Explain why, in contrast to meso-stilbene dibromide, the dibromide 49 exists in two enantiomeric forms.
The Correct Answer and Explanation is :
The key difference between meso-stilbene dibromide and the dibromide 49 lies in their stereochemistry. Meso-stilbene dibromide is achiral due to its symmetry, while dibromide 49 exists as two enantiomeric forms because it lacks such symmetry.
- Meso-stilbene dibromide:
Meso-stilbene dibromide arises when bromine adds to trans-stilbene via an anti-addition mechanism. The resulting product has two stereocenters, but the molecule is achiral due to an internal plane of symmetry. This internal symmetry renders the molecule superimposable on its mirror image, which makes it a meso compound rather than a pair of enantiomers. - Dibromide 49:
In contrast, dibromide 49 is a product of bromination involving a starting material or mechanism that does not lead to such symmetry. If dibromide 49 has two stereocenters but lacks an internal plane of symmetry, the molecule is chiral. A chiral molecule with two stereocenters can exist as a pair of nonsuperimposable mirror images, or enantiomers.
Why the Difference?
- Symmetry: Meso-stilbene dibromide has symmetry that negates chirality, while dibromide 49 does not.
- Stereocenter Configuration: In meso-stilbene dibromide, one stereocenter is the opposite configuration of the other (R and S), which cancels out chirality. For dibromide 49, the configurations of the stereocenters (e.g., R,R and S,S) result in a chiral molecule.
- Anti-addition Mechanism: Bromine addition to the starting material determines whether the product will be symmetrical or asymmetrical. For dibromide 49, the bromination likely leads to configurations that lack symmetry, giving rise to enantiomers.
Conclusion
Meso-stilbene dibromide is achiral due to symmetry, while dibromide 49 exists in enantiomeric forms due to its lack of symmetry and the presence of stereocenters that generate chirality.