Reactions of Haloalkanes - SN1 and SN2 Mechanisms - UNSOLVED PRACTICE SET
Chapter: Haloalkanes and Haloarenes | Topic: Reactions of Haloalkanes SN1 and SN2 Mechanisms
REACTIONS OF HALOALKANES - SN1 AND SN2 MECHANISMS - UNSOLVED PRACTICE SET
Topic: Reactions of Haloalkanes SN1 and SN2 Mechanisms
Multiple Choice Questions
Q1. In an SN2 reaction, the nucleophile attacks from:
- The same side as the leaving group
- The opposite side of the leaving group
- Any random direction
- Above the plane of the molecule
Q2. SN1 reactions proceed through:
- A carbocation intermediate
- A carbanion intermediate
- A free radical intermediate
- No intermediate
Q3. The rate of SN2 reaction depends on:
- Only the concentration of substrate
- Only the concentration of nucleophile
- Both substrate and nucleophile concentrations
- Neither substrate nor nucleophile concentration
Q4. Which substrate is most reactive towards SN1 reaction?
- CH₃Cl
- CH₃CH₂Cl
- (CH₃)₂CHCl
- (CH₃)₃CCl
Q5. Inversion of configuration occurs in:
- SN1 reactions only
- SN2 reactions only
- Both SN1 and SN2 reactions
- Neither SN1 nor SN2 reactions
Q6. Polar protic solvents favour:
- SN1 reactions
- SN2 reactions
- Both equally
- Neither
Short Answer Questions
Q7. What is the difference between SN1 and SN2 reactions in terms of:
(a) Kinetics
(b) Stereochemistry
(c) Nature of substrate
Q8. Why do tertiary alkyl halides undergo SN1 reactions while primary alkyl halides undergo SN2 reactions?
Q9. Explain why SN2 reactions are accompanied by inversion of configuration (Walden inversion).
Q10. What is a transition state? How does it differ from an intermediate in a reaction mechanism?
Q11. Why do polar aprotic solvents favour SN2 reactions while polar protic solvents favour SN1 reactions?
Q12. A student mixes (CH₃)₃CCl with aqueous NaOH and observes a precipitate of (CH₃)₃COH. Is this an SN1 or SN2 reaction? Give reason.
Long Answer Questions
Q13. (a) Describe the SN2 mechanism with a suitable example.
(b) Draw a diagram showing the transition state of an SN2 reaction.
(c) Explain why SN2 reactions show:
(i) Second-order kinetics
(ii) Inversion of configuration
(iii) No rearrangement
(d) Why does steric hindrance decrease the rate of SN2 reactions?
Q14. (a) Describe the SN1 mechanism with a suitable example.
(b) Draw energy profile diagrams for both SN1 and SN2 reactions.
(c) Explain why SN1 reactions show:
(i) First-order kinetics
(ii) Racemization (not complete inversion)
(iii) Carbocation rearrangement
(d) Why does the stability of the carbocation determine the rate of SN1 reactions?
Q15. (a) Compare SN1 and SN2 reactions with respect to:
(i) Kinetics
(ii) Stereochemistry
(iii) Substrate reactivity
(iv) Solvent effect
(v) Nature of nucleophile
(b) Predict whether the following reactions will proceed by SN1 or SN2 mechanism:
(i) CH₃Br + OH⁻ → CH₃OH + Br⁻
(ii) (CH₃)₃CBr + H₂O → (CH₃)₃COH + HBr
(iii) CH₃CH₂CH₂Cl + I⁻ → CH₃CH₂CH₂I + Cl⁻
(iv) C₆H₅CH₂Cl + CN⁻ → C₆H₅CH₂CN + Cl⁻
Numerical / Application-Based Problems
Q16. The following kinetic data was obtained for the reaction of alkyl halides with NaOH:
| Experiment | Substrate | [Substrate] (M) | [NaOH] (M) | Rate (M/s) |
|---|---|---|---|---|
| 1 | CH₃Cl | 0.1 | 0.1 | 2.0 × 10⁻⁴ |
| 2 | CH₃Cl | 0.2 | 0.1 | 4.0 × 10⁻⁴ |
| 3 | CH₃Cl | 0.1 | 0.2 | 4.0 × 10⁻⁴ |
| 4 | (CH₃)₃CCl | 0.1 | 0.1 | 1.5 × 10⁻³ |
| 5 | (CH₃)₃CCl | 0.2 | 0.1 | 3.0 × 10⁻³ |
| 6 | (CH₃)₃CCl | 0.1 | 0.2 | 1.5 × 10⁻³ |
(a) Determine the order of reaction with respect to substrate and nucleophile for CH₃Cl.
(b) Determine the order of reaction with respect to substrate and nucleophile for (CH₃)₃CCl.
(c) Identify the mechanism (SN1 or SN2) for each substrate.
(d) Calculate the rate constant for each reaction.
(e) Predict the rate if [CH₃Cl] = 0.3 M and [NaOH] = 0.15 M.
Q17. A student studies the reaction of 2-bromobutane with hydroxide ion.
(a) Write the reaction showing both enantiomers of 2-bromobutane reacting with OH⁻.
(b) If the reaction proceeds by SN2 mechanism, what is the stereochemistry of the product? Draw the structures.
(c) If the reaction proceeds by SN1 mechanism, what is the stereochemistry of the product? Explain why racemization occurs.
(d) The student uses a polar aprotic solvent (DMF) and a strong nucleophile (NaCN). Which mechanism is favoured? Explain.
(e) The student then uses a polar protic solvent (ethanol) and a weak nucleophile (H₂O). Which mechanism is favoured? Explain.
Q18. In a school laboratory, students compare the reactivity of different alkyl halides towards nucleophilic substitution.
(a) A student adds aqueous NaOH to CH₃CH₂CH₂Cl (1-chloropropane) and heats the mixture. The solution becomes cloudy and then clear. Write the reaction and explain the observations.
(b) Another student adds aqueous NaOH to (CH₃)₃CCl (tert-butyl chloride) at room temperature. The solution immediately becomes cloudy. Why does this reaction occur faster than with 1-chloropropane? Write the mechanism.
(c) A third student adds NaI in acetone to CH₃CH₂CH₂Cl. A white precipitate forms. Identify the precipitate and explain why this reaction occurs. (Hint: Consider solubility and nucleophilicity in acetone.)
(d) The teacher asks: "Why do we not use NaOH in acetone for SN2 reactions?" What is your answer?
(e) A student accidentally adds AgNO₃ to a mixture of 1-chlorobutane and 2-chloro-2-methylpropane. A white precipitate forms immediately with one compound but slowly with the other. Explain which compound reacts faster and why.