Problem 169 - A Complete Resource Book in Chemistry for JEE Main

Question

Luca's test of alcohols involves following reaction: \(\mathrm{R}-\mathrm{OH}+\mathrm{HCl} \frac{\text { anhydrous }}{\mathrm{ZnCl}_{2}} \underset{\text { white turbidity }}{\mathrm{RCl}+\mathrm{H}_{2} \mathrm{O}}\) Select the correct statement(s) for the Luca's test. (a) Lesser the acidic character of alcohol, greater is its reactivity towards Luca's reagent. (b) ROH behaves as a base. (c) \(\mathrm{CH}_{3} \mathrm{OH}\) gives Luca's test most quickly. (d) Reactivity of \(1^{\circ}, 2^{\circ}, 3^{\circ}\) alcohol, lies in the following sequence (for Luca's reaction \(3^{\circ}>2^{\circ}>1^{\circ}\) ).

Step-by-Step Solution

Answer

Statements (a), (b), and (d) are correct.

Step 1: Understand Luca's Test Mechanism

Luca's test is used to classify alcohols based on their reactivity towards hydrogen chloride (HCl) in the presence of anhydrous zinc chloride (ZnCl₂). Primary (1°) alcohols react slowly or not at all, secondary (2°) alcohols react at a moderate rate, and tertiary (3°) alcohols react immediately, forming a white turbidity due to the formation of alkyl chlorides.

Step 2: Analyze Alcohol Reactivity

In the reaction, the alcohol (ROH) acts as a base, accepting a proton from HCl. Tertiary alcohols react faster because they form more stable carbocation intermediates when the leaving group (water) departs.

Step 3: Evaluate the Given Statements

Let's examine each statement: (a) Correct - Lesser acidic (stronger base) alcohols like tertiary alcohols react faster due to stability. (b) Correct - ROH does behave as a base during the reaction, accepting a proton. (c) Incorrect - Methanol (CH₃OH), a primary alcohol, reacts slowly, not quickly. (d) Correct - The reactivity sequence for Luca's test is indeed 3° > 2° > 1° alcohols.

Key Concepts

Chemistry of Alcohols

Alcohols are organic compounds containing one or more hydroxyl (-OH) groups attached to a carbon atom. This functional group greatly influences the properties and reactivity of alcohols.
These properties play a pivotal role in reactions such as Luca's test.

• Alcohols can be classified based on the number of hydroxyl groups: monohydric, dihydric, and polyhydric alcohols.
• Another way to classify alcohols is by the substitution pattern on the carbon atom holding the -OH group: primary, secondary, and tertiary.

Understanding these classifications helps in predicting the behavior of alcohols in certain chemical tests and reactions.

Reaction Mechanisms

Understanding the reaction mechanisms is crucial for interpreting Luca’s test. The reaction involves certain key steps:
First, the alcohol behaves as a base, accepting a proton from hydrogen chloride (HCl).
In the presence of anhydrous zinc chloride (ZnCl₂), which acts as a catalyst, the -OH group of the alcohol is replaced by a chloride ion, forming alkyl chloride and water.
This process leads to the appearance of a white turbidity.

• The first step in the mechanism involves protonation of the -OH group, turning it into a better leaving group.
• Subsequently, the leaving group departs, leading to the generation of a carbocation.

This intermediary carbocation is crucial for the next concept we will discuss.

Carbocation Stability

Carbocations are pivotal intermediates in many organic reactions, including Luca's test. The stability of a carbocation significantly dictates the rate of reaction.
During Luca's test, the alcohol undergoes protonation, and then the departing water molecule leaves behind a carbocation.
The stability of these carbocations follows the order: tertiary > secondary > primary.

• Tertiary carbocations are the most stable due to electron-donating alkyl groups that stabilize the positive charge through hyperconjugation and inductive effects.
• Secondary carbocations are more stable than primary but less stable than tertiary.
• Primary carbocations are the least stable, making them less likely to form quickly.

Thus, tertiary alcohols undergo the reaction more rapidly, forming the visible turbidity more quickly.

Primary, Secondary, Tertiary Alcohols

Categorizing alcohols into primary (1°), secondary (2°), and tertiary (3°) helps predict their behavior in various reactions, including Luca's test.
Primary alcohols have the -OH group attached to a carbon atom that is bonded to only one other carbon atom, secondary alcohols to a carbon bonded to two other carbons, and tertiary to three.
In Luca’s test, the reactivity order is tertiary > secondary > primary.

• Primary alcohols react very slowly or not at all, as they form the least stable carbocations.
• Secondary alcohols react at a moderate rate because of their somewhat stable carbocations.
• Tertiary alcohols react rapidly due to the formation of highly stable carbocations.

This sequence is crucial for correctly interpreting results in practical applications of Luca's test.