Question

In the following compounds, what is the increasing order of their reactivity towards nucleophilic addition reactions?

• A. Benzaldehyde $<$ p-Tolualdehyde $<$ p-Nitrobenzaldehyde $<$ Acetophenone
• B. Acetophenone $<$ Benzaldehyde $<$ p-Tolualdehyde $<$ p-Nitrobenzaldehyde
• C. Benzaldehyde $<$ Acetophenone $<$ p-Tolualdehyde $<$ p-Nitrobenzaldehyde
• D. Acetophenone $<$ p-Tolualdehyde $<$ Benzaldehyde $<$ p-Nitrobenzaldehyde

Answer 1

The Correct Option is D

The reactivity of carbonyl compounds towards nucleophilic addition is influenced by two main factors: the electron-withdrawing effect and steric hindrance. Elecron-withdrawing groups increase reactivity by making the carbonyl carbon more electrophilic, while steric hindrance can decrease reactivity. Let's analyze the provided compounds:

Compound	Factors Influencing Reactivity
Acetophenone	Presence of an electron-donating alkyl group reduces the electrophilicity of the carbonyl carbon, decreasing reactivity.
p-Tolualdehyde	A methyl group on the benzene ring is slightly electron-donating, which makes it less reactive than benzaldehyde but more reactive than acetophenone.
Benzaldehyde	Without additional electron-donating groups, it's more reactive than acetophenone and p-tolualdehyde but less reactive than p-nitrobenzaldehyde.
p-Nitrobenzaldehyde	The nitro group is a strong electron-withdrawing group, increasing the reactivity of the carbonyl carbon the most.

Based on these factors, the increasing order of reactivity towards nucleophilic addition reactions is as follows:

Acetophenone < p-Tolualdehyde < Benzaldehyde < p-Nitrobenzaldehyde.

Answer 2

As discussed in the previous response, the reactivity towards nucleophilic addition reactions depends on the electronic and steric effects of the substituents on the carbonyl group.

Let's reiterate the analysis:

Benzaldehyde (C₆H₅CHO): A simple aromatic aldehyde.

p-Tolualdehyde (p-CH₃C₆H₄CHO): The methyl group (CH₃) is electron-donating, decreasing reactivity.

p-Nitrobenzaldehyde (p-NO₂C₆H₄CHO): The nitro group (NO₂) is electron-withdrawing, increasing reactivity.

Acetophenone (C₆H₅COCH₃): A ketone with steric hindrance and electron donation from the methyl group, decreasing reactivity.

The electron-withdrawing nitro group in p-nitrobenzaldehyde makes it the most reactive, while the electron-donating methyl group in p-tolualdehyde reduces reactivity. The ketone, acetophenone, is the least reactive due to steric hindrance and electron donation from the methyl group. Benzaldehyde is intermediate in reactivity.

Therefore, the increasing order of reactivity is:

Acetophenone < p-Tolualdehyde < Benzaldehyde < p-Nitrobenzaldehyde

The correct answer is:

Option 4: Acetophenone < p-Tolualdehyde < Benzaldehyde < p-Nitrobenzaldehyde