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What is Lucas test and what is its significance?

What is Lucas test and what is its significance?

The Lucas test is mainly used to differentiate and then to categorize theprimary, secondary and tertiary alcohols that too by using a solution of anhydrous zinc chloride that is in concentrated hydrochloric acid. This solution used in the reaction is known as the Lucas reagent. This classification and categorization of the alcohols into Primary, Secondary and Tertiary Alcohols are all based on their reactivity with the Lucas reagent.   The reaction process can also be named as a nucleophilic substitution reaction. The  primary product that is presentis Chloride in the zinc-chloride bond that is replaced with a hydroxyl group that will be originating from the alcohol.

This reaction will show the difference in reactivity of the different types of alcohol and at the same time will demonstrate the ease at which the corresponding carbocation’s of the alcohols are formed. In this reaction it can be observed that the clear and colourless characteristic of the solution changes to a turbid, cloudy, and hazy one. This implies that a chloroalkane has formed.  This also indicates that the Lucas test is being successfully performed and the alcohol participating in it can be categorized.

This test helps us to differentiate between the different alcohols based on the way that they will react with the reagent that present in the reaction.

INDICATORS OF LUCAS TEST FOR ALCOHOLS

There are certain indicators that can be used so as to differentiate the alcohol. There is  definite speed at which the alcohols react based on their category and that is the way that the people are able to differentiate among the different alcohols. This test will see the time taken by the Lucas reagent to turn the mixture from a clear solution to turn turbid.

There is a tabular format that is being formed so that the reaction process can be better tabulated and the reaction levels of different alcohols can be noted.

Primary Alcohol The solution remains colourless until and unless heat is increased into the solution.The solution forms an oily layer when heated to a certain extent showing the presence of primary alcohol. Example: 1-Pentanol.
Secondary Alcohol The solution tends turns turbid and then forms an oily layer in three to five minutes depending upon the solution and mixture quality indicating the presence of Secondary Alcohol. Example: 2-Pentanol.
Tertiary Alcohol The solution turns turbid and will form an oily layer immediately. This indicates the presence of Tertiary Alcohol as this is one alcohol that reacts the most quickly. Example: 2-methyl-2-butanol.

As we can see that there is a definite way that the alcohols do react and as such it is also important that we understand the properties of the alcohols as well.  Let us also try and understand the mechanism that is followed in the test.

MECHANISM OF LUCAS TEST

 TheLucas test basically works with the reaction of the Lucas reagent in the solution. Thus we can say that there is a definite mechanism that this test follows by which we will be able to differentiate the Alcohols.  There are some basic steps that this chemical reaction follows and that are where we are able to identify and differentiate between the different types of alcohols at the initial stages.

STEP 1

 This is the first step of the reaction mechanism. In this step, the OH group that belongs to the alcohol is protonated by the hydrochloric acid. It is noticed here that the reaction that takes place, since chlorine is a stronger nucleophile than water, it will replace the water molecule and then will get attached to the carbon in the group. This process will lead to the formation of carbocation.in the next process that is followed, the water molecule will get replaced in the reaction process and the solution comes in direct contact with the Lucas Reagent.

STEP 2:

In this step, the reaction takes the reaction goes in the next level and in  that the Chloride anion will  now attack the carbocation and leads to the  formation  an alkyl chloride. This new compound that is formed is said to be insoluble in water and that will lead to the solution becoming turbid. The whole mechanism is being depicted in the diagram below so that we can have a clear understating of the chemical reaction.

So, here with the reaction process there is the process of differentiation that is done based on the reaction level of the alcohols. The time taken by them to turn the solution turbid when reacted with the Lucas reagent is also being calculated at the same point of time for different alcohols.

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