Question

Write a short note on Lucas test.

Answer 1

Alcohols

• A type of organic molecule with at least one hydroxyl $\left(-\mathrm{OH}\right)$ functional group attached to a saturated carbon atom is called an alcohol.
• Primary, secondary, and tertiary alcohols are the three types of alcohol.
• Examples: Ethanol $\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\right)$ and Butanol $\left({\mathrm{C}}_4{\mathrm{H}}_9\mathrm{OH}\right)$.

Lucas test

• The Lucas test is used to distinguish between the many types of alcohol.
• The Lucas reagent is used to distinguish between primary, secondary, and tertiary alcohols.
• The Lucas test uses a solution of anhydrous zinc chloride $\left({\mathrm{ZnCl}}_2\right)$ in concentrated hydrochloric acid $\left(\mathrm{HCl}\right)$ to distinguish and classify primary, secondary, and tertiary alcohols.
• The Lucas reagent is a common name for this mixture.

Lucas test for tertiary alcohol

• Tertiary alcohol reacts right away to create an oily layer and an insoluble compound called alkyl halide.
• Examples: $2-\mathrm{methyl}-2-\mathrm{butanol}$.

Lucas test for secondary alcohol

• The reaction time of secondary alcohol is five minutes.
• The solution becomes turbid in three to five minutes and produces an oily layer.
• Example: $2-\mathrm{Pentanol}$.

Lucas test for primary alcohol

• Primary alcohols don't seem to react at all.
• Unless it is heated, the solution doesn't change color.
• When heated, the solution generates an oily layer.
• Example: $1-\mathrm{Pentanol}$.

Mechanism of Lucas test

• S_N1 nucleophilic substitution, a two-step reaction, is the reaction that takes place in this test.