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.

Experiment

Observation

Conclusion

General Structure

 

Unknown alcohol + Lucas Reagent (mix at room temperature)

 

Cloudiness appears immediately within 2-3 min.

 

3o alcohol

 

R3C-OH

Cloudiness appears after 5 min.

2o alcohol

R2CH-OH

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

1o alcohol

R-CH2-OH

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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