Q.Arrange the following compounds in increasing order of their reactivity in nucleophilic addition reactions.

Concept: Resonance Stabilization Effect — The reactivity of carbonyl compounds in nucleophilic addition depends on (a) steric hindrance around the carbonyl carbon and (b) electronic effects (inductive and resonance) that stabilise or destabilise the carbonyl group.

Reasoning:

1. For aliphatic aldehydes and ketones:

   • Aldehydes (ethanal, propanal) are more reactive than ketones (propanone, butanone) because the carbonyl carbon in ketones is more sterically hindered by two alkyl groups.
   • Among aldehydes, ethanal (H attached) is less hindered than propanal (CH₃ attached), so ethanal is more reactive.
   • Among ketones, propanone (two CH₃) is less hindered than butanone (CH₃ and C₂H₅), so propanone is more reactive.

2. For aromatic carbonyl compounds:

   • Acetophenone (ketone) is least reactive due to steric hindrance and +I effect of the methyl group.
   • Benzaldehyde has no such hindrance and is more reactive.
   • p-Tolualdehyde has an electron-donating CH₃ group (via hyperconjugation), which reduces electrophilicity of carbonyl carbon → less reactive than benzaldehyde.
   • p-Nitrobenzaldehyde has a strong electron-withdrawing NO₂ group, which increases electrophilicity via resonance → most reactive.

The Core Idea: Why Reactivity Varies

Nucleophilic addition to a carbonyl group ($\ce{C=O}$) is an attack by a nucleophile on the electrophilic carbon. Two factors determine how easily this attack happens:

1. Steric Effect: The carbonyl carbon is $sp^2$ hybridised and planar. Bulky groups attached to it physically block the approach of the nucleophile. More bulk = slower reaction.
2. Electronic Effect: Electron-withdrawing groups (EWGs) pull electron density away from the carbonyl carbon, making it more positive (more electrophilic) and thus more reactive. Electron-donating groups (EDGs) push electron density toward the carbon, making it less electrophilic and less reactive.

The trick is to apply these two effects together, because they often work in opposite directions.

Part (i): Aliphatic Aldehydes and Ketones

We have: Ethanal ($\ce{CH3CHO}$), Propanal ($\ce{CH3CH2CHO}$), Propanone ($\ce{CH3COCH3}$), Butanone ($\ce{CH3COCH2CH3}$).

Step 1: Separate aldehydes from ketones first.

Aldehydes have one alkyl group attached to the carbonyl carbon; ketones have two. The general rule: Aldehydes are more reactive than ketones in nucleophilic addition. Why? Two reasons:

• Steric: Ketones have two bulky groups crowding the carbon; aldehydes have only one.
• Electronic: Alkyl groups are weakly electron-donating (via hyperconjugation and inductive effect). Ketones get two such donations, making the carbonyl carbon less positive than in an aldehyde.

So all aldehydes here will be more reactive than all ketones.

Step 2: Compare the two aldehydes — Ethanal vs Propanal.

Ethanal has a $\ce{-CH3}$ group. Propanal has a $\ce{-CH2CH3}$ group. The ethyl group is bulkier than the methyl group. So steric hindrance is greater in propanal. Also, the ethyl group is slightly more electron-donating than methyl (inductive effect). Both factors make propanal less reactive than ethanal.

Step 3: Compare the two ketones — Propanone vs Butanone.

Propanone is acetone: $\ce{CH3-CO-CH3}$. Butanone is $\ce{CH3-CO-CH2CH3}$. In butanone, one methyl is replaced by a bulkier ethyl group. This increases steric hindrance. Also, the ethyl group donates slightly more electron density than methyl. So butanone is less reactive than propanone.

Step 4: Arrange the full order.

From most reactive to least:

Ethanal > Propanal > Propanone > Butanone.

Part (ii): Aromatic Carbonyl Compounds

We have: Benzaldehyde ($\ce{C6H5CHO}$), p-Tolualdehyde ($\ce{CH3-C6H4-CHO}$), p-Nitrobenzaldehyde ($\ce{NO2-C6H4-CHO}$), Acetophenone ($\ce{C6H5COCH3}$).

Step 1: Separate aldehyde from ketone again.

Acetophenone is a ketone (one phenyl, one methyl on carbonyl). The rest are aldehydes (one phenyl, one H). So acetophenone will be the least reactive due to steric hindrance from the phenyl group and the electron-donating methyl group — and the phenyl ring itself adds a further electronic effect, examined next.

Step 2: Understand the phenyl group's dual role.

A phenyl ring is bulky (large steric hindrance). But electronically, it can be either electron-withdrawing or electron-donating depending on the situation. Here, the carbonyl carbon is $sp^2$ and the phenyl ring is conjugated with it. The ring can donate electron density into the carbonyl via resonance (making the carbon less electrophilic). This resonance stabilises the carbonyl, making it less reactive than a typical aliphatic aldehyde. So benzaldehyde is less reactive than ethanal, but still more reactive than a ketone like acetophenone.

Step 3: Compare the substituted benzaldehydes.

• p-Nitrobenzaldehyde: The nitro group ($\ce{-NO2}$) is a strong electron-withdrawing group (by both inductive and resonance effects). It pulls electron density away from the ring, which in turn pulls density away from the carbonyl carbon. This makes the carbon more electrophilic. So p-nitrobenzaldehyde is the most reactive here.
• p-Tolualdehyde: The methyl group ($\ce{-CH3}$) is electron-donating (hyperconjugation + inductive). It pushes electron density into the ring, which then pushes more density toward the carbonyl carbon, making it less electrophilic. So p-tolualdehyde is less reactive than benzaldehyde.
• Benzaldehyde: No substituent — it sits in the middle.

Step 4: Arrange the order.

