Lucas Test
The Lucas Test is a qualitative analytical method used to distinguish between primary, secondary, and tertiary alcohols based on their reactivity with Lucas reagent (solution of anhydrous zinc chloride in concentrated hydrochloric acid). This test was first conducted in the year 1930. This test is based on the difference in reactivity of the three classes of alcohols with hydrogen halides via unimolecular nucleophilic substitution SN1 reaction.
Reaction of the Lucas Test
R-OH + HCl ---ZnCl2 → R-X + H2
Mechanism of the Lucas Test
The Lucas Test occurs via SN1 mechanism and takes place in the following steps:
Protonation: The hydroxyl group of the alcohol is protonated by hydrochloric acid, forming a better leaving group (water).
Carbocation Formation: The leaving group departs and form the carbocation.
Nucleophilic Attack: The chloride ion from the Lucas reagent attacks the carbocation, leading to the formation of the corresponding alkyl chloride.
Observations and Results
The results of the Lucas Test are based on how quickly turbidity appears in the solution after adding Lucas reagent:
Primary Alcohols: Reaction does not take palce immediately at room temperature; turbidity may only develop upon heating after 30-45 minutes.
Example: Ethanol
C2H5OH + HCl + ZnCl2 → No turbidity
Secondary Alcohols: Turbidity appears within 3-5 minutes at room temperature.
Example: 2-propanol.
(CH3)2CHOH + HCl + ZnCl2 → (CH3)2CHCl + ZnCl2 + H2O
The turbidity is due to the formation of (CH3)2CHCl.
Tertiary Alcohols: Turbidity appears immediately upon mixing with the reagent.
Example: 2-methyl-2-propanol.
(CH3)3COH+HCl + ZnCl2 → (CH3)3CCl+ ZnCl2 + H2O
The turbidity is due to the formation of (CH3)3CCl.
Also read Victor Meyer's Test
NEET-UG Exam Date: 5.5.2024
Which one of the following alcohols react instantaneously with Lucas reagent ?
Solution:
Option 4 is correct answer.
Tertiary alcohols reacts immediately when mixing with the Lucas reagent.
