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Lucas reagent is a solution of anhydrous ZnCl2 & conc.HCl. This solution is used to classify alcohols of low molecular weight. This reaction is substitution in which a chloride replaces hydroxyl group. It is to differentiate between Primary, Secondary & tertiary alcohols.
ROH + HCl ------> RCl + H2O
On reaction,
Primary- no effect
Secondary- solution turns cloudy after 3-5 minutes
Tertiary- solution turns cloudy immediately
Lucas reagent is a solution of anhydrous zinc chloride (Lewis acid) in concentrated hydrochloric acid. It is used as a reagent to test alcohols and classify them in accordance to their reactivity. The reaction is a substitution reaction where the chloride of the zinc chloride gets replaced by the hydroxyl group of the alcohol.
The reactivity of the alcohol with Lucas Reagent is measured by the degree of turbidity which may vary from colorless to turbid. The formation of turbid solution happens due to the formation of chloroalkane.
This experiment was done in 1930. Since then, it is utilized as a standard technique in organic chemistry experiments. But now this method is not widely used as large number of spectroscopic and chromatographic analytical methods have replaced it.
Lucas Reagent Formula
Lucas reagent is basically a solution which is formed by the combination of HCl and ZnCl
Lucas Test
This test is more often used to categorize the different types of alcohols based on the time taken to form a turbid solution or precipitation using the Lucas Reagent namely:
In Lucas test, Zinc Chloride acts as catalyst. The classification of the alcohols is usually done based on the difference in reaction with concentrated hydrochloric acid. A simple reaction is given below:
ZnCl
ROH + HCl—>RCl + H
O
The tertiary alcohol undergoes the most stable reaction and the primary alcohol undergoes the least stable reaction. This test can be conducted only with those alcohols which are soluble in Lucas reagent and with lower molecular weight. Alcohols generally with more than six carbon atoms cannot be tested.
Lucas Reagent Preparation
The Lucas reagent can be prepared by the following steps:
Lucas Reagent Mechanism
The reaction which normally occurs is a SN
nucleophilic substitution which is a two steps reaction. Alcohols which have a capability to form carbocation intermediates exhibit this reaction. Only Secondary and Tertiary alcohols exhibit SN
nucleophilic mechanism.
The two steps which are generally followed in this reaction:
Here the first step is generally the slowest step and is the rate determining step. As the tertiary carbocation is much stabilized, they are the ones which undergo reaction and form a turbid solution. The opposite of it happens in the case of primary alcohols.
Thus on the basis of the reactions and the rate of all the three reactions, we can not only measure the reactivity but also characterize the three groups of alcohols.
Lucas Reagent MSDS
Lucas reagent is highly toxic and corrosive and should be handled carefully while conducting the experiment. The toxicity and corrosiveness arises as a result of the constituents.
Apart from the constituents the vapors of the alcohol are slightly irritating to the eyes and nose. It should not be inhaled as it may turn fatal.
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