See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Concept: Singlet carbene (:CH2) reacts with alkenes via a concerted, stereospecific syn addition (cycloaddition) to form cyclopropanes. Because the addition is concerted (both new C-C bonds form simultaneously without rotation), the stereochemistry of the alkene is retained in the product. Step 1: Identify the alkene geometry. The starting alkene shows both methyl groups on the same side of the double bond (top) and both H atoms on the same side (bottom). This is (Z)-but-2-ene (cis-2-butene). Step 2: Singlet carbene reacts in a concerted, stereospecific syn fashion. The carbene adds to one face of the double bond, and since the addition is simultaneous, the relative configuration of the substituents is preserved. In cis-2-butene, both methyl groups are on the same face; after cyclopropanation, both methyls end up on the same face of the cyclopropane ring. Step 3: The product is cis-1,2-dimethylcyclopropane — both methyl groups on the same face of the ring (both on wedge bonds, both H on dash bonds), which corresponds to option (a). Step 4: Why other options fail: - Option (b) is trans-1,2-dimethylcyclopropane, which would be formed from trans (E)-but-2-ene with singlet carbene, not from cis-2-butene. - Option (c) (50:50 mixture) would result from a stepwise/non-stereospecific process (as with triplet carbene), not singlet carbene which is stereospecific. - Option (d) is a structural isomer (pent-2-ene) resulting from insertion, not cycloaddition, and is not the major product. Therefore, the correct answer is A.