The Lucas test is an important topic of Class XII Chemistry. Every year many questions are asked from this topic in the final exam. So, you need to give special attention to the preparation of this topic. In this article we will discuss Lucas test in detail with its mechanism.
Lucas test is performed to distinguish primary, secondary and tertiary alcohols and which alcohol gives fastest alkyl halide. Lucas test is based on the difference in reactivity of alcohols with hydrogen halide. Primary secondary and tertiary alcohols react with hydrogen halide (hydrochloric acid) at different rates. It follows the SN1 reaction mechanism.
The Lucas test was given by Howard Lucas in 1930. After that it soon became popular in organic chemistry for qualitative analysis. Although with the discovery of spectroscopic and chromatographic methods of qualitative analysis in organic chemistry, this test has taken a back seat and is generally used for teaching purposes in schools and colleges.
What is Lucas Test?
The Lucas Test is the test which is performed by using Lucas reagent with alcohols to distinguish primary, secondary and tertiary alcohols. In this carbocation is formed as intermediate and it follows unimolecular nucleophilic substitution reaction mechanism.
As primary, secondary and tertiary alcohols differ in their reactivity with Lucas reagent, so they give different results as well and it forms the base for Lucas Test. A positive test indicates change in color of the sample from clear and colorless to turbid signaling formation of a chloroalkane.
What is Lucas Reagent?
The solution of concentrated hydrochloric acid with zinc chloride is called Lucas reagent. Both conc. HCl and ZnCl2 are taken in equimolar quantities to make the reagent. In Lucas test, Lucas reagent reacts with alcohols and gives different results on the basis of stability of carbocation intermediate formed during the reaction. Chloride ion of hydrochloric acid reacts with alkyl group of alcohol and forms alkyl chloride while zinc chloride is used as a catalyst. Rate of reaction of primary, secondary, and tertiary alcohols with Lucas reagent differ which forms the base of the Lucas Test. General reaction involved is represented below –
ROH + HCl ZnCl2→ RCl + H2O
Alcohol Hydrochloric Alkyl chloride water
How to Perform Lucas Test?
Lucas test is performed by following steps –
Preparation of Lucas Reagent – Take equimolar quantities of zinc chloride and conc. HCl and make a solution.
Take a very small quantity of the given sample in a test tube.
Now add ~2ml Lucas reagent in the test tube containing the given sample and mix them.
Record the time until the solution becomes turbid or cloudy.
Result of Lucas Test if Sample contains 1° alcohol
If the sample contains primary alcohol, then it will not give a turbid or cloudy solution as a result at room temperature. If we give heat to the solution, then after 30-45mins turbidity comes. General reaction can be represented as follows –
Sample containing primary alcohol + Lucas Reagent 🡪 No turbidity in the solution
After giving heat/ 30-45min –
Sample containing primary alcohol + Lucas Reagent 🡪 Turbidity in the solution
For example, if an ethanol solution reacts with lucas reagent at room temperature, then it doesn’t give any turbid solution.
Result of Lucas Test if Sample contains 2° alcohol
If the sample contains secondary alcohol, then the test will give a turbid or cloudy solution as a result at room temperature after 3-5minutes. General reaction can be represented as follows –
Sample containing secondary alcohol + Lucas Reagent 3-5min.→ Turbidity in the solution
For example, if isopropyl alcohol is present in the sample solution then after adding Lucas reagent in it, it will give a turbid solution after 3-5min. Reaction is given below –
(CH3)2CHOH HCl+ZnCl2→ (CH3)2CHCl + H2O + ZnCl2
Isopropyl 2°alkyl chloride (turbid solution)
Result of Lucas Test if Sample contains 3° alcohol
If the sample contains tertiary alcohol, then the test will instantly give a turbid or cloudy solution as a result at room temperature. General reaction can be represented as follows –
Sample containing tertiary alcohol + Lucas Reagent Instantly→ Turbidity in the solution
For example, if tertiary butyl alcohol is present in the sample solution then after adding Lucas reagent in it, it will give a turbid solution instantly. Reaction is given below –
(CH3)3COH HCl+ZnCl2→ (CH3)3CCl + H2O + ZnCl2
t-butyl alcohol t-alkyl chloride (turbid solution)
Explanation of Difference in Reactivity of 1°,2° & 3° alcohols with Lucas Reagent
Reaction of primary, secondary and tertiary alcohols with Lucas reagent takes place through unimolecular nucleophilic substitution reaction mechanism. Lucas reagent forms carbocation as intermediate with all three alcohols. But stability of carbocation intermediate differs in all three reactions. Tertiary alcohol gives instant results with Lucas reagent as its carbocation is highly stable. While secondary alcohol gives result with Lucas reagent after 3-5mins as its carbocation intermediate is moderately stable and primary alcohol don’t give any result with Lucas reagent at room temperature because its carbocation is highly unstable. Thus, we can write stability of carbocation intermediate of primary, secondary and tertiary alcohol is –
Mechanism of Lucas Test Reaction
Lucas reagent reaction with alcohols is a type of SN1 reaction. In this reaction chloride ion of HCl substitutes a hydroxyl group of alcohols. It takes place by following two steps –
Loss of leaving group and formation of carbocation
Loss of leaving group and formation of carbocation – In this step zinc chloride reacts with alcohol and forms carbocation intermediate and loss of leaving group takes place. ZnCl2 behaves as lewis acid. Zinc gains electrons from the oxygen atom and gets bonded with it. Thus, zinc gets negative charge while oxygen atom gets positive charge. Now the electron deficient oxygen atom being an electronegative element gains electrons from the alkyl group. It leads to the formation of carbocation. This is the slowest step of the reaction. So, it is the rate determining step. Thus, we can say the rate of reaction depends on formation of carbocation and its stability. Reaction is given below –
Nucleophilic attack – Cl- acts as nucleophile and attacks on carbocation and forms alkyl chloride. Due to higher entropy of water, H+ of HCl reacts with the hydroxyl group and forms water. Catalyst zinc chloride gets removed as it is. Reaction is given below –
Applications of Lucas Test
Lucas test has following applications –
It is used to distinguish primary, secondary and tertiary alcohols in the sample.
It gives information about which alcohol gives fastest alkyl halides. By Lucas test we can write order of giving alkyl halides by primary, secondary and tertiary alcohols. Tertiary alcohol gives the fastest alkyl halide.
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