An organic compound (A) reacts with methylmagnesium iodide, and acid hydrolysis of the product gives propan-2-ol (isopropyl alcohol). Propan-2-ol reacts with hydrogen iodide to give (B). Compound (B) on reaction with potassium cyanide gives (C). Compound (C) on reduction gives an amine (D).
Identify the compound (A) in the reaction sequence given in the passage.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Acetaldehyde
A Grignard reagent adds to the C=O group of an aldehyde or ketone, and hydrolysis of the adduct gives an alcohol.
Product: propan-2-ol, \( \ce{CH3-CH(OH)-CH3} \), is a secondary alcohol, which comes from an aldehyde other than methanal.
Carbon count: propan-2-ol has 3 C atoms; one \( \ce{CH3} \) comes from \( \ce{CH3MgI} \), so (A) has 2 C atoms: (A) is \( \ce{CH3CHO} \), acetaldehyde.
Reaction:
\( \displaystyle \ce{CH3CHO + CH3MgI -> (CH3)2CH-OMgI} \xrightarrow{\ce{H2O}} \ce{(CH3)2CH-OH + Mg(OH)I} \)
Why the other options are incorrect
Option (1) Formaldehyde — Incorrect. Methanal gives a primary alcohol: with \( \ce{CH3MgI} \) the product is ethanol, \( \ce{CH3CH2OH} \), not propan-2-ol.
Option (3) Ethanol — Incorrect. An alcohol has no C=O group; its acidic O–H hydrogen converts the Grignard reagent into a hydrocarbon (methane here), so no new alcohol is formed.
Option (4) Dimethyl ether — Incorrect. An ether has no C=O group for the reagent to add to; Grignard reagents are in fact prepared in dry ether.
NCERT (2026-27 edition):
8.4 Chemical Reactions: 1. Nucleophilic addition reactions – “(c) Addition of Grignard reagents: (refer Unit 7, Class XII).”
7.4.1 Preparation of Alcohols: 3. From Grignard reagents – “Alcohols are produced by the reaction of Grignard reagents … with aldehydes and ketones. … You will notice that the reaction produces a primary alcohol with methanal, a secondary alcohol with other aldehydes and tertiary alcohol with ketones.”