Lucas Test

The Lucas Test is the test that 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 a 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 the change in colour of the sample from clear and colourless to turbid signalling formation of a chloroalkane. 

Lucas test is performed to distinguish primary, secondary and tertiary alcohols and which alcohol gives the 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.

The Lucas test is an important topic of Class XII Chemistry. Every year many questions are asked about 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 the Lucas test in detail with its mechanism.

What is Lucas Reagent? 

The solution of concentrated hydrochloric acid with zinc chloride is called Lucas reagent. Thus it can also be defined as a solution of anhydrous zinc chloride present in the concentrated Hydrochloric acid. It is used to classify the alcohols that have a lower molecular weight. It is a substitution reaction where the chloride finally replaces the hydroxyl group. The reaction results from the clear and colourless solution to the turbid that indicates the formation of chloroalkanes. In this reaction, the tertiary alcohols from their respective halides are much faster than the primary or the secondary alcohols because the intermediate tertiary carbocation is much stable in tertiary alcohol. Both concentrated HCl and ZnCl₂ are taken in equimolar quantities to make the reagent. 

Since the test was first conducted in the year 1930, the test became the standard test for identification of primary, secondary and test the test was first conducted in the year 1930 and since then it became the standard method for the identification of primary, secondary and tertiary alcohol. But with the development of new testing methods like spectroscopy and various chromatography, the Luca’s test is becoming less popular.   In the Lucas test, Lucas reagent reacts with alcohols and gives different results on the basis of stability of carbocation intermediate formed during the reaction. The Chloride ion of hydrochloric acid reacts with an alkyl group of alcohol and forms alkyl chloride while zinc chloride is used as a catalyst. The rate of reaction of primary, secondary, and tertiary alcohols with Lucas reagent differ which forms the base of the Lucas Test. The simple reaction involved is represented below –

ROH + HCl →  RCl + H₂O

(In presence of  ZnCl₂)

Lucas Test

Lucus test is performed to differentiate between primary, secondary and tertiary alcohols. This test is based on the difference in the reactivity of the primary, secondary and tertiary alcohol with hydrogen halide by SN1 reaction. 

ROH + HCl →  RCl + H₂O

(In presence of  ZnCl₂)

The difference in the reactivity of the degrees of alcohol provides the differing ease of formation of the corresponding carbocations. The primary carbocation is least stable followed by secondary carbocation and the tertiary alcohols form the most stable tertiary carbocations. 

The reagent for the test is the equimolar mixture of ZnCl₂ and concentrated HCl. the alcohol becomes protonated and the water molecule that is formed leaves, that makes the carbocation, and the nucleophile Cl⁻ (which is present in excess) readily attacks the carbocation, forming the chloroalkane. Due to the low solubility of the organic chloride in the aqueous mixture, the tertiary alcohol reacts immediately within five minutes that becomes evident by turbidity.  

How to Perform Lucas Test? 

Lucas test is performed by following steps –

• Preparation of Lucas Reagent – Take equimolar quantities of zinc chloride and concentrated HCl and make a solution.

• Take a very small quantity of the given sample in a test tube.

• Now add ~2ml of the Lucas reagent in the test tube containing the given sample and mix them.

• Record the time until the solution become turbid or cloudy.

Result of the 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. The reaction is given below –

(CH₃)₂CHOH + HCl+ZnCl2→ (CH₃)₂CHCl + H₂O + ZnCl₂

Isopropyl alcohol                 2°alkyl chloride (turbid solution)

Result of the 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. The reaction is given below –

(CH₃)₃COH + HCl + ZnCl2 → (CH₃)₃CCl + H₂O + ZnCl₂

Explanation of Difference in Reactivity of 1°,2° & 3° Alcohols with Lucas Reagent 

The reaction of primary, secondary and tertiary alcohols with Lucas reagent takes place through a unimolecular nucleophilic substitution reaction mechanism. Lucas reagent forms carbocation as intermediate with all three alcohols. But the 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 results 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 –

3°>2°>1°

Mechanism of Lucas Test Reaction

The reaction generally occurs in the SN1 nucleophilic reaction which is a two step reaction. The alcohols that are highly reactive become carbocation intermediates and then exhibit an immediate reaction. The SN1 nucleophilic reaction is exhibited by the primary and the secondary alcohols.

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The two-step is generally as follows:-

1. In the first step, the H⁺ proton from the hydrogen halide will first protonate the negatively charged hydroxyl ion that is OH⁻ in the alcohol. Thus the water, thus, attached to the carbon atom is a weaker nucleophile than the chlorine ion and thus the chlorine ion replaces the water. This results in the formation of a carbocation. 

2. In the second step the Cl⁻ attacks the carbocation and thus forms alkyl chloride.

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 a negative charge while oxygen atom gets a positive charge. Now the electron-deficient oxygen atom being an electronegative element gains electrons from the alkyl group. It leads to the formation of a 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 the formation of carbocation and its stability. The reaction is given below –

Nucleophilic attack – Cl⁻ acts as nucleophile and attacks on carbocation and forms alkyl chloride. Due to the higher entropy of water, H⁺ of HCl reacts with the hydroxyl group and forms water. Catalyst zinc chloride gets removed as it is. 

Applications of Lucas Test

Lucas test has the following applications –

It is used to distinguish primary, secondary and tertiary alcohols in the sample.

• It gives information about which alcohol gives the fastest alkyl halides. By Lucas test, we can write the order of giving alkyl halides by primary, secondary and tertiary alcohols. Tertiary alcohol gives the fastest alkyl halide.

3°>2°>1°