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
Primary, secondary and tertiary alcohols can be distinguished from each other in the laboratory using Lucas reagent (conc. HCl and ZnCl2). (i) An unknown alcohol is treated with Lucas reagent.
The reaction involved is:
$R - OH \underset{ ZnCl _2}{\stackrel{ HCl }{\longrightarrow}} R - Cl$
(ii) Depending on the observation an unknown alcohol can be identified as primary, secondary or tertiary.
(iii) The reaction between isopropyl alcohol and Lucas reagent can be given as:
The reagent will turns turbid slowly, without heating.
The reaction involved is:
$R - OH \underset{ ZnCl _2}{\stackrel{ HCl }{\longrightarrow}} R - Cl$
(ii) 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. |
The reagent will turns turbid slowly, without heating.
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