Question

An unknown alcohol is treated with Lucas reagent. Explain how you will determine whether the alcohol is primary, secondary or tertiary. Indicate by chemical equation the reaction between isopropyl alcohol and Lucas reagent.

Answer 1

Primary, secondary and tertiary alcohols can be distinguished from each other in the laboratory using Lucas reagent (conc. HCl and ZnCl2).

1. An unknown alcohol is treated with Lucas reagent.
   The reaction involved is: \[\ce{R - OH ->[HCl][ZnCl2] R - Cl}\]
2. Depending on the observation an unknown alcohol can be identified as primary, secondary or tertiary.
   

   Test	Observation	Inference
   Unknown Alcohol + Lucas reagent	Reagent turns turbid on heating.	Primary alcohol is present.
   Unknown Alcohol + Lucas reagent	Reagent turns turbid slowly, without heating.	Secondary alcohol is present.
   Unknown Alcohol + Lucas reagent	Reagent turns turbid immediately, without heating.	Tertiary alcohol is present.

3. The reaction between isopropyl alcohol and Lucas reagent can be given as:
   \[\begin{array}{cc}
   \ce{CH3 - CH - CH3 + HCl + ZnCl2 -> CH3 - CH - CH3}\\
   \phantom{..}|\phantom{......................................}|\phantom{..}\\
   \phantom{.....}\ce{\underset{\text{Isopropyl alcohol (2° alcohol)}}{OH} \phantom{....................}\underset{\text{Isopropyl chloride}}{Cl}}\phantom{........}
   \end{array}\]
   The reagent will turns turbid slowly, without heating.