See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Cold dilute KMnO4 (Baeyer's reagent) performs syn-dihydroxylation of alkenes, adding two OH groups to the same face of the double bond (syn addition), producing a diol. For the product to be a meso compound, two conditions must be met: 1. The diol must have two stereocenters. 2. The molecule must have an internal plane of symmetry (making it achiral despite having stereocenters). Step 1 - Understand syn dihydroxylation: Both OH groups add to the same face, giving syn diol. Step 2 - Determine which alkene gives a meso diol upon syn addition: - A meso compound requires that the two stereocenters be identical in substitution but have opposite configurations that cancel each other out via an internal mirror plane. - For syn addition to produce a meso compound, the alkene must be cis (same substituents on same side), and the carbon chain must be symmetric about the double bond. Step 3 - Analyze each option: (a) cis-2-pentene: CH3-CH=CH-CH2CH3 (cis). The substituents on the two carbons of the double bond are CH3 and CH2CH3 — these are different. Syn dihydroxylation gives a diol with two different substituents at each stereocenter, so the two stereocenters are not identical. This cannot be meso. (b) cis-2-hexene: CH3-CH=CH-CH2CH2CH3 (cis). The substituents are CH3 and n-propyl — different. Not meso. (c) cis-4-octene: CH3CH2CH2-CH=CH-CH2CH2CH3 (cis). Both carbons of the double bond bear identical n-propyl groups. Syn dihydroxylation adds two OH groups on the same face. The resulting diol is: (4R,5S) or (4S,5R) — but because both stereocenters bear identical substituents (n-propyl and H and OH), and since syn addition on a symmetric cis alkene gives the same configuration at both centers from the same-face addition... wait, let me reconsider. For cis-4-octene (symmetric alkene), syn addition gives the (4R,5S) diol which has an internal plane of symmetry — this IS the meso compound. The molecule is CH3CH2CH2-C(OH)(H)-C(OH)(H)-CH2CH2CH3, which is symmetric. With syn addition, both OH groups are on the same face, giving configurations that are mirror images of each other across the internal plane — hence meso. (d) trans-2-hexene: Syn addition on a trans alkene with different substituents does not give meso. Step 4 - Why cis-4-octene specifically: The key is symmetry of the alkene. cis-4-octene is symmetric (n-propyl on both sides) AND cis. Syn dihydroxylation of a symmetric cis alkene gives the meso diol because the two OH groups added syn to one face create (R,S) configuration at the two identical stereocenters, resulting in an internal plane of symmetry. Note: For a symmetric cis alkene, syn addition → meso diol. For a symmetric trans alkene, syn addition → (R,R)/(S,S) racemic mixture. Options (a), (b), (d) either lack symmetry or are trans. Therefore, the correct answer is C.