Answer the questions on the basis of passage given below:
As aldehydes & ketones possess the carbonyl functional group, they show similar reactions. They exhibit nucleophilic addition reactions, reduction, oxidations, reactions due to α-hydrogen atom, and many other reactions, like Cannizzaro reaction to electrophilic substitution reactions (in case of aromatic aldehydes & ketones). These reactions help in the preparation of a number of important compounds, like acetic acid, vinyl acetate, polymers and in perfume and dye industries. Nucleophilic addition reactions form cyanohydrins, acetals, alcohols and N-substituted derivatives like imine, oxime, semicarbazone, phenylhydrazone & Schiff’s base. Reactions due to α-hydrogen atoms help in formation of aldols and α,β-unsaturated aldehydes & ketones.
In the following reaction
'A' is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → CH3COCH2CH(CH3)C(OH)(CH3)CH2CH(Br)CH3
4-Bromopentan-2-one, \( \ce{CH3COCH2CH(Br)CH3} \), has both a C–Br bond and a ketone group. With magnesium in dry ether, the C–Br end forms a Grignard reagent:
\( \displaystyle \ce{CH3COCH2CH(Br)CH3 + Mg} \xrightarrow{\text{dry ether}} \ce{CH3COCH2CH(MgBr)CH3} \)
A Grignard reagent cannot survive next to a carbonyl group. Its nucleophilic carbon at once adds to the C=O of another molecule of the bromoketone still present; an attack on its own C=O would have to close a strained three-membered ring.
\( \displaystyle \ce{CH3COCH2CH(MgBr)CH3 + CH3COCH2CH(Br)CH3 -> CH3COCH2CH(CH3)C(OMgBr)(CH3)CH2CH(Br)CH3} \)
Hydrolysis of the adduct gives the tertiary alcohol, which keeps one C=O group and one C–Br group:
\( \displaystyle \ce{CH3COCH2CH(CH3)C(OMgBr)(CH3)CH2CH(Br)CH3 + H2O -> CH3COCH2CH(CH3)C(OH)(CH3)CH2CH(Br)CH3 + Mg(OH)Br} \)
Why the other options are incorrect
Option (1) 1,2-dimethylcyclopropan-1-ol — Incorrect. This would need the Grignard carbon to attack the C=O of its own molecule, closing a highly strained three-membered ring; attack on a neighbouring molecule is the major path.
Option (2) a diol still carrying –MgBr — Incorrect. After treatment with water no C–MgBr bond can remain, and no second –OH group is formed.
Option (3) CH3COCH2CH(MgBr)CH3 — Incorrect. This is only the Grignard reagent itself; it reacts with the ketone group present, and water would in any case destroy the C–Mg bond.
NCERT (2026-27 edition):
From Grignard Reagents – “Alcohols are produced by the reaction of Grignard reagents (Unit 6, Class XII) with aldehydes and ketones. The first step of the reaction is the nucleophilic addition of Grignard reagent to the carbonyl group to form an adduct. Hydrolysis of the adduct yields an alcohol.”