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

Question

Luca's test of alcohols involves following reaction: \(\mathrm{R}-\mathrm{OH}+\mathrm{HCl} \frac{\text { anhydrous }}{\mathrm{ZnCl}_{2}}{\underline{\phantom{xx}}}_{\text {white turbidity }} \mathrm{RCl}+\mathrm{H}_{2} \mathrm{O}\) 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 (b) and (d) are correct for Luca's test.

Step 1: Understanding Luca's Test

Luca's test is a chemical test that distinguishes primary, secondary, and tertiary alcohols based on their reactivity with hydrogen chloride (HCl) in the presence of zinc chloride (ZnCl₂). The reaction proceeds with the formation of an alkyl chloride and water, and is often indicated by the formation of a white turbidity or cloudiness.

Step 2: Analyzing Statement (a)

Statement (a) suggests that lesser acidic alcohols are more reactive. In Luca's test, the reactivity is not directly related to acidity but rather to the structure of the alcohol. Tertiary alcohols are more reactive due to the carbocation stability, not acidity. Thus, this statement is incorrect.

Step 3: Evaluating Statement (b)

In statement (b), ROH (alcohol) acting as a base implies it donates electrons. Alcohols do react with acids like HCl where they can act as a Lewis base, forming a carbocation intermediate. This statement is correct but only in the context of Bronsted acids and bases.

Step 4: Reviewing Statement (c)

Statement (c) claims that methanol, \(\mathrm{CH}_{3} \mathrm{OH}\), reacts most quickly. However, methanol being a primary alcohol is the least reactive in Luca's test among primary, secondary, and tertiary alcohols. Therefore, this statement is incorrect.

Step 5: Checking Statement (d)

Statement (d) describes the reactivity order of alcohols. In Luca's test, tertiary alcohols react more quickly due to the formation of more stable carbocations, followed by secondary and then primary alcohols. Thus, the reactivity order is \(3^{\circ}>2^{\circ}>1^{\circ}\), making this statement correct.

Key Concepts

Understanding Alcohol Reactivity

The reactivity of alcohols in chemical reactions depends on their ability to convert into other substances. In the context of the Lucas Test, alcohol reactivity refers to how quickly and effectively an alcohol can be converted to an alkyl chloride in the presence of hydrochloric acid and zinc chloride. This transformation is key to determining whether an alcohol is primary, secondary, or tertiary. In general, tertiary alcohols react more swiftly in the Lucas Test. This is because they form stable intermediates during the reaction, which facilitates the reaction's progress. Secondary alcohols follow, with primary alcohols being the least reactive. This decreasing order of alcohol reactivity - tertiary, then secondary, then primary - is crucial for accurately performing and interpreting the Lucas Test.

The Stability of Carbocations

Carbocations are positively charged ions that play a significant role in many organic reactions, including the Lucas Test. The stability of a carbocation significantly impacts how quickly an alcohol reacts in this test. Tertiary alcohols form tertiary carbocations, which are more stable due to hyperconjugation and the inductive effect from three alkyl groups.

Secondary carbocations, having two alkyl groups, are less stable than tertiary carbocations but more stable than primary ones, which are only supported by a single alkyl group.

This stability sequence is pivotal as the more stable the carbocation, the more rapidly it will form, and thus, the faster the Lucas Test will proceed.

• Tertiary carbocations (most stable)
• Secondary carbocations
• Primary carbocations (least stable)

Understanding this sequence helps explain why tertiary alcohols react first, followed by secondary, with primary being the last in the Lucas Test.

Acid-Base Reactions with Alcohols

Alcohols, when reacted with acids, typically participate in acid-base reactions due to their ability to donate electrons, thus functioning as Lewis bases. In the context of the Lucas Test, alcohols react with hydrochloric acid, forming a complex species where the alcohol initially acts as an electron pair donor.

This interaction is crucial because it sets the stage for the formation of a carbocation, which then leads to the eventual production of an alkyl chloride. Although the test is often discussed in terms of carbocation formation, the initial step is an acid-base reaction between the alcohol and hydrochloric acid.

• Alcohols act as Lewis bases by donating electrons.
• This electron donation helps form carbocations.
• The process is part of the acid-base interaction with HCl in the Lucas Test.

Recognizing the role of alcohols as bases enhances our understanding of how they react in various chemical contexts, not just in acid-base reactions but in broader organic chemistry.

Primary, Secondary, and Tertiary Alcohols

Alcohols can be classified based on the carbon atom bonded to the hydroxyl (-OH) group. Primary alcohols have the hydroxyl group attached to a carbon atom that is also connected to only one other carbon. Secondary alcohols have their hydroxyl group connected to a carbon bonded to two other carbons. Tertiary alcohols are where the hydroxyl group's carbon is connected to three other carbon atoms.

The classification affects how these alcohols behave in the Lucas Test and other reactions.

• Primary alcohols: Least reactive in the Lucas Test.
• Secondary alcohols: Moderately reactive, forming less stable carbocations than tertiary ones.
• Tertiary alcohols: Most reactive due to forming the most stable carbocations.

This understanding is essential for predicting the outcome of reactions involving alcohols, allowing chemists to choose appropriate conditions and reagents for the desired chemical transformations.