Assertion (A): 2,2-Dimethylpropanal undergoes the Cannizzaro reaction with concentrated NaOH.
Reason (R): The Cannizzaro reaction is a disproportionation reaction.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are true but (R) is not the correct explanation of (A).
The Cannizzaro reaction is given by aldehydes with no α-hydrogen atom, on heating with concentrated alkali.
Assertion (A): 2,2-Dimethylpropanal undergoes the Cannizzaro reaction with concentrated NaOH. — True. In \( \ce{(CH3)3C-CHO} \) the α-carbon carries three \( \ce{CH3} \) groups and no hydrogen, so the aldehyde undergoes the reaction:
\( \displaystyle \ce{2(CH3)3C-CHO + NaOH} \xrightarrow[\Delta]{\text{conc.}} \ce{(CH3)3C-CH2OH + (CH3)3C-COONa} \)
Reason (R): The Cannizzaro reaction is a disproportionation reaction. — True. One molecule of the aldehyde is reduced to an alcohol while another is oxidised to the salt of a carboxylic acid: a self oxidation-reduction.
Why R does not explain A?
(R) only describes what kind of reaction it is; being a disproportionation does not decide which aldehydes can undergo it.
2,2-Dimethylpropanal undergoes it because it has no α-hydrogen.
NCERT (2026-27 edition):
8.4 Chemical Reactions: 5. Other reactions – “(i) Cannizzaro reaction: Aldehydes which do not have an α-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.”