CHEMICAL TEST : LUCAS TEST
Purpose : Observations : Used to differentiate classes of alcohols. Reagent and condition: Conc. HCl, ZnCl2 Class of Alcohol Observation 1 ° No cloudy solution forms within 5 − 10 minutes even after heating. 2 ° Solution turns cloudy within 5 – 10 minutes 3 ° Solution turns cloudy immediately
R OH + conc. HCl R Cl haloalkane + H2O two layers or cloudiness appear due haloalkane formed is insoluble in water ZnCl2 ZnCl2 General equation : | | alcohol Lucas reagent consists of concentrated HCl and ZnCl2 as catalyst. Alcohols with 6 or a fewer carbon atoms dissolve in Lucas reagent, but the haloalkanes formed do not. Two layers or a cloudiness appears during the reaction. The time taken for the cloudiness to appear is used to differentiate between different classes of alcohols.
EXERCISE: P, Q and R are isomeric alcohols with the molecular formula C4H10O. Reactions of these isomers with Lucas reagent give the following observations. Write all the reaction equations involved. Compound Observation P CH3CH2CH2CH2OH No cloudy solution forms within 5-10 minutes even after heating Q CH3CH2CH(CH3 )OH Cloudy solution forms within 5-10 minutes. R C(CH3 )3OH Cloudy solution forms immediately.
ANSWER : CH3CH2CH2CH2OH + conc. HCl ZnCl2 No reaction CH3CH2CH(CH3 )OH + conc. HCl + H2O ZnCl2 CH3CH2CH(CH3 )Cl C(CH3 )3OH + conc. HCl + H2O ZnCl2 C(CH3 )3Cl 1 2 3