In addition reactions, two reactants add together to form a single product with no left over atoms. Alkenes are known to participate in addition reactions.Select the major product of the following addition reaction:

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The reaction shown is the addition of Br2 to an alkene (styrene-type), which proceeds via a bromonium ion intermediate followed by backside (anti) opening by the bromide ion. This mechanism dictates that both bromine atoms add across the same double bond, forming a vicinal dibromide with anti stereochemistry. In other words, the two new C–Br bonds end up on opposite faces of the former double bond.

Option analysis:
- Option a: If this structure shows a single Br added to one carbon (monobrominated product) or shows Br atoms on the same face (syn) or on non-adjacent carbons, it would be inconsistent with the anti, vicinal dibromide outcome expected from Br2 addition to an alkene. This option is incorrect because it does not depict the required two bromines across the double bond in an anti arrangement.
- Option b: If this option presents a dibromide with both bromines on the correct adjacent carbons but in a syn (same-face) arrangement, it would contradict the typical bromonium ion mechanism where opening by Br− occurs from the opposite face, giving anti addition. This makes it incorrect.
- Option c: If this shows a dibromide that is on the correct adjacent carbons but with one bromine attached to the benzylic carbon and the other to the vinylic carbon in the anti orientation, this matches the expected anti addition and vicinal dibromide product. This would be a correct depiction.
- Option d: If this option displays a dibromide but with a rearranged carbon skeleton, or with both bromines on the same carbon (geminal dibromide) or missing one bromine, it would be incorrect because Br2 adds across the double bond, not to give geminal dibromide or monobrominated product.
- Option e: If this shows a trans arrangement that violates anti addition (e.g., both bromines drawn on the same face as if they were syn), it’s not consistent with the mechanism and is therefore incorrect.
- Option f: If this option presents the correct vicinal dibromide, but the substituent positions are misrepresented (e.g., Br placed on the wrong carbon relative to the phenyl group), this would be wrong because regiochemistry matters: bromine adds to both carbons of the original double bond, preserving the carbon skeleton and giving the vicinal dibromide with Br on each former vinylic carbon.

In summary, the major product should be the anti, vicinal dibromide resulting from anti addition of Br2 across the C=C bond of the alkene. The correct option among a–f will be the one showing two bromine atoms on adjacent carbons (the former double-bond carbons) with anti orientation relative to each other, matching the bromonium ion mechanism. The other options fail because they depict monobromination, syn addition, geminal products, rearrangements, or incorrect regiochemistry.

My LMS Subjects Quiz 1

In addition reactions, two reactants add together to form a single product with no left over atoms. Alkenes are known to participate in addition reactions.Select the major product of the following addition reaction:

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