See image — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

Concept: Grignard reagents add to aldehydes and ketones to form alcohols. The target molecule is 4-heptanol: (CH3CH2CH2)2CHOH, a secondary alcohol where the carbon bearing the OH group (C4) has two n-propyl groups attached to it and one hydrogen. Step 1: Identify the target structure. 4-heptanol is (CH3CH2CH2)2CHOH. The carbon bearing OH has: one H, two n-propyl (CH3CH2CH2-) groups. This is a secondary alcohol. Step 2: Retrosynthetic analysis. For a Grignard synthesis of a secondary alcohol, we disconnect at the C-C bond formed between the Grignard carbon and the carbonyl carbon. The OH-bearing carbon came from the carbonyl carbon of an aldehyde. Breaking one C-C bond at C4 gives: CH3CH2CH2MgBr (propyl Grignard) + CH3CH2CH2CHO (butanal, an aldehyde with 4 carbons including the carbonyl carbon). Step 3: Verify option (b). CH3CH2CH2MgBr (1 mole) reacts with butanal CH3CH2CH2CH=O. The Grignard carbon (from propyl MgBr) attacks the carbonyl carbon of butanal. This gives after H3O+ workup: CH3CH2CH2-CH(OH)-CH2CH2CH3 = (CH3CH2CH2)2CHOH = 4-heptanol. This matches the target exactly. Step 4: Why other options fail. - Option (a): 2 moles of CH3CH2CH2MgBr + formaldehyde (H2C=O). Formaldehyde has only 1 carbon; one mole of Grignard reacts with one mole of formaldehyde to give a primary alcohol CH3CH2CH2CH2OH (1-butanol), not 4-heptanol. Two moles cannot both add to one formaldehyde molecule in a single step. - Option (c): CH3CH2CH2CH2MgBr (butyl Grignard, 4 carbons) + acetone (CH3)2C=O. Product would be (CH3)2C(OH)(CH2CH2CH2CH3), a tertiary alcohol with two methyl groups and one butyl group — not 4-heptanol. - Option (d): (CH3CH2CH2)2CHMgBr + formaldehyde. The Grignard here has 7 carbons total; adding to formaldehyde gives (CH3CH2CH2)2CHCH2OH, an 8-carbon primary alcohol — not 4-heptanol. Therefore, the correct answer is B.