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Lucas Test: Lucas Reagent, Procedure, Observations, Mechanism, Applications, Practice Problems and Frequently Asked Questions(FAQs)

Lucas Test: Lucas Reagent, Procedure, Observations, Mechanism, Applications, Practice Problems and Frequently Asked Questions(FAQs)

"Have you visited a vegetable market, and got confused between Mint (Pudina) and coriander(Dhaniya), and bought the wrong one that your mother asked for?

Well, it's a common issue among the bachelors. But as a Chemist you must not use wrong chemicals in experiments and you should be able to differentiate between different chemicals."

The Lucas test is a test performed on alcohol using Lucas reagents to distinguish between primary, secondary and tertiary alcohols. Carbocations are formed as intermediates, followed by a unimolecular nucleophilic substitution reaction mechanism.

Due to the different reactivity of primary, secondary, and tertiary alcohols with Lucas reagents, they also give different results and form the basis of the Lucas test. A positive test indicates that the color of the sample has changed from clear and colorless to cloudy, indicating the formation of chloroalkanes.

Table of content:

  • Lucas reagent
  • Procedure of Lucas’ test
  • Observations from Lucas test
  • Mechanism of the reaction
  • Applications of Lucas reagent
  • Practice problems
  • Frequently asked questions(FAQs)

Lucas reagent:

Anhydrous zinc chloride (Lewis acid) is dissolved in strong hydrochloric acid to form the Lucas reagent. Low molecular weight alcohols are tested using the Lucas Reagent. According to their degree of reactivity, it is used to divide alcohols into primary, secondary, and tertiary alcohols.

Anhydrous zinc chloride and hydrochloric acid are combined to form the Lucas Reagent. Collect both reaction species—zinc chloride and hydrochloric acid—in equimolar amounts in order to make the Lucas reagent. The reaction comes after a carbocation intermediate-forming unimolecular nucleophilic substitution process, also known as the SN1 reaction. The choice between primary, secondary, and tertiary alcohols is heavily influenced by the stability of this carbocation.

Formula of Lucas reagent : Anhydrous ZnCl2+HCl; In Lucas reagent, the chloride ion of hydrochloric acid reacts with an alkyl group to form an alkyl halide, and zinc chloride acts as a catalyst.

Procedure Of Lucas’ Test:

  • The Lucas reagent must first be prepared before the Lucas test can be carried out.
  • Make a solution using equimolar quantities of anhydrous zinc chloride and hydrochloric acid to create the Lucas reagent.
  • Take a small amount of sample to be tested in a test tube.
  • Add about 23 ml of Lucas reagent to the unknown sample.
  • To make the solution effective, thoroughly combine it. Keep track of the passing time and check for turbidity or a lack thereof.
  • Lucas' test response is given as follows:

ROH+ Anhydrous ZnCl2+HCl → RCl+H2O

Observations From Lucas’ Test:

Below are the observations from the Lucas test when the unknown sample contains:

Primary alcohol: At room temperature, after adding the Lucas reagent, the solution won't turn turbid if the unknown sample includes primary alcohol. But occasionally the solution will get turbid if it is heated for 30 to 45 minutes.

Example: C2H5OH +HCl+ ZnCl2→ No turbidity

Secondary alcohol: After adding the Lucas reagent, the solution will turn turbid after 3 to 5 minutes at room temperature if the unknown sample includes secondary alcohol.

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

The turbidity is due to the formation of (CH3)2CHCl.

Tertiary Alcohol: If a sample is unknown, adding the Lucas reagent at room temperature will cause the solution to soon become cloudy if tertiary alcohol is present.

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

The production of (CH3)3CCl is the cause of the turbidity. Through a single molecule nucleophilic substitution reaction mechanism, the Lucas reagent combines with primary, secondary, and tertiary alcohols to generate a carbocation intermediate with each of the three alcohols. As you are aware, the stability of the intermediates of a carbocation is 3o>2o>1o. There is immediate turbidity in tertiary alcohols as a result of the Lucas reagent's formation of an extremely stable 3o cation.

Conversely, secondary alcohols produce reasonably stable 2o carbocation intermediates with the Lucas reagent within 3-5 minutes. On the other hand, when the Lucas reagent is applied to the test tube at room temperature, primary alcohols do not exhibit any evidence of turbidity. This occurs because an extremely unstable intermediate 1o carbocation is created when the primary alcohol and Lucas reagent react.

Mechanism Of The Reaction:

Lucas reagents react with alcohols via a unimolecular nucleophilic substitution reaction mechanism or the SN1 reaction mechanism. The overall mechanism occurs in two steps.

Step I: Formation of a carbocation intermediate

In this step, zinc chloride (from the Lucas reagent) is reacted with the alcohol to promote cleavage of the leaving group and formation of the carbocation intermediate. As Lewis acid, zinc chloride binds oxygen by accepting electrons. As a result, oxygen acquires a positive charge and zinc acquires a negative charge. This positive charge of oxygen induces the transfer of electrons from the alkyl group to oxygen, leading to the formation of carbocations. This reaction step in the Lucas reagent test is the slowest step. So, it is the rate-determining step of the reaction.

