Definition of Lucas Reagent

An equimolar (1:1) mixture of a Lewis acid anhydrous ZnCl2 and concentrated HCl- Lucas Reagent, is used to identify and classify unknown alcohol (R-OH) as primary (1o), secondary (2o) or tertiary (3o). The Lucas test is based on the speed at which corresponding insoluble alkyl chlorides (R-Cl) are formed post reaction. The formation of alkyl chlorides solution appears as cloudiness/emulsion, and it is correlated with the reactivity of the alcohol, 3o alcohol being the most and 1o being the least.




General Structure


Unknown alcohol + Lucas Reagent (mix at room temperature)


Cloudiness appears immediately within 2-3 min.


3o alcohol



Cloudiness appears after 5 min.

2o alcohol


No cloudiness appears at room temperature and the solution remains clear. But on heating cloudiness appears.

1o alcohol


Here, the alcohols react by SN1 mechanism and role of anhydrous ZnCl2 is to help cleave the Carbon-Oxygen bond to attain the carbocation formation. One limitation of the test is that any alcohol greater than six carbon fail to respond to this test. 


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