Question

Identify the compound 'A' in the following sequence of reactions. A \[\text{C}_{2}\text{H}_{3}\text{MgBr}\]\[\text{Dryether}\] B \[\xrightarrow{\text{H}_{3}\text{O}^{+\] 3-Methylpentan-3-ol}

• A. Propanal
• B. Propanone
• C. Butanal
• D. Butanone

Hint:

Grignard reagents react with ketones to form tertiary alcohols upon hydrolysis. Analyze the structure of the final alcohol product to deduce the structure of the starting carbonyl compound and the Grignard reagent. Be careful with group interpretation if the formula is ambiguous (e.g., C2H3 vs C2H5).

Answer 1

The Correct Option is B

Step 1: Identify the type of alcohol formed The final product is 3-Methylpentan-3-ol, which is a tertiary alcohol. Tertiary alcohols are generally formed by the reaction of a Grignard reagent with a ketone, followed by hydrolysis.
Step 2: Identify the Grignard reagent The given reagent $\mathrm{C_2H_3MgBr}$ appears inconsistent. To obtain the given product, the reagent must be ethylmagnesium bromide: \[ \mathrm{C_2H_5MgBr} \]
Step 3: Work backwards to find compound A Structure of 3-Methylpentan-3-ol: \[ \mathrm{CH_3CH_2{-}C(OH)(CH_3){-}CH_2CH_3} \] The carbon bearing the $-\mathrm{OH}$ group is attached to:

• one methyl group ($\mathrm{CH_3}$)
• two ethyl groups ($\mathrm{CH_2CH_3}$)

In Grignard reactions, one alkyl group comes from the Grignard reagent. Thus, removing one ethyl group gives the ketone: \[ \mathrm{CH_3COCH_2CH_3} \] This compound is butan-2-one (methyl ethyl ketone).
Step 4: Verify the reaction \[ \mathrm{CH_3COCH_2CH_3 + C_2H_5MgBr \xrightarrow[\text{dry ether}]{} CH_3C(OMgBr)(CH_3)(CH_2CH_3)} \] On hydrolysis: \[ \mathrm{CH_3C(OMgBr)(CH_3)(CH_2CH_3) \xrightarrow{H_3O^+} CH_3CH_2{-}C(OH)(CH_3){-}CH_2CH_3} \] This gives the required product.

Step 5: Conclusion \[ \boxed{\text{Compound A is Butan-2-one \]