From most reactive to least:

p-Nitrobenzaldehyde > Benzaldehyde > p-Tolualdehyde > Acetophenone.

Method: Combined Steric and Electronic Effect Analysis

This method evaluates two competing factors — steric hindrance around the carbonyl carbon and electronic effects (inductive and resonance) that alter the electrophilicity of the carbonyl group.

Steps for any carbonyl compound:

1. Identify the carbonyl carbon — the site of nucleophilic attack.
2. Check steric hindrance — more bulky groups attached to carbonyl carbon reduce reactivity.
3. Check electronic effects:
   • Electron-withdrawing groups (EWG) increase positive charge on carbonyl carbon → increase reactivity.
   • Electron-donating groups (EDG) decrease positive charge → decrease reactivity.
4. For aromatic aldehydes/ketones: Consider resonance stabilization — conjugation with the benzene ring reduces electrophilicity.
5. Combine both effects to rank reactivity.

(i) Ethanal, Propanal, Propanone, Butanone

Step 1: Identify functional group — all are aliphatic carbonyls.

Step 2: Steric effect

• Aldehydes ($RCHO$) have one alkyl group → less hindered.
• Ketones ($RCOR'$) have two alkyl groups → more hindered.
• Order of steric hindrance: Ethanal < Propanal < Propanone < Butanone

Step 3: Electronic effect — alkyl groups are weakly electron-donating (inductive effect), slightly reducing electrophilicity. But steric effect dominates here.

Step 4: Final order (increasing reactivity = least reactive first):

Butanone < Propanone < Propanal < Ethanal

(ii) Benzaldehyde, p-Tolualdehyde, p-Nitrobenzaldehyde, Acetophenone

Step 1: Identify type — aromatic carbonyls.

Step 2: Resonance effect — all have conjugation with benzene ring, which reduces electrophilicity compared to aliphatic aldehydes.

Step 3: Substituent effects on benzene ring:

• p-Nitrobenzaldehyde — $NO_2$ is strong EWG (withdraws electron density via resonance and induction) → most reactive.
• Benzaldehyde — no substituent → moderate.
• p-Tolualdehyde — $CH_3$ is EDG (hyperconjugation/induction) → slightly less reactive than benzaldehyde.
• Acetophenone — has a methyl group on carbonyl side (steric hindrance) plus resonance with benzene → least reactive.

Step 4: Final order (increasing reactivity):

Acetophenone < p-Tolualdehyde < Benzaldehyde < p-Nitrobenzaldehyde

Key Concept Summary

Always check steric first, then electronic — resonance effects often dominate in aromatic systems.

Common Mistake #1: Ignoring the Steric Effect in Part (i)

The Mistake:

Students rank reactivity purely by the number of alkyl groups, forgetting that bulkier groups hinder the nucleophile from attacking the carbonyl carbon.

Correct Understanding:

In nucleophilic addition, the carbonyl carbon changes from $sp^2$ to $sp^3$ — the groups around it get pushed closer together. Bulky groups (like ethyl, isopropyl) create steric hindrance, slowing the reaction.

How to Avoid:

• For aldehydes vs. ketones: Aldehydes are more reactive because they have at least one H (small) attached to the carbonyl.
• For similar compounds: more alkyl groups → more steric hindrance → less reactivity.

Example (Part i):

Ethanal (1 alkyl) > Propanal (1 alkyl, but slightly bulkier) > Propanone (2 methyls) > Butanone (methyl + ethyl)

Correct order: Ethanal < Propanal < Propanone < Butanone?

No! Actually: Ethanal > Propanal > Propanone > Butanone

(Decreasing reactivity)

Common Mistake #2: Confusing Electronic Effects in Part (ii)

The Mistake:

Students think all aromatic aldehydes are equally reactive, or they misapply +I / -I effects.

Correct Understanding:

• Electron-withdrawing groups (EWG) like $-NO_2$ increase reactivity by making the carbonyl carbon more δ+.
• Electron-donating groups (EDG) like $-CH_3$ decrease reactivity by reducing δ+.
• Benzaldehyde has no substituent — intermediate.
• Acetophenone is a ketone — less reactive than any aldehyde due to steric + electronic effects.

How to Avoid:

• Draw resonance structures: $-NO_2$ pulls electrons via -R effect; $-CH_3$ pushes via +I effect.
• Remember: More δ+ on C = faster attack by nucleophile.

Example (Part ii):

p-Nitrobenzaldehyde (strong EWG) > Benzaldehyde > p-Tolualdehyde (weak EDG) > Acetophenone (ketone)

Correct order: Acetophenone < p-Tolualdehyde < Benzaldehyde < p-Nitrobenzaldehyde

(Increasing reactivity)

Common Mistake #3: Forgetting Resonance Stabilization in Aromatic Systems

The Mistake:

Students treat benzaldehyde like a simple aldehyde, ignoring that the aromatic ring stabilizes the carbonyl via conjugation, making it less reactive than aliphatic aldehydes.

Correct Understanding:

The lone pair on the carbonyl oxygen can delocalize into the ring — this reduces the δ+ on carbon, making nucleophilic attack harder.

How to Avoid:

• Always compare: aliphatic aldehyde > aromatic aldehyde (for same alkyl chain length).
• In part (ii), benzaldehyde is less reactive than ethanal, but more reactive than acetophenone.

Quick Summary Table

Final Exam-Ready Answer

(i) Increasing reactivity:

Butanone < Propanone < Propanal < Ethanal

(ii) Increasing reactivity:

Acetophenone < p-Tolualdehyde < Benzaldehyde < p-Nitrobenzaldehyde

Key takeaway: Always check both steric and electronic effects — and never forget that aldehydes beat ketones in nucleophilic addition reactions.