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Lucas Tests














Lucas reagent is a solution of anhydrous zinc chloride in concentrated hydrochloric acid and is used primarily to classify alcohols of low molecular weight.

 The result is observed by any change in the turbulence of the medium. This turbulence is due to the presence of a chloroalkane in the solution.

 Alcohols are mainly of three types:
1. Primary alcohols
2. Secondary alcohols
3. Tertiary alcohols

 Furthermore any alcohol can be distinguished from other organic compounds from the molecular formula and the presence of a hydroxyl (OH) group.

 Primary alcohols-
Primary alcohols contain the functional group -CH2OH. Examples of primary alcohols are ethanol and butanol.

        Ethanol






Secondary alcohols-

 Secondary alcohols contain the functional group -CHROH.  Here R = CH3. Examples of secondary alcohols are Propan - 2 - ol and Butan -2-ol.



  Propan - 2- ol




Tertiary alcohols - 
Tertiary alcohols contain the functional group -CR2OH. Here R = CH3. Examples of tertiary alcohols are  2-Methylbutan-2-ol  and  2-Methylpentan-2-ol .



    2-Methylbutan-2-ol  






Lucas test in alcohols is a test to distinguish between primary, secondary, and tertiary alcohols. The experiment is based on the difference in reactivity of the three classes of alcohols with hydrogen halides.

 When an alcohol reacts with a hydrogen halide it converts into the alkile halide and producing water as a by product.

 For example if we perform an experiment with ethanol and hydrogen chloride, the reaction will be as follows:

 CH3CH2OH  + HCl     →      CH3CH2Cl + H2O

 In the above reaction, first the hydroxyl part (OH-) breaks off ethanol. This leads to the formation of the carbocation - CH3CH2 + .

Then HCl breaks down into H+  and Cl-. Finally the carbocation mixes with the chloride ion and the hydroxyl ion and the hydrogen ion forms water.

 The most important part of the above reaction is the formation of the carbocation. 
This is because the three types of alcohols are separated using the Lucas reagent using this carbocation.

 Since we have three types of alcohols, the carbocations formed in the reactions involving hydrogen halides are also of three types.
They are:
1. Primary carbocations
2. Secondary carbocations
3. Tertiary carbocations

 Tertiary carbocations are far more stable than secondary carbocations, and primary carbocations are the least stable.

 As mentioned above, Lucas reagent is a mixture of ZnCl2 and HCl. 

 Tertiary alcohols react immediately to the Lucas reagent. Furthermore, turbidity, as explained above is the key factor that distinguishes the different alcohols.

 Depending upon the turbidity and the reaction rate, the three types of alcohols display the following properties:


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