Elimination Reactions - UNSOLVED PRACTICE SET
Chapter: Haloalkanes and Haloarenes | Topic: Elimination Reactions
ELIMINATION REACTIONS - UNSOLVED PRACTICE SET
Topic: Elimination Reactions
Multiple Choice Questions
Q1. In dehydrohalogenation of alkyl halides, the reagent used is:
- Dilute NaOH
- Alcoholic KOH
- Aqueous KOH
- Dilute H₂SO₄
Q2. The Saytzeff rule states that in dehydrohalogenation:
- The less substituted alkene is formed predominantly
- The more substituted alkene is formed predominantly
- A mixture of all possible alkenes is formed equally
- No alkene is formed
Q3. The E2 mechanism is:
- A two-step process with a carbocation intermediate
- A one-step concerted process
- A radical process
- An ionic substitution process
Q4. In the E2 mechanism, the H and X atoms must be:
- On the same carbon
- On adjacent carbons and anti-periplanar
- On adjacent carbons and syn-periplanar
- Anywhere in the molecule
Q5. Which alkyl halide is most reactive towards E1 elimination?
- CH₃Cl
- CH₃CH₂Cl
- (CH₃)₂CHCl
- (CH₃)₃CCl
Q6. The reaction of tert-butyl chloride with alcoholic KOH gives:
- tert-butyl alcohol
- Isobutylene
- n-butyl alcohol
- No reaction
Short Answer Questions
Q7. Distinguish between E1 and E2 mechanisms in terms of:
(a) Kinetics
(b) Nature of substrate
(c) Stereochemistry
Q8. What is the Saytzeff rule? Give an example illustrating this rule.
Q9. Why does alcoholic KOH favour elimination while aqueous KOH favours substitution?
Q10. Explain why the E2 mechanism requires the H and X atoms to be anti-periplanar.
Q11. What is Hofmann elimination? When does it occur instead of Saytzeff elimination?
Q12. A student treats 2-bromobutane with alcoholic KOH. Two alkenes are possible. Which one is the major product according to Saytzeff rule? Write the structures.
Long Answer Questions
Q13. (a) Describe the E2 mechanism of dehydrohalogenation with a suitable example.
(b) Draw a diagram showing the anti-periplanar arrangement of H and X in the E2 transition state.
(c) Explain why the E2 mechanism shows:
(i) Second-order kinetics
(ii) Stereospecificity
(iii) No rearrangement
(d) Why is a strong base required for E2 elimination?
Q14. (a) Describe the E1 mechanism of dehydrohalogenation with a suitable example.
(b) Draw an energy profile diagram for E1 elimination.
(c) Explain why E1 reactions show:
(i) First-order kinetics
(ii) Saytzeff product preference
(iii) Carbocation rearrangement
(d) Compare E1 and E2 mechanisms with respect to substrate, base, solvent, and stereochemistry.
Q15. (a) Explain the competition between substitution and elimination reactions.
(b) What factors favour elimination over substitution?
(c) Predict the major product when 2-bromo-2-methylbutane is treated with:
(i) Aqueous NaOH at low temperature
(ii) Alcoholic KOH at high temperature
(d) Explain why tertiary alkyl halides favour elimination while methyl halides favour substitution.
Numerical / Application-Based Problems
Q16. The following kinetic data was obtained for the reaction of 2-bromo-2-methylpropane with ethoxide ion:
| Experiment | [(CH₃)₃CBr] (M) | [C₂H₅O⁻] (M) | Rate (M/s) |
|---|---|---|---|
| 1 | 0.1 | 0.1 | 1.2 × 10⁻³ |
| 2 | 0.2 | 0.1 | 2.4 × 10⁻³ |
| 3 | 0.1 | 0.2 | 1.2 × 10⁻³ |
(a) Determine the order of reaction with respect to substrate and base.
(b) Is this E1 or E2 mechanism? Explain your reasoning.
(c) Write the mechanism and identify the rate-determining step.
(d) Calculate the rate constant for this reaction.
(e) If the reaction is carried out with a stronger base (tert-butoxide) at higher temperature, how would the rate and product distribution change?
Q17. A student studies the dehydrohalogenation of 2-bromobutane.
(a) Write the structures of all possible alkene products.
(b) Identify the major product according to Saytzeff rule and the minor product.
(c) Calculate the percentage of each product if the ratio of major to minor is 4:1.
(d) If the reaction is carried out with a bulky base like tert-butoxide, which product becomes major? Explain (Hofmann rule).
(e) The student uses cisand trans-2-bromo-1-methylcyclohexane as substrates. Which isomer reacts faster in E2 elimination? Explain using stereochemistry.
Q18. In a school laboratory, students prepare alkenes from alkyl halides.
(a) A student heats 2-bromopropane with alcoholic KOH. The gas evolved decolourizes bromine water. Identify the gas and write the reaction. Why does it decolourize bromine water?
(b) Another student tries to prepare cyclohexene from bromocyclohexane. She uses alcoholic KOH and distills the product as it forms. Why is distillation important in this preparation?
(c) A third student treats 1-bromo-1-methylcyclohexane with alcoholic KOH. Two alkenes are possible. Draw their structures and identify the major product according to Saytzeff rule.
(d) The teacher asks: "Why can't we use aqueous NaOH to prepare alkenes from alkyl halides?" What is your answer?
(e) In industry, alkenes are prepared by cracking of petroleum rather than from alkyl halides. Why? (Consider economic and environmental factors.)