Why are Aldehydes More Reactive than Ketones?
Aldehydes undergo nucleophilic addition reactions more readily than ketones because of two major factors:
+
2. Steric Effect
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More readily undergoes nucleophilic addition
The carbonyl group contains a polar C=O bond because oxygen is more electronegative than carbon.
Therefore, the carbonyl carbon carries a partial positive charge and acts as an electrophilic centre.
A nucleophile attacks this electron-deficient carbon atom.
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Nucleophilic Addition
Aldehyde
An aldehyde has one alkyl group and one hydrogen attached to the carbonyl carbon.
Ketone
A ketone has two alkyl groups attached to the carbonyl carbon.
Alkyl groups show a +I (electron-releasing) effect. They push electron density towards the carbonyl carbon.
↓
Greater +I effect
↓
Carbonyl carbon becomes less δ⁺
↓
Nucleophilic attack becomes difficult
Aldehyde
1 alkyl group + 1 H
Hydrogen is very small. Therefore, there is comparatively less crowding around the carbonyl carbon.
Ketone
2 alkyl groups
The two alkyl groups occupy more space around the carbonyl carbon. Therefore, the nucleophile faces greater steric hindrance while approaching the carbonyl carbon.
Ketone → Greater steric hindrance
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Nu⁻ attacks carbonyl carbon
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Tetrahedral intermediate
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R₂C(O⁻)(Nu)
The nucleophile attacks the electrophilic carbonyl carbon. The π bond of C=O breaks and the electron pair moves towards oxygen.
| Factor | Aldehydes | Ketones |
|---|---|---|
| General structure | R–CHO | R–CO–R′ |
| Alkyl groups attached to C=O | One | Two |
| Steric hindrance | Lower | Higher |
| +I effect | Lower | Higher |
| Electrophilicity of carbonyl C | Higher | Lower |
| Nucleophilic addition | More readily | Less readily |
For simple aliphatic carbonyl compounds, the general order of reactivity towards nucleophilic addition is:
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Other Aldehydes
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Ketones
Formaldehyde is particularly reactive because its carbonyl carbon is attached to two hydrogen atoms, giving minimum steric hindrance and minimum electron-releasing effect.
Aldehydes are more reactive than ketones towards nucleophilic addition because the carbonyl carbon in aldehydes is both less sterically hindered and more positively polarised/electrophilic.
Q. Why do aldehydes undergo nucleophilic addition more readily than ketones?
Answer:
Aldehydes have only one alkyl group attached to the carbonyl carbon, whereas ketones have two. Hence aldehydes have less steric hindrance and a smaller +I effect. Their carbonyl carbon is therefore more electrophilic and is attacked more readily by nucleophiles.
Q. Give two reasons why aldehydes are more reactive than ketones towards nucleophilic addition.
Answer:
- Steric effect: Aldehydes contain one alkyl group and one small hydrogen atom, so the carbonyl carbon is less sterically hindered.
- Electronic effect: Ketones have two electron-releasing alkyl groups, which reduce the positive character of the carbonyl carbon.
- Hence, the carbonyl carbon of aldehydes is more accessible and more electrophilic.
Q. Explain the greater reactivity of aldehydes towards nucleophilic addition reactions on the basis of steric and electronic effects.
Answer:
The carbonyl carbon is electrophilic because the C=O bond is polar. A nucleophile therefore attacks the carbonyl carbon.
In aldehydes, the carbonyl carbon is attached to one alkyl group and one hydrogen atom. In ketones, it is attached to two alkyl groups.
The two alkyl groups in ketones cause greater steric hindrance and also exert a stronger +I effect. This decreases the positive character of the carbonyl carbon.
→ Aldehydes undergo nucleophilic addition more readily.
Q. Explain in detail why aldehydes are more reactive than ketones towards nucleophilic addition reactions.
1. Polar Nature of C=O
The carbonyl carbon is electron deficient and is therefore attacked by nucleophiles.
2. Steric Effect
Aldehydes contain one alkyl group and one hydrogen atom. Ketones contain two alkyl groups.
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Less steric hindrance
↓
Greater steric hindrance
3. Electronic Effect
Alkyl groups have a +I effect. Therefore, ketones have a greater electron-releasing effect and reduce the electrophilicity of the carbonyl carbon.
4. Result
More electrophilic + Less steric hindrance
↓
Greater reactivity towards nucleophilic addition
Q. Discuss the factors responsible for the greater reactivity of aldehydes than ketones towards nucleophilic addition reactions.
1. Carbonyl Group is Electrophilic
Because oxygen is more electronegative than carbon, the carbonyl bond is polar.
The carbonyl carbon is therefore the site of nucleophilic attack.
2. Structural Difference
Ketone: R–CO–R′
An aldehyde has one alkyl group and one hydrogen, while a ketone has two alkyl groups.
3. Steric Effect
The hydrogen atom in aldehyde is very small. Consequently, the nucleophile can approach the carbonyl carbon more easily. Two alkyl groups in a ketone create greater steric hindrance.
4. Electronic Effect
Alkyl groups exert a +I effect and release electron density towards the carbonyl carbon. Ketones have two alkyl groups and hence a greater +I effect than aldehydes.
Thus, the carbonyl carbon of a ketone has a smaller partial positive charge and is less electrophilic.
5. Overall Reactivity
(i) less steric hindrance
(ii) greater electrophilicity of the carbonyl carbon.
1. Aldehydes generally undergo nucleophilic addition:
✅ Answer
B) More readily than ketones
2. The carbonyl carbon is:
✅ Answer
B) Electrophilic
3. The carbonyl bond is polar because:
✅ Answer
A
4. General structure of an aldehyde is:
✅ Answer
B) R–CHO
5. General structure of a ketone is:
✅ Answer
B) R–CO–R′
6. An aldehyde contains how many alkyl groups attached to carbonyl carbon?
✅ Answer
B) One
7. A ketone contains:
✅ Answer
B) Two alkyl groups
8. Alkyl groups generally exhibit:
✅ Answer
A) +I effect
9. Greater +I effect makes the carbonyl carbon:
✅ Answer
B) Less electrophilic
10. Which has greater steric hindrance at the carbonyl carbon?
✅ Answer
B) Ketone
11. The small atom attached to carbonyl carbon in an aldehyde is:
✅ Answer
A) Hydrogen
12. Nucleophile attacks the:
✅ Answer
A) Carbonyl carbon
13. Which is generally the most reactive towards nucleophilic addition?
✅ Answer
A) Formaldehyde
14. Correct general reactivity order is:
✅ Answer
B
15. Which factor favours nucleophilic attack on aldehydes?
✅ Answer
B) Less steric hindrance
16. Ketones have lower reactivity because:
✅ Answer
A
17. Nucleophilic addition involves attack on an:
✅ Answer
A) Electrophilic centre
18. Which carbonyl compound has minimum steric hindrance?
✅ Answer
A) Formaldehyde
19. In ketones, two alkyl groups:
✅ Answer
A
20. The two principal reasons for greater aldehyde reactivity are:
✅ Answer
A
🎯 Quick Revision
Ketone: R–CO–R′
Carbonyl carbon: Electrophilic
Nucleophile: Attacks carbonyl carbon
Aldehyde: One alkyl group + one H
Ketone: Two alkyl groups
Steric hindrance: Aldehyde < Ketone
+I effect: Aldehyde < Ketone
Reactivity: HCHO > RCHO > RCOR′
Conclusion: Aldehydes undergo nucleophilic addition more readily than ketones.
