What's The Formation Two Layers In Lucas Test?


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Deborah Mann answered
  • The Reagent Test
The Lucas reagent uses ZnCl in an aqueous HCl solution. The reagent reacts with alcohols in a reaction (SN1), forming an alkyl chloride. This forms as a cloudy layer first because it is immiscible with water. This eventually separates out to form the other layer.

  • Limitations
The test needs dissolved compounds to work because solid alcohols are difficult to dissolve in the first place and multifunctional compounds can give some very unusual results. The Lucas Test will not detect phenols as they do not react with the reagent.

Secondary, tertiary or benzylic alcohols do react positively, but primary alcohols give negative results. Where alcohol is the primary it will dissolve into the reagent, giving no real positive result. Alcohol molecules with more than six carbon atoms cannot always be tested using the Lucas Test.

  • Procedure
Use two drops of the test liquid to 0.5ml of Lucas reagent, using a small vial to contain it. Shake the vial vigorously to mix.  Secondary alcohols generate a cloudy solution, and may separate out into layers after a wait of around five minutes. Benzylic, allylic and Tertiary alcohols do give a cloudy layer result, creating a separate layer almost immediately.

Note: Due to the chemicals involved in the Lucas reagent tests, make sure the vial is stoppered correctly with a cork rather than using your thumb because this can be hazardous.

  • Positive Test
Three percent alcohols will show an immediate result; otherwise you will have to wait for two or three minutes.

Two percent concentrations of alcohol will demonstrate a result in five to ten minutes.

One percent will not give any result, as explained above in the limitations section.

  • Layer Composition
The layer that forms is alkyl chloride; this is the insoluble, immiscible layer or emulsion that forms as a consequence of the test.

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