Question

Lucas test is used to differentiate and categorize primary, secondary and tertiary alcohols using a solution of anhydrous zinc chloride in concentrated hydrochloric acid. This solution is commonly referred to as “Lucas Reagent”.

2-Methyl-2-butonol can be differentiated from 3-methyl-2-butanol using Lucas reagent because:

• A. 2-Methyl-2-butanol gives instant turbidity with Lucas reagent
• B. 3-Methyl-2-butanol gives instant turbidity with Lucas reagent
• C. 3-Methyl-2-butanol gives turbidity after 5-10 min with Lucas reagent
• D. 2-Methyl-2-butanol gives turbidity after 5-10 min with Lucas reagent

Answer 1

Answer: (A), (C)

3-Methyl-2-butanol gives turbidity after 5-10 min with Lucas reagent

The difference between primary, secondary and tertiary alcohol can be distinguished by Lucas test on the basis of their reactivity with Lucas reagent.
Generally, reaction takes place via $S_{N}1$ mechanism.
$\text{Rate of}{S}_{N}1\text{reaction}\propto \text{Stability of carbocation formed}$
Therefore, the order of reactivity of alcohols with Lucas reagent is $3^{\circ }>2^{\circ }>1^{\circ }$.

Hence, $3^{\circ }$ alcohol (2-methyl-2-butanol) gives instant turbidity with Lucas reagent, $2^{\circ }$ alcohol (3-methyl-2-butanol) gives turbidity after 5-10 min with Lucas reagent and $1^{\circ }$ alcohol doesn't give turbidity with Lucas reagent.