See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1 – Formation of (A): Cyclohexene reacts with NBS (N-bromosuccinimide) via allylic bromination to give 3-bromocyclohexene. Treatment with Mg/ether converts this to a Grignard reagent: 3-cyclohexenylmagnesium bromide (allylic Grignard). The Grignard reagent exists as a resonance-delocalized allyl-Grignard, meaning the metal can be at C1 or C3 of the allylic system, but effectively gives 3-cyclohexenylmagnesium bromide (A). Step 2 – Formation of (B): The Grignard (A) reacts with isobutyraldehyde (H-C(=O)-CH(CH3)2) in the presence of ethyl bromide (CH3-CH2-Br). The Grignard adds to the aldehyde to give, after workup, a secondary alcohol: the product is 1-(cyclohex-2-en-1-yl)-2-methylpropan-1-ol. However, re-reading the reagents: the aldehyde shown is H-C(=O)-CH(CH3)(CH3) which is isobutyraldehyde (2-methylpropanal), and CH3CH2Br is ethyl bromide. In this reaction, the Grignard reacts with the aldehyde; the ethyl bromide serves to quench or is part of a two-step sequence. The product (B) contains a cyclohexene ring with an attached carbon bearing OEt and two methyl groups — this suggests the aldehyde reacts with the Grignard, then etherification or the ethyl bromide alkylates the resulting alkoxide. The alkoxide formed from Grignard addition to (CH3)2CH-CHO gives (CH3)2CH-CH(O^-)-cyclohexenyl; treatment with EtBr alkylates the oxygen to give the ethyl ether: (CH3)2CH-CH(OEt)-cyclohexenyl. But the product structures show a quaternary or tertiary carbon bearing OEt and two methyls directly on the ring carbon, suggesting the Grignard adds to give a tertiary alcohol carbon attached to the ring. Actually, the Grignard (cyclohexenyl-MgBr) adds to isobutyraldehyde (Me2CHCHO) → product alcohol is Me2CH-CH(OH)-(cyclohex-2-en-1-yl); alkoxide + EtBr → Me2CH-CH(OEt)-(cyclohex-2-en-1-yl). This places a -CH(OEt)(CHMe2) substituent at C1 of cyclohexene ring, giving (B) as 1-[1-ethoxy-2-methylpropyl]cyclohex-2-ene. Step 3 – Formation of (C): OsO4/H2O2 performs syn-dihydroxylation of the double bond (C2=C3 of cyclohexene) in (B), giving a cis-diol. The double bond is between C2 and C3, so dihydroxylation gives cis-OH groups at C2 and C3. Step 4 – Stereochemistry analysis: The substituent at C1 is -CH(OEt)(CHMe2). This carbon (exocyclic) bears H, OEt, and the isobutyl portion — it is a chiral center with one H. The OsO4 dihydroxylation adds two OH groups syn from either face. The key question is whether C1 has a quaternary or tertiary (with H) carbon bearing OEt. Since the aldehyde carbon (CHO) becomes CH(OEt) after Grignard addition and etherification, it bears one H. Thus the carbon bearing OEt also has one H, matching option (b) which shows H on that carbon. Option (a) shows no H (quaternary), which would require a ketone substrate instead of aldehyde — incorrect. The syn-dihydroxylation gives cis-diol (both OH on same face, shown as bold wedges), matching option (b). Option (c) shows trans relationship of the two OH groups, which cannot result from OsO4 (which gives exclusively syn addition). Therefore, the correct answer is B.