Fundamentals of Organic ChemistryChemistryAlcohols Phenols and Ethers
Arrange the following alcohols in order of increasing ease of acid-catalysed dehydration: (i) CH ₃ CH ₂ OH (ii) C ₆ H ₅ CH ₂ OH (iii) Cl ₃ CCH ₂ OH (iv) F ₃ CCH ₂ OH
Options
- A(iv) < (iii) < (i) < (ii)
- B(iv) < (iii) < (ii) < (i)
- C(ii) < (i) < (iv) < (iii)
- D(ii) < (i) < (iii) < (iv)
Correct answer
A. (iv) < (iii) < (i) < (ii)
Step-by-step solution
The ease of acid-catalysed dehydration of alcohols is directly proportional to the stability of the carbocation intermediate formed after the protonation and loss of a water molecule. The carbocations formed from the given alcohols are: (i) CH ₃ CH ₂⁺ (ii) C ₆ H ₅ CH ₂⁺ (iii) Cl ₃ CCH ₂⁺ (iv) F ₃ CCH ₂⁺ The benzyl carbocation (ii) is highly stabilized by resonance with the benzene ring, making it the most stable. The ethyl carbocation (i) is stabilized by the +I effect and hyperconjugation of the methyl group. The