In the reaction R–OH + HX → R–X + H2O, the order of reactivity of the alcohols is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Tertiary > secondary > primary
An alcohol reacts with a halogen acid (HX) when its –OH group is replaced by the halogen, for example:
\( \displaystyle \ce{(CH3)3C-OH + HCl -> (CH3)3C-Cl + H2O} \)
The more easily the C–OH bond breaks, the faster the alcohol reacts. The ease of reaction shows in the conditions each class of alcohol needs with HCl:
Tertiary alcohols — first (most reactive). They react on shaking with concentrated HCl at room temperature, with no catalyst; in the Lucas test they give turbidity at once.
Secondary alcohols — second. They need the catalyst \( \ce{ZnCl2} \) to react with HCl, and react more slowly than tertiary alcohols.
Primary alcohols — third (least reactive). They also need \( \ce{ZnCl2} \), and even with the Lucas reagent they give no turbidity at room temperature.
NCERT (2026-27 edition):
6.4.1 From Alcohols – “… 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°.”
1. Reaction with hydrogen halides – “… The difference in reactivity of three classes of alcohols with HCl distinguishes them from one another (Lucas test). Alcohols are soluble in Lucas reagent (conc. HCl and ZnCl2) while their halides are immiscible and produce turbidity in solution. In case of tertiary alcohols, turbidity is produced immediately as they form the halides easily. Primary alcohols do not produce turbidity at room temperature.”