When trans-1-phenylpropene is treated with bromine, some syn addition is observed. Explain why the presence of a phenyl group causes a loss of stereospecificity. Br Br Br2 + En + En Br Br trans-1-Phenylpropene Anti addition products (83%) Syn addition products (17%) 0 o The bromonium ion can open at the benzylic position. The resulting planar carbocation can be attacked from either face by bromide, o A bromonium ion will not form due to steric hindrance with the benzene ring. Both positions of the alkene are secondary carbons, so a mixture of stereochemistry is expected. o The bromonium ion is meso. Attack from either side by bromide results in diastereomers being formed. o The bottom face of the bromonium ion is blocked by the benzene ring, so the bromide attacks the top face.
The Correct Answer and Explanation is:
Correct Answer:
The bromonium ion can open at the benzylic position. The resulting planar carbocation can be attacked from either face by bromide.
When trans-1-phenylpropene is treated with Br₂, the typical expectation is anti addition, wherein the bromine molecule adds across the double bond to form a three-membered bromonium ion intermediate. This intermediate is then attacked from the opposite face by a bromide ion, leading to anti stereochemistry in the product.
However, in this case, a small amount of syn addition product is also observed (17%), indicating that the reaction is not completely stereospecific. The key to understanding this lies in the effect of the phenyl group on the reaction mechanism.
The phenyl group is conjugated with the double bond of the alkene, especially at the benzylic (allylic to phenyl) carbon. This conjugation stabilizes a carbocation that may form if the bromonium ion is partially or fully opened. Specifically, the positive charge can delocalize into the aromatic ring, making this pathway energetically favorable.
Thus, instead of a completely stable bromonium ion, the intermediate may undergo partial ring opening to generate a benzylic carbocation. This carbocation is planar, allowing nucleophilic attack from either face by bromide ion. As a result, this opens the door to syn addition products forming alongside the expected anti products.
This loss of stereospecificity is a direct consequence of the stabilization provided by the phenyl group to a potential carbocation intermediate. Without such stabilization, the reaction would proceed solely through the bromonium ion, resulting in exclusive anti addition.
In summary, the presence of the phenyl group promotes partial carbocation character during the reaction, allowing attack from either face and thus reducing the stereospecificity typically seen in halogen addition to alkenes.
