Step 1 (Governing Principle): [1 Mark]

The reactivity of carbonyl compounds towards nucleophilic addition reactions is governed by two primary factors:

• Steric Effect: The presence of bulky alkyl groups around the carbonyl carbon ($C=O$) hinders the approach of the nucleophile. As the number of alkyl groups increases, steric hindrance increases, decreasing reactivity.
• Electronic Effect: Alkyl groups are electron-donating ($+I$ effect). They decrease the electrophilicity of the carbonyl carbon by reducing the partial positive charge ($\delta+$) on it, thereby decreasing reactivity.

The general order of reactivity is: Aldehydes > Ketones, and within each group, reactivity decreases as the size of the alkyl group increases.

Step 2 (Step-by-Step Analysis): [1 Mark]

Let us analyze the structures of the given compounds:

• Ethanal ($CH_3CHO$): One methyl group attached to the carbonyl carbon. (Least steric hindrance, highest electrophilicity).
• Propanal ($CH_3CH_2CHO$): One ethyl group attached. (Slightly more steric hindrance than ethanal).
• Propanone ($CH_3COCH_3$): Two methyl groups attached. (Ketone, higher steric hindrance and $+I$ effect).
• Butanone ($CH_3COCH_2CH_3$): One methyl and one ethyl group attached. (Ketone, highest steric hindrance).

Comparing the electronic and steric factors:

Step 3 (Final Concluded Answer): [1 Mark]

The increasing order of reactivity towards nucleophilic addition reactions is:

Butanone < Propanone < Propanal < Ethanal

💡 EL10 Concept Insight: Think of the carbonyl carbon as a target for a nucleophile; aldehydes are like small, open targets, while ketones are like targets surrounded by bulky "bodyguards" (alkyl groups) that block the nucleophile from getting close, making them much less reactive.