See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Step 1 – Formation of Grignard reagent A: The starting material is isobutyl bromide (1-bromo-2-methylpropane, (CH3)2CHCH2Br). Treatment with Mg in Et2O gives the Grignard reagent A: isobutylmagnesium bromide, (CH3)2CHCH2MgBr. Step 2 – Grignard opening of the epoxide: The epoxide shown is 1-methyl-2-oxabicyclo[3.1.0]hexane (or equivalently, a cyclopentane oxide with a CH3 substituent). More precisely, the structure is a cyclopentane ring bearing an epoxide (oxirane) fused at one bond (C1–C6 bridge with oxygen), and a CH3 group on a dashed bond at C1. This is 1-methylcyclopent-1,2-epoxide (1-methylcyclopentene oxide), where CH3 is on a dash (alpha/below plane) at the epoxide carbon bearing the methyl. Grignard reagents open epoxides by backside (SN2) attack at the less hindered carbon of the epoxide. The isobutyl Grignard attacks at the less substituted carbon (C2, the unsubstituted epoxide carbon) from the back face. This places the isobutyl group trans to the oxygen-derived OH group. After aqueous workup (H3O+), the product is a cyclopentanol. Stereochemical analysis: - The epoxide has CH3 on a dashed bond at C1 (below plane) and the oxygen bridge above the plane (on the same face as the wedge bonds). - Backside attack by the isobutyl group at C2 (the less substituted epoxide carbon) occurs from the face opposite to the oxygen bridge. - After ring opening, C1 bears OH (from the oxygen, now on the top/wedge face) and CH3 (on the dashed/bottom face); C2 bears the isobutyl group introduced from the bottom face (trans to OH at C1). - The product is 1-hydroxy-1-methyl-2-isobutylcyclopentane with OH and isobutyl trans to each other relative to the ring, and CH3 trans to OH at C1. This matches option (a): cyclopentane with the isobutyl group on a bold (wedge) bond at C2, OH on a bold bond at C1, and CH3 on a dashed bond at C1 – consistent with the trans relationship of OH and isobutyl and cis relationship of OH and the added group geometry. Why other options fail: - (b) shows OH and isobutyl on the same face (cis), which would require frontside attack – not possible for SN2 Grignard opening. - (c) shows an ether product, meaning the epoxide was not fully opened or rearranged – not consistent with Grignard/H3O+ workup giving an alcohol. - (d) shows a hydroxymethyl group, implying a different carbon framework than expected from this Grignard + epoxide combination. Therefore, the correct answer is A.