Draw the mechanism of the bromination of cis-stilbene and the mechanism of the bromination of trans-stilbene. Why are the products of the bromination of trans-stilbene achiral, while the products of the bromination of cis-stilbene are chiral?
The correct answer and explanation is :
Mechanism of Bromination of Cis-Stilbene and Trans-Stilbene
The bromination of stilbene involves the addition of bromine (Br2) to the carbon-carbon double bond. This reaction proceeds through an electrophilic addition mechanism. Here’s a breakdown of the steps involved in the reaction:
1. Formation of a Bromonium Ion:
- The double bond of stilbene reacts with Br2, where the electron-rich π-bond attacks the bromine (Br2), creating a bromonium ion. The bromonium ion is a three-membered ring with a positively charged bromine atom attached to both carbon atoms that were previously part of the double bond.
2. Nucleophilic Attack:
- A bromide ion (Br-) then attacks the more substituted carbon of the bromonium ion (according to Markovnikov’s rule), resulting in the opening of the three-membered bromonium ring. This attack leads to the formation of a vicinal dibromide (a compound with two bromine atoms attached to adjacent carbon atoms).
Mechanism for Cis-Stilbene:
- In cis-stilbene, the two phenyl groups are on the same side of the molecule. When the bromine atom attacks the bromonium ion, it leads to the formation of a chiral center at the carbon where the nucleophile attacks. Since the two phenyl groups are already in the same orientation, this leads to a product with chirality.
Mechanism for Trans-Stilbene:
- In trans-stilbene, the two phenyl groups are opposite to each other. This leads to the formation of a meso compound when the bromine attacks the bromonium ion because the attack is symmetrical. The resulting product has two chiral centers but is achiral overall due to internal symmetry (it is superimposable on its mirror image).
Why the Products Differ in Chirality:
- Cis-Stilbene Bromination: The reaction of cis-stilbene creates a chiral product because the two phenyl groups are on the same side, causing the addition of bromine to occur in a way that generates two different spatial arrangements at the reaction site, leading to chirality in the final product.
- Trans-Stilbene Bromination: The reaction of trans-stilbene leads to a meso compound, which is achiral. This happens because the two bromines end up being placed on opposite sides of the ring, and the molecule has an internal mirror plane, resulting in no net chirality in the product.
In summary, the difference in chirality arises due to the spatial arrangement of the substituents on the stilbene molecule. The cis-isomer creates chiral products, while the trans-isomer gives a meso (achiral) product due to symmetry.