Question

Luca's test is used for the identification of ________

• A. Aldehydes
• B. Ketones
• C. Alcohols
• D. Carboxylic acids

Answer 1

Answer:

C. Alcohols

• Lucas' test is a chemical test used to distinguish between primary, secondary, and tertiary alcohols.

• Reagent: The reagent used is Lucas reagent, which is a mixture of concentrated hydrochloric acid (HCl) and anhydrous zinc chloride (ZnCl₂).

• Chemical Principle: The reaction relies on the substitution of the hydroxyl (-OH) group with a chlorine atom, converting the alcohol into an alkyl chloride. Alkyl chlorides are insoluble in the aqueous Lucas reagent, resulting in the formation of a cloudy emulsion or turbidity.

Classification Based on Reactivity

• Tertiary (3°) Alcohols: Reacts immediately with Lucas reagent at room temperature. This results in the instant formation of a cloudy or milky layer due to the rapid formation of alkyl chlorides.

• Secondary (2°) Alcohols: Reacts more slowly, typically showing turbidity after 5 to 10 minutes at room temperature.

• Primary (1°) Alcohols: Do not react at room temperature. No turbidity or cloudiness appears; the solution remains clear unless heated, which is generally not required for the standard identification test.