Question

Grove's method is used for the preparation of alkyl halides from alcohols using $\mathrm{HX}$ and ${\mathrm{ZnCl}}_2$. The order of reactivity is :

• A. $\mathrm{HCl}>\mathrm{HBr}>\mathrm{HI}$
• B. $\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}$
• C. $\mathrm{HCl}>\mathrm{HI}>\mathrm{HBr}$
• D. $\mathrm{HI}>\mathrm{HCl}>\mathrm{HBr}$

Answer 1

The correct option is B

$\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}$

Explanation for Correct Option:

Option(B): ($\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}$)

• Grove's method is used for the preparation of alkyl halides$(\mathrm{R}-\mathrm{X})$ reaction from alcohols$\left(\mathrm{ROH}\right)$ using $\mathrm{HX}\mathrm{and}{\mathrm{ZnCl}}_2$. The general reaction is :

$\underset{\left(\mathrm{Alcohol}\right)}{\mathrm{ROH}}+\underset{\left(\mathrm{Halo}\mathrm{acid}\right)}{\mathrm{HX}}\to \underset{\left(\mathrm{Alkyl}\mathrm{halide}\right)}{\mathrm{R}-\mathrm{X}}+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}}$

• The reaction is a nucleophilic substitution reaction.
• The bond dissociation energy of the $\mathrm{HX}$bond is given as the order: $\mathrm{HCl}>\mathrm{HBr}>\mathrm{HI}.$
• Hence, the reactivity order of the respective halogen acids are:$\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}.$

The explanation for the incorrect Option:

Option(A): ($\mathrm{HCl}>\mathrm{HBr}>\mathrm{HI}$)

• Here among the Halogen halides, $\mathrm{HCl}$ is the least reactive in nature and $\mathrm{HI}$ is the most reactive.
• The option given here doesn't match with the correct option.

Option(C): ($\mathrm{HCl}>\mathrm{HI}>\mathrm{HBr}$)

• Here Chloride ion is a weaker nucleophile than Bromide than the Iodine ion.
• The order here given in the option doesn't satisfy the reactivity order.

Option(D): ($\mathrm{HI}>\mathrm{HCl}>\mathrm{HBr}$)

• As the bond dissociation energy of the $\mathrm{HX}$ bond follows the order: $\mathrm{HCl}>\mathrm{HBr}>\mathrm{HI}.$
• The reactivity order will be reversed to the order of bond dissociation energy order, but the option given here doesn't coincide with the respective order.

Hence, The order of reactivity for the Grove's methods is :$\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}.$