Step II: Attack of nucleophile

In this step, the nucleophile chloride ion attacks the carbocation intermediate to form an alkyl chloride. Zinc chloride as a catalyst is removed unaffected.

Applications of Lucas test:

The most important application of the Lucas test is to use Lucas reagent to distinguish between primary, secondary and tertiary alcohols.

Type of alcohol present in the sample

Reaction with Lucas reagent

Observation after adding Lucas reagent

Carbocation

formed

Primary alcohol ( 1o)

C2H5OH+HCl+ZnCl2Noturbidity

No turbidity in the solution or colorless solution as no reaction takes place at room temperature.

Primary carbocation

(highly unstable)

Secondary alcohol

(2o)

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

Turbid solution after 3-5 min. white cloudy solution formed at room temperature.

Secondary carbocation

(moderately stable)

Tertiary alcohol

(3o)

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

Turbid solution instantly at room temperature. White cloudy solution formed immediately.

Tertiary carbocation

(highly stable)

Practice problems:

Q.1. Ethanal cannot show which of the following tests?

(A) Iodoform test (B) Lucas test

(C) Fehling's test (D) Tollens test

Answer: (B)

Solution: As ethanal comes under the aldehydes it cannot give Lucas test. Lucas test is only given by primary, secondary and tertiary alcohols with low molecular weights.

Q.2. Which of the following will react very fast with Lucas’ reagent?

(A) CH3OH (B) CH3CH2OH

(C) (CH3)2CHOH (D) (CH3)3COH

Answer: (D)

Solution: Tertiary alcohol is more reactive in Lucas test due to the formation of stable tertiary carbocation and forms immediate turbidity with the Lucas reagent.

Q.3. Alcohol (A) will be Lucas tested within 5 mins. 7.4 g of alcohol releases 1120 mL of H2 with STP when treated with sodium metal. What is the molecular formula of alcohol (A)?

(A) CH3(CH2)3OH (B) CH3CH(OH)CH2CH3

(C) (CH3)3COH (D) CH3CH(OH)CH2CH2CH3

Answer: (B)

Solution: ROH+Na    RONa+12 H2

We have to get the molecular mass of alcohol corresponding to half mole of H2 only

112011200=7.4M

M=74

CnH2n+1OH=74

                             CnH2n+1=74-17

CnH2n=57-1

12n+2n=14 n=56

n=4

Thus molecular formula of A is C4H9OH. As (A) gives Lucas test within 5 min, thus secondary alcohol corresponding to the molecular formula C4H9OHisCH3CH(OH)CH2CH3 (butan-2-ol).

Q.4. The compound that reacts fast with Lucas reagent at room temperature is

(A) Butan-1-ol (B) Butan-2-ol

(C) 2-Methylpropan-1-ol (D) 2-Methylpropan-2-ol

Answer: (D)

Solution: 2-Methylpropan-2-ol is a tertiary alcohol and it will be tertiary carbocation which is more stable. So, 2-Methylpropan-2-ol will react faster with lucas reagent.

Frequently asked questions(FAQs):

Q1: Does phenol give Lucas test?
Answer:
No, Lucas Reagent is a solution of zinc chloride in concentrated hydrochloric acid. It is used to distinguish between low molecular weight primary alcohols, secondary alcohols and tertiary alcohols. The low molecular weight requirement derives from the fact that alcohol must be soluble in aqueous/acidic solutions. Tertiary alcohols react almost instantly to form the corresponding alkyl halides. Haloalkanes do not dissolve and cause haze. However, the reaction of secondary alcohols is much slower. Cloudy appears, but this lasts for a few minutes. Primary alcohol fails the test.

The Lucas test requires the formation of carbonium ions where the "OH" group was. This is not possible with phenol under reaction conditions, so phenols will fail the test.

Q2: Will benzyl alcohol (C6H5CH2OH) give Lucas test?
Answer:
Yes, even though it is primary alcohol it will give Lucas test instantly because of the formation of the stable benzyl (C6H5CH2+) carbocation.

Q3: How can one tell whether the Lucas test result is positive?
Answer:
If a cloudy turbid characteristic forms after the reaction in less than 5 or 6 minutes, it is a positive indication. Negative signs are present if there is no visible change.

Q4: What is the role of ZnCl2 in the Lucas test?
Answer:
It is necessary to replace the OH group of alcohol with Cl. This is a type of nucleophilic substitution reaction. Cl needs to replace the OH group.

However, OH is a poor leaving group due to the high electronegativity of oxygen, and Cl is a moderate (not very good) nucleophile. So it's a little difficult.

Therefore, Lucas reagent (ZnCl2+HCl) is used. ZnCl2 is Lewis acid as the d-orbital of Zn is empty. Oxygen in OH forms a coordinate bond with Zn. That is, it forms a complex with zinc. Oxygen acquires a positive charge and Zn acquires a negative charge.

So basically what happens is that the bad leaving group OH is converted to a good leaving group because of lewis acid ZnCl2 .

Related Topics:

Glycerin

Methanol

Aldol condensation

Tollens test

Acetone

phenol

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