Lucas Test is a chemical test that differentiates and categorizes primary, secondary, and tertiary alcohols using a solution of anhydrous zinc chloride in concentrated hydrochloric acid. This solution also goes by the name Lucas reagent.
Lucas test was introduced by and named after Howard Lucas in 1930.
More about Lucas Test
Alcohols are classified into the three different classes mentioned above, based on their reactivity with the Lucas reagent. The test is based on the difference in reactivity of the three classes of alcohols with hydrogen halides via an SN1 reaction.
In the reaction, the chloride in the zinc-chloride bond is replaced with a hydroxyl group that originates from the given alcohol.
ROH + HCl + ZnCl2 → RCl + H2O
The Lucas reagent is a solution of concentrated hydrochloric acid and anhydrous zinc chloride. Equimolar quantities of concentrated HCl and ZnCl2 are taken to make the reagent.
Lucas test for alcohols gives a positive indication when a clear and colorless characteristic of the reagent changes to a turbid, cloudy, and hazy characteristic, implying the formation of a chloroalkane.
The rate of formation of the chloroalkane determines the type of alcohol. It has been observed that tertiary alcohols react the fastest due to the fact that the organic chloric has relatively low solubility in the aqueous mixture.
Lucas Test Mechanism
Step 1: The OH group of the alcohol is protonated by hydrochloric acid. Due to chlorine being a stronger nucleophile than water, it replaces the resulting water molecular attached to the carbon. This forms a carbocation.
Step 2: The carbocation is attacked by the chloride anion thus forming an alkyl chloride. This alkyl chloride being insoluble turns the solution turbid.
The reaction of primary, secondary, and tertiary alcohols with Lucas reagent takes place via a unimolecular nucleophilic substitution reaction mechanism. A carbocation is formed as an intermediate with all three types of alcohol. The stability of this carbocation differs.
Tertiary alcohol gives instant results as the carbocation formed is highly stable. For secondary alcohols, the carbocation is moderately stable thus giving a result after an approximate time period of 5 minutes, whereas the primary alcohol doesn’t give any result due to a highly unstable carbocation.
Thus the stability of the carbocation can be written as follows:
3o > 2o > 1o
|Type of Alcohol||Observation after reaction|
|Primary||The solution remains colorless until heated. Upon heating, an oily layer is formed.|
|Secondary||The solution turns turbid and an oily layer is formed within 3 to 5 minutes.|
|Tertiary||The solution turns turbid and an oily layer is formed immediately.|
The observed visual change from a clear and colorless characteristic to a turbid, cloudy, and hazy characteristic, in a positive Lucas test, is caused by the Lucas reagent.
Lucas test is used to differentiate and categorize the given alcohol into three classes, namely, primary, secondary, and tertiary.
Upon reaction with the Lucas reagent, if a cloudy, turbid characteristic is observed within 5-6 minutes, then it is a positive indication. If there is no change visually, then it is a negative indication.