See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: MCPBA is a peracid that performs syn-epoxidation of alkenes. The oxygen is delivered to both faces of the double bond simultaneously from the same face, preserving the stereochemical relationship of substituents. Step 1: Identify the starting alkene. The structure shown is (Z)-2-butene (cis-2-butene), where both CH3 groups are on the same side of the double bond (both H atoms on each carbon are shown, with CH3 groups implied at the top of each sp2 carbon). Looking at the structure: each carbon of the double bond bears one CH3 and one H, and the geometry is cis (Z) - the two CH3 groups are on the same side. Step 2: Apply syn-epoxidation stereochemistry. MCPBA adds oxygen in a syn fashion - the two C-O bonds form simultaneously from the same face. This means the relative stereochemistry of substituents is retained. In cis-2-butene, both CH3 groups are on the same side of the double bond. Upon syn-epoxidation, the oxygen bridges from one face, and the two CH3 groups end up on the same face of the epoxide ring. Step 3: Identify the product. The product is cis-2,3-dimethyloxirane (meso compound), where both CH3 groups are on the same side of the epoxide ring. This corresponds to option (b), where both CH3 groups appear on the same face of the epoxide (one CH3 on top-left carbon and one CH3 on top-right carbon of the epoxide). Step 4: Eliminate other options. - Option (a): Shows CH3 groups on opposite sides (trans epoxide) - this would be the product from trans-2-butene, not cis-2-butene. - Option (c) and (d): Show incorrect substitution patterns inconsistent with 2-butene epoxidation. Therefore, the correct answer is B.