role in organic chemistry and are key to understanding molecular structures. Chiral centers, also known as stereocenters, occur when a carbon atom is bonded to four different groups. This unique arrangement allows for the possibility of isomers — different compounds with the same molecular formula but different structures.
In the molecule 2,3-dibromo-2,3-dichlorobutane, the chiral centers are located at the second and third carbon atoms. Each of these carbon atoms is connected to four distinct groups, namely a hydrogen atom, a bromine atom, a chlorine atom, and the remainder of the carbon chain.
Identifying chiral centers is crucial because they determine the three-dimensional arrangement of a molecule, affecting its optical activity and interaction with polarized light. This spatial arrangement can lead to different stereoisomers, such as enantiomers and meso compounds.

Understanding Enantiomers

Enantiomers are a type of stereoisomer that are mirror images of each other but cannot be superimposed. This is much like how your left and right hands are mirror images but are not identical. Enantiomers often exhibit different behaviors in biological systems due to their interaction with other chiral molecules.
To visualize enantiomers for 2,3-dibromo-2,3-dichlorobutane, consider the two chiral centers. If the substituents on one center are switched, say switching the positions of bromine and chlorine on one carbon, it creates a non-superimposable mirror image.
This change results in two distinct molecules with unique optical activities, sometimes referred to as optical isomers. Each enantiomer will rotate plane-polarized light in opposite directions but to the same extent, a property that is critical in fields such as pharmaceutical chemistry.

The Concept of Meso Compounds

Meso compounds present a fascinating exception among stereoisomers. Despite containing chiral centers, a meso compound is considered achiral due to the presence of an internal plane of symmetry. This symmetry makes the entire molecule superimposable on its mirror image, eliminating optical activity.
The internal plane of symmetry divides the molecule into two equal halves, with one half being a mirror image of the other. In the case of 2,3-dibromo-2,3-dichlorobutane, arranging the substituents in such a way that the identical groups are opposite each other at each chiral center achieves this.
Meso compounds demonstrate that chirality is not solely about the presence of chiral centers but also how these centers interact within the molecule. This understanding is crucial when analyzing stereochemistry and predicting the behavior of molecules in different environments.