Mechanism of Dehydration - UNSOLVED PRACTICE SET
Chapter: Alcohols Phenols and Ethers | Topic: Mechanism of Dehydration
MECHANISM OF DEHYDRATION - UNSOLVED PRACTICE SET
Topic: Mechanism of Dehydration
Multiple Choice Questions
Q1. The mechanism of acid-catalyzed dehydration of alcohols involves:
- Only E1 mechanism
- Only E2 mechanism
- E1 for tertiary and secondary, E2 for primary
- E2 for all alcohols
Q2. In the E1 mechanism of dehydration, the rate-determining step is:
- Formation of protonated alcohol
- Loss of water to form carbocation
- Loss of proton to form alkene
- All steps are equally slow
Q3. The ease of dehydration of alcohols follows the order:
- Primary > Secondary > Tertiary
- Tertiary > Secondary > Primary
- Secondary > Tertiary > Primary
- All are equally easy
Q4. Rearrangement of carbocation during dehydration leads to:
- More stable alkene
- Less stable alkene
- No change in product
- Formation of ether
Q5. The dehydration of primary alcohols at 140Β°C with conc. HβSOβ gives:
- Alkene
- Ether
- Aldehyde
- Carboxylic acid
Q6. In the dehydration of alcohols, the function of HβSOβ is to:
- Act as a dehydrating agent
- Protonate the -OH group
- Both (a) and (b)
- Act as an oxidizing agent
Short Answer Questions
Q7. Write the mechanism for the acid-catalyzed dehydration of propan-2-ol to propene.
Q8. Why do tertiary alcohols dehydrate more easily than primary alcohols? Explain in terms of carbocation stability.
Q9. What is a carbocation rearrangement? Give an example where rearrangement occurs during dehydration.
Q10. Why does the dehydration of ethanol at 170Β°C give ethene, while at 140Β°C it gives ethoxyethane?
Q11. Explain why the dehydration of butan-2-ol can give both but-1-ene and but-2-ene. Which is the major product?
Q12. Your teacher shows you that 3,3-dimethylbutan-2-ol on dehydration gives 2,3-dimethylbut-2-ene as the major product, not 3,3-dimethylbut-1-ene. Explain this observation.
Long Answer Questions
Q13. (a) Describe the E1 mechanism for the acid-catalyzed dehydration of a tertiary alcohol.
(b) Draw the energy profile diagram for this mechanism.
(c) Explain why the E1 mechanism is favoured for tertiary alcohols.
(d) What is the role of the acid catalyst in this mechanism?
Q14. (a) Describe the E2 mechanism for the base-induced dehydration (dehydrohalogenation) of alcohols.
(b) How does the E2 mechanism differ from the E1 mechanism in terms of:
(i) Kinetics
(ii) Stereochemistry
(iii) Substrate preference
(c) Why is the E2 mechanism less common for alcohol dehydration than the E1 mechanism?
Q15. (a) Explain the mechanism of dehydration of ethanol to ethene at 170Β°C.
(b) Explain the mechanism of formation of ethoxyethane from ethanol at 140Β°C.
(c) Why does temperature control the product in these reactions?
(d) What would happen if the temperature exceeded 170Β°C during ethanol dehydration?
Numerical / Application-Based Problems
Q16. The dehydration of 2-methylbutan-2-ol gives two alkenes:
Product A: 2-methylbut-2-ene (major)
Product B: 2-methylbut-1-ene (minor)
(a) Write the mechanism showing the formation of both products.
(b) Explain why Product A is the major product according to Saytzeff rule.
(c) If 44 g of 2-methylbutan-2-ol (molar mass = 88 g/mol) is dehydrated and 18.2 g of Product A is obtained, calculate the percentage yield of Product A.
(d) The total mass of alkenes obtained is 25.2 g. Calculate the percentage of Product A and Product B in the mixture.
(e) A student claims that if a more bulky base is used, Product B would become major. Is this correct? Explain (Hofmann elimination concept).
Q17. The following energy profile diagram shows the dehydration of two alcohols:
Alcohol X (tertiary): Activation energy = 80 kJ/mol, ΞH = +20 kJ/mol
Alcohol Y (primary): Activation energy = 120 kJ/mol, ΞH = +25 kJ/mol
(a) Which alcohol dehydrates faster? Explain using activation energy.
(b) Calculate the ratio of rate constants at 300 K assuming the pre-exponential factor is the same for both. (Use Arrhenius equation concept)
(c) Both reactions are endothermic. What does this tell you about the stability of alkenes compared to alcohols?
(d) Draw a rough energy profile diagram for both reactions on the same axes.
(e) A catalyst reduces the activation energy of Alcohol Y to 90 kJ/mol. Calculate the new ratio of rate constants at 300 K.
Q18. In a school laboratory, students study the dehydration of alcohols.
(a) A student heats butan-1-ol with concentrated HβSOβ at 170Β°C. Two alkenes are possible. Draw their structures and identify the major product.
(b) Another student heats butan-1-ol with concentrated HβSOβ at 140Β°C. A different product forms. Identify it and write the mechanism.
(c) A third student uses 2-methylpropan-2-ol (tert-butanol) for dehydration. The reaction occurs at much lower temperature than with butan-1-ol. Explain why.
(d) The teacher demonstrates that 1-phenylethanol dehydrates to give mainly 1-phenylethene (styrene), even though a more substituted alkene is possible. Explain this observation. (Hint: Consider conjugation.)
(e) The students learn that in industry, ethanol is dehydrated to ethene on a large scale using alumina (AlβOβ) catalyst at 350Β°C. Why is alumina preferred over HβSOβ for industrial production? (Consider corrosion, separation, and environmental factors.)