Propanal is shaken with a saturated aqueous solution of sodium hydrogensulphite. The addition compound formed is then warmed with dilute hydrochloric acid. What is obtained in the second step?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Propanal
Step 1 – Addition of NaHSO3: sodium hydrogensulphite adds to the carbonyl group of propanal by nucleophilic addition. The addition is reversible, and for most aldehydes the equilibrium lies well to the right. The crystalline addition compound carries an OH group and an –OSO2Na group on the former carbonyl carbon (scheme below the extract):
\( \displaystyle \ce{CH3CH2CHO + NaHSO3 <=> CH3CH2CH(OH)OSO2Na} \)
Step 2 – Warming with dilute acid: warming the addition compound with dilute mineral acid breaks it down and gives back the original aldehyde:
\( \displaystyle \ce{CH3CH2CH(OH)OSO2Na + HCl} \xrightarrow{\Delta} \ce{CH3CH2CHO + NaCl + SO2 + H2O} \)
Final product: the original aldehyde, propanal (Option 2). The addition compound is water soluble, so it can be taken away from other substances and then turned back into the aldehyde. This is why the reaction is used to separate and purify aldehydes.
Why the other options are incorrect
Option (1) Propan-1-ol — Incorrect. Propan-1-ol is the reduction product of propanal, formed with NaBH4 or LiAlH4:
\( \displaystyle \ce{CH3CH2CHO} \xrightarrow{\ce{NaBH4}} \ce{CH3CH2CH2OH} \)
Option (3) Propanoic acid — Incorrect. Propanoic acid needs an oxidising agent, such as Tollens’ reagent or acidified potassium dichromate. Dilute HCl does not oxidise the aldehyde.
Option (4) 2-Hydroxybutanoic acid — Incorrect. This forms when propanal adds HCN and the cyanohydrin is then hydrolysed; the extra carbon comes from the cyanide group:
\( \displaystyle \ce{CH3CH2CHO} \xrightarrow{\ce{HCN}} \ce{CH3CH2CH(OH)CN} \xrightarrow{\ce{H3O^+}} \ce{CH3CH2CH(OH)COOH} \)
NCERT (2026-27 edition):
8.4 Chemical Reactions – “The position of the equilibrium lies largely to the right hand side for most aldehydes and to the left for most ketones due to steric reasons. The hydrogensulphite addition compound is water soluble and can be converted back to the original carbonyl compound by treating it with dilute mineral acid or alkali. Therefore, these are useful for separation and purification of aldehydes.”
