See image — Alcohols Phenols and Ethers Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the target product: The final product is 3,4-dimethyl-3-hexanol. Its structure is CH3CH2C(CH3)(OH)CH(CH3)CH2CH3. This is a tertiary alcohol with a hydroxyl group on C3, methyl groups on C3 and C4, and ethyl groups extending from C3 and C4. Step 2 - Retrosynthetic analysis of the Grignard reaction: A tertiary alcohol from a Grignard reaction (RMgX + ketone or aldehyde) means we need to identify the ketone (B) and the Grignard reagent from alcohol (A). The product 3,4-dimethyl-3-hexanol can be formed by addition of an ethyl Grignard (CH3CH2MgBr) to 3-methylpentan-2-one (CH3-CO-CH(CH3)-CH2-CH3), OR by addition of a sec-butyl Grignard to a ketone. Step 3 - Trace back from (A): Alcohol (A) is oxidized by Na2Cr2O7/H2SO4 to give ketone (B). A secondary alcohol gives a ketone. Alcohol (A) is also converted via PBr3 to alkyl bromide (C), then treated with Mg/ether to give Grignard reagent. Step 4 - Identify (A) as option (d): Option (d) is 4-methylpentan-2-ol: (CH3)2CHCH2CH(OH)CH3. Oxidation gives 4-methylpentan-2-one: (CH3)2CHCH2COCH3. The Grignard reagent from (A) via PBr3 then Mg would be 4-methylpentan-2-yl magnesium bromide... However, re-examining: option (d) appears to be a secondary alcohol. If A = 3-methylpentan-2-ol (CH3CH(OH)CH(CH3)CH2CH3), oxidation gives 3-methylpentan-2-one (CH3COCH(CH3)CH2CH3 = B), and the Grignard from A would be CH3CH(MgBr)CH(CH3)CH2CH3. Adding this Grignard to B gives: the carbon bearing MgBr attacks the carbonyl carbon, producing 3,4-dimethyl-3-hexanol after H3O+ workup. This matches the target product exactly. Step 5 - Confirm option (d): The structure shown in (d) is a secondary alcohol corresponding to 3-methylpentan-2-ol. Oxidation gives 3-methylpentan-2-one. The Grignard from (d) via PBr3/Mg is CH3C(MgBr)HCH(CH3)CH2CH3. Addition to 3-methylpentan-2-one gives 3,4-dimethyl-3-hexanol upon workup. This is consistent. Step 6 - Why other options fail: Option (a) is a primary alcohol giving an aldehyde upon oxidation; the Grignard addition would not yield the correct tertiary alcohol. Option (b) n-butanol gives butanal; the combination does not give 3,4-dimethyl-3-hexanol. Option (c) gives a different ketone and Grignard combination that does not produce the target. Therefore, the correct answer is D.