See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Concept: 2-Pentanone is CH3-C(=O)-CH2-CH2-CH3, a methyl ketone where the carbonyl is at C2 with a methyl group on one side and a propyl chain on the other. Step 1 – Identify the target molecule: 2-pentanone = CH3COCH2CH2CH3. It has 5 carbons total, with the ketone at position 2. Step 2 – Analyze option (c): The starting material is butanenitrile (CH3CH2CH2C≡N, i.e., n-butyl cyanide, which has 4 carbons in the chain plus the nitrile carbon = 4 carbons total in the chain: CH3-CH2-CH2-C≡N). Reaction with CH3MgI (Grignard reagent) adds a methyl group to the electrophilic nitrile carbon, forming a metalloimine (imine salt) intermediate: CH3CH2CH2-C(=NMgI)-CH3. Upon aqueous acid workup (H3O+), the imine hydrolyzes to give a ketone: CH3CH2CH2-C(=O)-CH3, which is 2-pentanone. This is a reliable method to convert nitriles to methyl ketones using CH3MgI followed by hydrolysis. Step 3 – Analyze option (a): CH3CH2CH2CHO (butanal, 4 carbons) + CH3MgI → after H3O+ workup gives CH3CH2CH2CH(OH)CH3, a secondary alcohol (2-pentanol), NOT a ketone. This does not yield 2-pentanone. Step 4 – Analyze option (b): The epoxide shown appears to be 1,2-epoxybutane (or a methylated epoxide). Opening of a simple epoxide with CH3MgI gives an alcohol, not directly a ketone. This would not yield 2-pentanone under these conditions. Step 5 – Analyze option (d): Propanal (CH3CH2CHO, 3 carbons) + CH3MgI → after H3O+ gives CH3CH2CH(OH)CH3, a secondary alcohol (2-butanol), NOT 2-pentanone. Step 6 – Conclusion: Only option (c), reacting butanenitrile (CH3CH2CH2CN) with CH3MgI followed by acid hydrolysis, directly gives 2-pentanone via imine intermediate hydrolysis. Grignard addition to nitriles stops at the imine (ketone) stage upon hydrolysis, making this an excellent synthesis route for methyl ketones. Therefore, the correct answer is C.