See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Step 1 – Identify Compound A: Molecular formula C3H8O with degree of unsaturation = (2×3 + 2 - 8)/2 = 0, so A is a saturated alcohol. C3H8O alcohols are 1-propanol (CH3CH2CH2OH), 2-propanol ((CH3)2CHOH). Step 2 – Identify Compound B: Oxidation of A with acidified K2Cr2O7 gives B with formula C3H6O (one degree of unsaturation, likely C=O). B gives a silver mirror test (Tollens' test), meaning B is an aldehyde. The only C3 aldehyde is propanal (CH3CH2CHO). Therefore A must be 1-propanol (CH3CH2CH2OH), which oxidizes to propanal. Step 3 – Reaction of B with semicarbazide (NH2NHCONH2) in the presence of sodium acetate: Aldehydes and ketones react with semicarbazide (NH2NHCONH2) to form semicarbazones. The reaction is: R-CHO + NH2NHCONH2 → R-CH=NNHCONH2 + H2O. For propanal (CH3CH2CHO), the product C is CH3CH2CH=NNHCONH2. Step 4 – Match with options: Option (a) CH3CH2CH=NNHCONH2 corresponds exactly to the semicarbazone of propanal. Step 5 – Why other options fail: (b) and (c) show a branched C3 skeleton (acetone-derived, i.e., (CH3)2C=), which would come from 2-propanol oxidized to acetone (a ketone). But acetone does not give a silver mirror test, contradicting the given data for B. (d) CH3CH2CH=NCONHNH2 has the connectivity reversed (N-C-N order wrong for a semicarbazone; this resembles a different derivative). Therefore, the correct answer is A.