Match the reagents used for converting alcohols into haloalkanes in List-I with their descriptions in List-II.
| List-I (Reagent) | List-II (Description) |
|---|---|
| (A) Thionyl chloride | (I) Produces the halogenating reagent in situ (in the reaction mixture) |
| (B) Zinc chloride | (II) Acts as the catalyst when primary and secondary alcohols react with HCl |
| (C) Concentrated hydrochloric acid | (III) Converts tertiary alcohols into alkyl chlorides just by shaking at room temperature |
| (D) Red phosphorus with bromine | (IV) Forms the alkyl halide along with two gaseous products that escape, so the product is pure |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A-IV, B-II, C-III, D-I
(A) Thionyl chloride — This matches (IV) Forms the alkyl halide along with two gaseous products that escape. The by-products SO2 and HCl are gases, so they leave the mixture and a pure alkyl chloride remains; this is why thionyl chloride is preferred.
\( \displaystyle \ce{R-OH + SOCl2 -> R-Cl + SO2 ^ + HCl ^} \)
(B) Zinc chloride — This matches (II) Acts as the catalyst when primary and secondary alcohols react with HCl. Primary and secondary alcohols react slowly with HCl, so ZnCl2 is added as the catalyst.
\( \displaystyle \ce{R-OH + HCl} \xrightarrow{\ce{ZnCl2}} \ce{R-Cl + H2O} \)
(C) Concentrated hydrochloric acid — This matches (III) Converts tertiary alcohols into alkyl chlorides just by shaking at room temperature. Tertiary alcohols are the most reactive (3° > 2° > 1°), so no catalyst is needed. HCl is the reactant here, not a catalyst.
\( \displaystyle \ce{R3C-OH + HCl} \xrightarrow{\text{room temperature}} \ce{R3C-Cl + H2O} \)
(D) Red phosphorus with bromine — This matches (I) Produces the halogenating reagent in situ. Red phosphorus and bromine form phosphorus tribromide in the reaction mixture, and PBr3 converts the alcohol into the alkyl bromide.
\( \displaystyle \ce{2P + 3Br2 -> 2PBr3} \)
\( \displaystyle \ce{3R-OH + PBr3 -> 3R-Br + H3PO3} \)
NCERT (2026-27 edition):
6.4.1 From Alcohols – “…Thionyl chloride is preferred because in this reaction alkyl halide is formed along with gases SO2 and HCl. The two gaseous products are escapable, hence, the reaction gives pure alkyl halides. The reactions of primary and secondary alcohols with HCl require the presence of a catalyst, ZnCl2. With tertiary alcohols, the reaction is conducted by simply shaking the alcohol with concentrated HCl at room temperature.” … “The order of reactivity of alcohols with a given haloacid is 3°>2°>1°. Phosphorus tribromide and triiodide are usually generated in situ (produced in the reaction mixture) by the reaction of red phosphorus with bromine and iodine respectively.”