Question

Which of the following reactions will be the fastest under similar conditions ?

• A. $C_2H_5OH + HF \rightarrow C_2H_5F + H_2O$
• B. $C_2H_5OH + HCl \rightarrow C_2H_5Cl + H_2O$
• C. $C_2H_5OH + HBr \rightarrow C_2H_5Br + H_2O $
• D. $C_2H_5OH + HI \rightarrow C_2H_5I + H_2O $

Answer 1

Answer: (D)

$HI$ is the strongest acid and hence it protonates the alcohol most readily. Further, since $I^{-}$ is the strongest nucleophile. Therefore, it attacks the protonated alcohol molecule most easily forming the corresponding alkyl iodide.
$C_2H_5OH \xrightarrow[ - I^{-}]{HI} C_2H_5 - O^{+}H_2 \xrightarrow[Fast] {I^{-}}C_2H_5I + H_2O$