Match the reagent in List-I, which on reaction with alcohol (ROH) giv…

The correct answer is Option (3) → A-I, B-III, C-II, D-IV

Alcohols are converted to alkyl halides by replacing the –OH group with a halogen. Phosphorus halides and thionyl chloride do this, and each reagent gives its own by-products:

• A. \( \ce{PCl3} \) — This matches I. RCl + H₃PO₃. One \( \ce{PCl3} \) converts three molecules of alcohol and leaves phosphorous acid.

      \( \displaystyle \ce{3ROH + PCl3 -> 3RCl + H3PO3} \)

• B. \( \ce{PCl5} \) — This matches III. RCl + POCl₃ + HCl. The by-products are phosphoryl chloride and hydrogen chloride.

      \( \displaystyle \ce{ROH + PCl5 -> RCl + POCl3 + HCl} \)

• C. Red P/\( \ce{Br2} \) — This matches II. RBr. Red phosphorus and bromine form PBr₃ in the reaction mixture, which converts the alcohol into an alkyl bromide.

      \( \displaystyle \ce{2P + 3Br2 -> 2PBr3},\quad \ce{3ROH + PBr3 -> 3RBr + H3PO3} \)

• D. \( \ce{SOCl2} \) — This matches IV. RCl + SO₂ +HCl. Both by-products are gases and escape, leaving pure alkyl chloride.

      \( \displaystyle \ce{ROH + SOCl2 -> RCl + SO2 ^ + HCl ^} \)

Shortcut: The pair A–I appears in Options (3) and (4), and B–III then leaves only Option (3).

NCERT (2026-27 edition):
From Alcohols – “The hydroxyl group of an alcohol is replaced by halogen on reaction with concentrated halogen acids, phosphorus halides or thionyl chloride. … 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.

    3R–OH + PX₃ → 3R–X + H₃PO₃    (X = Cl, Br)
    R–OH + PCl₅ → R–Cl + POCl₃ + HCl
    R–OH → R–X    (red P/X₂; X₂ = Br₂, I₂)
    R–OH + SOCl₂ → R–Cl + SO₂ + HCl”