13C isotope ratio reveals 4 carbons; oxidation test identifies functional group Steps: - The M+1/M+ ratio of 4.91% arises from 13C isotope abundance (1.1% per carbon), so number of carbons n ≈ 4.91 / 1.1 = 4.5, indicating 4 carbons. - Options with 3 carbons (C, D) are eliminated as their ratio would be ~3.3%. - Acidified K2Cr2O7 oxidation to carboxylic acid fits aldehydes or primary alcohols with 4 carbons (A, B). - Alcohols show weak M+ peaks due to fragmentation (e.g., loss of H2O), making isotope ratios unreliable; aldehydes have prominent M+ peaks, fitting the given data. Why A is correct: - Butanal (C4H8O) has 4 carbons (expected M+1/M+ ≈ 4.4%, near 4.91%) and, as an aldehyde, oxidizes to butanoic acid per standard Cr(VI) oxidation of aldehydes to COOH. Why the others are wrong: - B. Butanol (C4H10O) has 4 carbons but weak M+ peak in mass spectrum prevents reliable M+1/M+ measurement. - C. Propan-1-ol has 3 carbons (M+1/M+ ≈ 3.3%, not 4.91%). - D. Propanenitrile has 3 carbons (M+1/M+ ≈ 3.3%, not 4.91%) and hydrolyzes to acid only …