Reactivity - Aldehydes vs Ketones - UNSOLVED PRACTICE SET
Chapter: Aldehydes Ketones and Carboxylic Acids | Topic: Reactivity Aldehydes vs Ketones
REACTIVITY - ALDEHYDES VS KETONES - UNSOLVED PRACTICE SET
Topic: Reactivity Aldehydes vs Ketones
Multiple Choice Questions
Q1. Aldehydes are more reactive than ketones towards nucleophilic addition because:
- Aldehydes have less steric hindrance
- Aldehydes have more electrophilic carbon
- The transition state in aldehydes is more stable
- All of the above
Q2. Which of the following is most reactive towards nucleophilic addition?
- CH₃CHO
- CH₃COCH₃
- C₆H₅CHO
- C₆H₅COCH₃
Q3. The order of reactivity of aldehydes and ketones towards nucleophilic addition is:
- Aliphatic aldehydes > Aliphatic ketones > Aromatic aldehydes > Aromatic ketones
- Aromatic ketones > Aromatic aldehydes > Aliphatic ketones > Aliphatic aldehydes
- Aliphatic aldehydes > Aromatic aldehydes > Aliphatic ketones > Aromatic ketones
- Aliphatic ketones > Aliphatic aldehydes > Aromatic ketones > Aromatic aldehydes
Q4. Aromatic aldehydes are less reactive than aliphatic aldehydes because:
- The benzene ring donates electrons by resonance
- The benzene ring withdraws electrons by inductive effect
- Steric hindrance is greater in aromatic aldehydes
- Aromatic aldehydes are more stable
Q5. Which of the following does NOT give a positive Tollens' test?
- Formaldehyde
- Acetaldehyde
- Acetone
- Benzaldehyde
Q6. The reactivity of carbonyl compounds towards nucleophilic addition is affected by:
- Electronic factors only
- Steric factors only
- Both electronic and steric factors
- Neither electronic nor steric factors
Short Answer Questions
Q7. Why are aldehydes more reactive than ketones towards nucleophilic addition reactions? Explain in terms of electronic and steric factors.
Q8. Why does acetaldehyde give a positive Tollens' test while acetone does not?
Q9. Arrange the following in order of increasing reactivity towards nucleophilic addition: CH₃CHO, CH₃COCH₃, C₆H₅CHO, C₆H₅COCH₃. Give reasons.
Q10. Why is formaldehyde more reactive than acetaldehyde towards nucleophilic addition?
Q11. Explain why aromatic ketones are less reactive than aliphatic ketones towards nucleophilic addition.
Q12. Your teacher shows you that benzaldehyde does not undergo aldol condensation but acetaldehyde does. Explain why.
Long Answer Questions
Q13. (a) Compare the reactivity of aldehydes and ketones towards nucleophilic addition reactions.
(b) Explain the factors affecting reactivity:
(i) Steric hindrance
(ii) Electronic effects (inductive and resonance)
(iii) Stability of the product
(c) Why are aromatic aldehydes less reactive than aliphatic aldehydes?
Q14. (a) Explain why:
(i) Formaldehyde is more reactive than acetaldehyde
(ii) Acetaldehyde is more reactive than acetone
(iii) Acetone is more reactive than benzophenone
(b) Arrange the following in order of increasing reactivity towards nucleophilic addition and explain:
HCHO, CH₃CHO, CH₃COCH₃, C₆H₅CHO, C₆H₅COCH₃, (CH₃)₃CCOCH₃
Q15. (a) Compare the reactivity of aldehydes and ketones towards:
(i) Oxidation
(ii) Reduction
(iii) Nucleophilic addition
(iv) Aldol condensation
(b) Why do aldehydes undergo oxidation more readily than ketones?
(c) Why do methyl ketones give iodoform test but other ketones do not?
Numerical / Application-Based Problems
Q16. The following rate constants (k) were measured for the reaction of different carbonyl compounds with HCN at 25°C:
| Carbonyl Compound | k (M⁻¹s⁻¹) |
|---|---|
| HCHO | 1.0 × 10⁴ |
| CH₃CHO | 2.0 × 10² |
| CH₃COCH₃ | 3.8 × 10⁻¹ |
| C₆H₅CHO | 1.5 × 10⁻¹ |
| C₆H₅COCH₃ | 1.2 × 10⁻³ |
(a) Arrange the compounds in order of decreasing reactivity.
(b) Calculate the ratio of reactivity of HCHO to CH₃COCH₃. What does this tell you?
(c) Plot a graph of log k vs. the number of alkyl/aryl groups attached to the carbonyl carbon.
(d) Explain why C₆H₅CHO is less reactive than CH₃CHO despite both being aldehydes.
(e) Predict the approximate rate constant for (CH₃)₃CCHO based on the trend. Explain your prediction.
Q17. The following data relates to the equilibrium constants for cyanohydrin formation:
| Carbonyl Compound | Keq |
|---|---|
| HCHO | 1.0 × 10⁴ |
| CH₃CHO | 3.0 × 10² |
| CH₃COCH₃ | 2.0 × 10¹ |
| C₆H₅CHO | 5.0 × 10⁻¹ |
| C₆H₅COCH₃ | 1.0 × 10⁻² |
(a) Calculate the standard free energy change (ΔG°) for each reaction at 25°C. (ΔG° = -RT ln Keq, R = 8.314 J K⁻¹ mol⁻¹)
(b) Which compound forms the most stable cyanohydrin? Which forms the least stable?
(c) Explain why the equilibrium constant decreases from HCHO to C₆H₅COCH₃.
(d) A student claims that if Keq < 1, the reaction is not useful. Is this correct? Explain.
(e) In industry, cyanohydrins are used to prepare α-hydroxy acids and amino acids. Why is the cyanohydrin of acetaldehyde particularly important?
Q18. In a school chemistry lab, students compare the reactivity of different carbonyl compounds.
(a) A student adds Tollens' reagent to acetaldehyde and acetone. A silver mirror forms in one tube but not the other. Identify which compound gives the positive test and explain why the other does not.
(b) Another student adds Fehling's solution to formaldehyde and benzaldehyde. A red precipitate forms with one but not the other. Explain the difference. (Hint: Consider the structure of benzaldehyde.)
(c) A third student performs the iodoform test on acetaldehyde, acetone, and benzaldehyde. Yellow precipitate forms with two compounds but not the third. Identify which compound does not give the test and explain why.
(d) The students learn that in the body, aldehydes are more toxic than ketones because they are more reactive. Why are aldehydes more reactive towards biological nucleophiles like amino groups in proteins?
(e) The teacher asks: "If you were designing a drug that needs to react specifically with aldehydes but not ketones in the body, what structural features would you incorporate?" Give your answer with reasoning.