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Reaction 1: cis-but-2-ene reacts with Br2 in CCl4 to give product mixture 1. Reaction 2: trans-but-2-ene reacts with KMnO4 / OH⁻ at 273 K to give product mixture 2. Which statement is correct?

1. Product mixture 1 is optically active.
2. Product mixture 2 is optically active.
3. Product mixture 1 is Erythro.
4. Product mixture 2 is Threo.

Correct answer: Product mixture 1 is Erythro.

Solution

Cis-but-2-ene undergoes anti addition of Br2, placing the two Br atoms on opposite faces. Starting from the cis alkene, this produces meso-2,3-dibromobutane, which is the erythro form and is optically inactive. Trans-but-2-ene undergoes syn dihydroxylation (same-face addition), producing meso-2,3-butanediol (erythro, optically inactive). So product 1 is erythro (correct), and product 2 is erythro (not threo).

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