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.