What is the correct order of reactivity of the following towards nucleophilic addition?

Methanal > Ethanal > Acetone

The correct answer is option 1. Methanal > Ethanal > Acetone

Let us delve deeper into why methanal (formaldehyde), ethanal (acetaldehyde), and acetone exhibit different reactivities towards nucleophilic addition:

Methanal (Formaldehyde):

Structure: Methanal has the structure HCHO.

Reactivity:

Electrophilicity: Methanal is highly reactive due to its structure where the carbonyl carbon is directly bonded to two hydrogen atoms.

Steric Hindrance: There is minimal steric hindrance around the carbonyl carbon, allowing nucleophiles to approach and attack the electrophilic carbon atom readily.

Effect on Reactivity: This makes methanal highly susceptible to nucleophilic attack, resulting in rapid addition reactions with nucleophiles.

Ethanal (Acetaldehyde):

Structure: Ethanal has the structure CH₃CHO.

Reactivity:

Electrophilicity: Ethanal is also reactive, but slightly less so than methanal due to the presence of a methyl group (–CH₃) adjacent to the carbonyl carbon.

Steric Hindrance: The methyl group provides some steric hindrance, which reduces the electrophilicity of the carbonyl carbon compared to methanal.

Effect on Reactivity: Nucleophilic addition to ethanal is slower compared to methanal but still occurs readily under appropriate conditions.

Acetone:

Structure: Acetone has the structure (CH₃)₂CO.

Reactivity:

Electrophilicity: Acetone is the least reactive towards nucleophilic addition among the three compounds.

Steric Hindrance: Acetone has two methyl groups attached to the carbonyl carbon, which greatly increases steric hindrance.

Effect on Reactivity: The significant steric hindrance around the carbonyl carbon in acetone reduces the electrophilicity of the carbonyl carbon. This makes acetone less susceptible to nucleophilic attack compared to methanal and ethanal.

Summary:

Methanal is the most reactive towards nucleophilic addition due to its high electrophilicity and minimal steric hindrance.

Ethanal is moderately reactive, with some steric hindrance from the methyl group.

Acetone is the least reactive due to significant steric hindrance from the two methyl groups attached to the carbonyl carbon.

This differential reactivity can be attributed to the structural differences and the resultant electronic and steric effects around the carbonyl carbon in each compound.