Lucas Test - Procedure, Observations, Example, Uses, FAQs

In this article we will discuss Lucas reagent Lucas test reaction and Lucas test mechanism.

Lucas test

Lucas reagent test involves a reaction in which a substitution reaction takes place. It is based on the rate at which alkyl halides are formed by primary, secondary and tertiary alcohols.

The Lucas test experiment was done in 1930 by Howard Lucas. After that, it is utilized as a standard qualitative analysis in organic chemistry experiments. But due to advancements in the number of spectroscopic and chromatographic analytical methods, this method is not widely used as before and is mainly used for teaching purposes. Lucas test reaction follows S_N1 mechanism or unimolecular nucleophilic substitution reaction mechanism where the chloride of the hydrogen chloride, from Lucas reagent, gets replaced by the hydroxyl group of the alcohol.

Carbocation is formed as an intermediate species during unimolecular nucleophilic substitution reaction or S_N1 reaction. Lucas reagent reacts with primary, secondary and tertiary alcohols to form solutions of different degrees of turbidity. The turbidity of solution may differ from colorless to turbid. The formation of alkyl halide (here, chloroalkane) leads to the formation of turbid solution.

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Lucas reagent

Lucas reagent is a solution of anhydrous zinc chloride (Lewis acid) in concentrated hydrochloric acid. Lucas reagent is used as a reagent to test alcohols of low molecular weight. Lucas reagent is used to classify alcohols in primary, secondary and tertiary alcohols according to their reactivity. Lucas reagent is a solution mixture of hydrochloric acid and zinc chloride. To make the Lucas reagent, both of the reacting species that are zinc chloride and hydrochloric acid are taken in equimolar quantities.

The reaction follows a unimolecular nucleophilic substitution reaction or S_N1 reaction in which a carbocation intermediate is formed. The stability of this carbocation is the key factor in determining which type of alcohol is used that is primary, secondary or tertiary alcohol. Lucas reagent formula: ZnCl₂ + HCl; in Lucas reagent the chloride ion of hydrochloric acid reacts with alkyl group or substitute alcoholic functional group to form alkyl halide while zinc chloride acts as an catalyst.

Lucas reagent test procedure

• Procedure to perform Lucas test initiates with preparation of Lucas reagent.

• To prepare Lucas reagent take an equimolar amount of zinc chloride and hydrochloric acid to form a solution.

• Take a small amount of sample that needs to be examined in a test tube.

• To the unknown sample add about 2-3 ml Lucas reagent.

• Mix the solution well and let the solution sit.

• Record the time taken and observe the presence of turbidity (cloudiness) or no turbidity.

Lucas test reaction can be given as:

ROH + HCl ZnCl₂→ RCl + H₂O

Also, students can refer,

• NCERT solutions for Class 12 Chemistry Chapter 11 Alcohols, Phenols and Ethers
• NCERT Exemplar Class 12 Chemistry Solutions Chapter 11 Alcohols, Phenols and Ethers
• NCERT notes Class 12 Chemistry Chapter 11 Alcohols, Phenols and Ethers

Observations of Lucas test

Following are observation of Lucas test if unknown sample contains:

Primary alcohol: if the unknown sample contains primary alcohol the solution after adding Lucas reagent will not turn turbid at room temperature. However, if the solution is heated for a good 30-45 minutes turbidity in the solution appears.

Example: C₂H₅OH + HCl +ZnCl₂ room temperature→ NO TURBIDITY

Secondary alcohol: if the unknown sample contains secondary alcohol the solution after adding Lucas reagent will turn turbid at room temperature after 3-5 minutes.

Example: (CH₃)₂CHOH HCl+ ZnCl₂ (after 3-5 min) → (CH₃)₂CHCl + H₂O + ZnCl₂

Turbidity is due to formation of (CH₃)₂CHCl.

Tertiary alcohol: if the unknown sample contains secondary alcohol the solution after adding Lucas reagent will turn turbid at room temperature immediately.

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

Turbidity is due to formation of (CH₃)₃CCl.Lucas reagent reacts with primary, secondary and tertiary alcohols through unimolecular nucleophilic substitution reaction mechanism and forms carbocation as intermediate with all three alcohols. As we know, the stability of the carbocation intermediate is 3°>2°>1°. Lucas reagent gives instant turbidity with tertiary alcohol due to formation of highly stable 3° cation.

While secondary alcohol forms moderately stable 2° carbocation intermediate and gives result with Lucas reagent after 3-5mins. Primary alcohol on the other hand, shows no signs of turbidity at room temperature when Lucas reagent is added to the test tube. This is because primary alcohol reacts with Lucas reagent to form 1° carbocation intermediate which is highly unstable.

Lucas test mechanism

Lucas reagent reacts with alcohol through a unimolecular nucleophilic substitution reaction mechanism or S_N1 reaction mechanism. The overall mechanism takes place in two steps:

1. Step involving formation of carbocation intermediate by loss of leaving group.

This step involves reaction of zinc chloride (from Lucas reagent) with alcohol to facilitate detachment of the leaving group and formation of carbocation intermediate. As a Lewis acid, zinc chloride bonds with oxygen by gaining electrons. Due to this oxygen gets a positive charge while zinc gets a negative charge. This positive charge on oxygen induces transfer of electrons from the alkyl group to oxygen leading to formation of carbocation. This reaction step of Lucas reagent test is rate determining step of the reaction and thus it is the slowest step.

2. Step involving nucleophilic attack

This step involves an attack of chloride ion, which is a nucleophile, on the carbocation intermediate to form alkyl chloride. The hydrogen ion form due to ionization of hydrogen chloride reacts with leaving hydroxyl groups to form water. Zinc chloride being a catalyst gets removed unaffected.

Uses of Lucas test

Most important application of the Lucas test is to distinguish between primary, secondary and tertiary alcohol using Lucas reagent.

Also check-

• NCERT Exemplar Class 11th Chemistry Solutions
• NCERT Exemplar Class 12th Chemistry Solutions
• NCERT Exemplar Solutions for All Subjects

NCERT Chemistry Notes:

• NCERT Notes Class 11th Chemistry
• NCERT Notes Class 12th Chemistry
• NCERT Notes For All Subjects