What is the correct order of reactivity towards nucleophilic addition?

Benzaldehyde > Acetophenone > Benzophenone

The correct answer is option 4. Benzaldehyde > Acetophenone > Benzophenone.

Let us break down the reasoning behind the reactivity order of the given carbonyl compounds towards nucleophilic addition in more detail.

Reactivity of Carbonyl Compounds:

Nucleophilic Addition Mechanism:

Nucleophiles attack the electrophilic carbon in the carbonyl group (C=O). The reactivity towards nucleophilic addition depends on both electronic factors (how electrophilic the carbonyl carbon is) and steric factors (how accessible the carbonyl carbon is).

Electronic Effects:

Electron-donating groups decrease the electrophilicity of the carbonyl carbon. Electron-withdrawing groups increase the electrophilicity of the carbonyl carbon.

Steric Effects:

Larger groups attached to the carbonyl carbon can hinder the approach of nucleophiles, decreasing reactivity. Smaller groups or fewer substituents mean less steric hindrance and higher reactivity.

Analysis of Each Compound:

Benzaldehyde \((C_6H_5CHO)\):

Structure:

Electronic Effects: The benzene ring is slightly electron-withdrawing by resonance (but weakly), which increases the electrophilicity of the carbonyl carbon.

Steric Effects: Only one substituent (the benzene ring) attached to the carbonyl carbon, making it less sterically hindered.

Conclusion: Benzaldehyde has a highly electrophilic carbonyl carbon and low steric hindrance, making it the most reactive towards nucleophilic addition.

Acetophenone \((C_6H_5COCH_3)\):

Structure:

Electronic Effects: The benzene ring is electron-withdrawing by resonance, increasing the electrophilicity of the carbonyl carbon. The methyl group \((CH_3)\) is weakly electron-donating by induction, slightly decreasing electrophilicity but not enough to counter the benzene ring's effect.

Steric Effects: Two substituents (the benzene ring and the methyl group) create more steric hindrance compared to benzaldehyde.

Conclusion: Acetophenone is less reactive than benzaldehyde due to increased steric hindrance and slightly reduced electrophilicity.

Benzophenone \((C_6H_5COC_6H_5)\):

Structure:

Electronic Effects: Both benzene rings are electron-withdrawing by resonance, which would increase electrophilicity. However, having two such groups also stabilizes the carbonyl carbon significantly.

Steric Effects: Two large substituents (two benzene rings) create significant steric hindrance, making it difficult for nucleophiles to approach the carbonyl carbon.

Conclusion: Benzophenone has the lowest reactivity towards nucleophilic addition due to high steric hindrance and a more stabilized carbonyl carbon.

Summary of Reactivity:

Combining both electronic and steric factors, the order of reactivity towards nucleophilic addition is:

Benzaldehyde: Most reactive due to high electrophilicity and low steric hindrance.

Acetophenone: Moderately reactive due to intermediate steric hindrance and electrophilicity.

Benzophenone: Least reactive due to high steric hindrance and stabilized carbonyl carbon.

Thus, the correct order of reactivity towards nucleophilic addition is: Benzaldehyde > Acetophenone > Benzophenone