Final answer:
The correct order of reactivity towards nucleophilic addition reactions is A. HCHO > CH₃CHO > CH₃COCH₃ > CH₃COC₂H₅, based on the levels of steric hindrance and electronic effects on the carbonyl carbon.
Step-by-step explanation:
The correct order of reactivity towards nucleophilic addition reactions for the given substances (1) HCHO, (2) CH₃CHO, (3) CH₃COCH₃, (4) CH₃COC₂H₅ is determined by the steric hindrance and electronic effects influencing the carbonyl carbon. Higher reactivity indicates a greater susceptibility of the carbonyl carbon towards attack by nucleophiles.
The substance HCHO (formaldehyde) has no alkyl groups to donate electron density or create steric hindrance, making it the most reactive. The next in reactivity is CH₃CHO (acetaldehyde), which has a single alkyl group increasing electron density and some steric hindrance. CH₃COCH₃ (acetone) has two methyl groups, further increasing electron density and steric hindrance, and making it less reactive. Finally, CH₃COC₂H₅ (ethyl methyl ketone) has both methyl and ethyl groups, providing the greatest steric hindrance and electron donating effects, making it the least reactive towards nucleophilic addition. Therefore, the correct order is A. 1>2>3>4.