Write a Short Note on Lucas Test: What is the Lucas Test: Lucas Test for Primary, Secondary, and Tertiary Alcohols (For CBSE, ICSE, IAS, NET, NRA 2022)

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Title: Write a Short Note on Lucas Test

  • Lucas test is used to differentiate and categorize primary, secondary, and tertiary alcohols using a solution of anhydrous zinc chloride in concentrated hydrochloric acid.
  • This solution is commonly referred to as the Lucas reagent.
  • A positive test is indicated by a change from clear and colorless to turbid, signaling formation of a chloroalkane.
  • The best results for this test are observed in tertiary alcohols, as they form the respective alkyl halides fastest due to higher stability of the intermediate tertiary carbocation.
  • Test was reported in 1930 and became a standard method in qualitative organic chemistry
  • This test is used to differentiate between the different classes of alcohol. This test helps to know whether the alcohol is primary, secondary, or tertiary one.
  • Lucas reagent is a mixture of concentrated .
  • Tertiary alcohol reacts immediately forms alkyl halide which is insoluble gives turbidity and forms an oily layer.
  • Secondary alcohol takes about 5 minutes to react.
  • Primary alcohols do not react noticeably.
  • Lucas test in alcohols is a test to differentiate between primary, secondary, and tertiary alcohols.
  • It is based on the difference in reactivity of the three classes of alcohols with hydrogen halides via an SN1 reaction

What is the Lucas Test?

  • Primary, Secondary and Tertiary Alcohols are classified based on their reactivity with the Lucas reagent.
  • The reaction that occurs in the Lucas test be a nucleophilic substitution reaction.
  • In this reaction, the Chloride in the zinc-chloride bond is replaced with a hydroxyl group originating from the given alcohol.
  • The reaction displays the difference in reactivity of the different types of alcohol as well as the difference in the ease at which corresponding carbocations of the alcohols are formed.
  • For example, primary alcohols do not react readily at room temperature with the added Lucas reagent whereas tertiary alcohols react immediately.
  • The observation of a change where the clear and colorless characteristic of the solution changes to a turbid, cloudy, and hazy one implies that a chloroalkane has formed.
  • This observation is a positive indication for the Lucas test.

Lucas Test for Primary, Secondary, and Tertiary Alcohols

As discussed earlier, the test can be used to differentiate the reaction speed of the alcohol with the given Lucas reagent. This is done by measuring the time taken for the clear solution to turn turbid.

Observations for Different Types of Alcohols

Observations for Different Types of Alcohols
Primary AlcoholThe solution remains colorless unless it is subjected to heat. The solution forms an oily layer when heated. Example: 1-Pentanol.
Secondary AlcoholThe solution turns turbid and forms an oily layer in three to five minutes (varies based on the solubility) . Example: 2-Pentanol.
Tertiary AlcoholThe solution turns turbid and forms an oily layer immediately. Example: 2-methyl-2-butanol.

Lucas Test Mechanism

Step 1

  • The OH group belonging to the alcohol is protonated by the hydrochloric acid.
  • Now, since chlorine is a stronger nucleophile than water, it replaces the resulting water molecule attached to the carbon.
  • This leads to the formation of a carbocation.

Step 2

  • The chloride anion now attacks the carbocation and forms an alkyl chloride.
  • This alkyl chloride is insoluble and hence turns the solution turbid.
  • The net mechanism of the Lucas test can be illustrated as follows.
Mechanism of the Lucas Test