Mass spectrum M+:(M-1)+ ratio and oxidation test identify aldehyde

Steps:

• Oxidation with acidified K2Cr2O7 to carboxylic acid shows J is a primary alcohol or aldehyde.
• Mass spectrum ratio M+:(M-1)+ = 4:1 indicates significant [M-H]+ peak from easy H loss.
• Aldehydes lose aldehydic H• to form stable acylium ion RCO+ at M-1, giving ~20-25% intensity relative to M+.
• Primary alcohols have weak M+ and insignificant M-1; they fragment via M-18 (water loss).
• Nitriles require concentrated acid hydrolysis, not K2Cr2O7 oxidation.

Why A is correct:

• Butanal (CH3CH2CH2CHO) is an aldehyde that oxidizes to butanoic acid and shows prominent M-1 (71) at 25% of M+ (72) due to stable C3H7CO+ formation.

Why the others are wrong:

• B. Butanol is a primary alcohol; weak M+ and no prominent M-1 peak.
• C. Propan-1-ol is a primary alcohol; same mass spec issue as B.
• D. Propanenitrile hydrolyzes to propanoic acid only with hot conc. HCl, not acidified K2Cr2O7.

Final answer: A