Problem 24 - A Complete Resource Book in Chemistry for JEE Main

Question

The compound which reacts fastest with Lucas reagent at room temperature is (a) butan - 1-ol (b) butan-2-ol (c) 2 -methylpropan-2-ol (d) 2-methylpropan-1-ol

Step-by-Step Solution

Answer

2-methylpropan-2-ol reacts fastest with Lucas reagent because it is a tertiary alcohol.

Step 1: Understanding Lucas Reagent

Lucas reagent is a mixture of concentrated HCl and ZnCl₂, and it is used to test alcohol reactivity based on their degree. It reacts with alcohols to form alkyl chlorides, and the reactivity order is tertiary > secondary > primary alcohols.

Step 2: Identifying Alcohol Types

First identify each compound's structure and its degree as an alcohol. (a) butan-1-ol is a primary alcohol. (b) butan-2-ol is a secondary alcohol. (c) 2-methylpropan-2-ol is a tertiary alcohol. (d) 2-methylpropan-1-ol is a primary alcohol.

Step 3: Predicting Reactivity with Lucas Reagent

Based on the reactivity order (tertiary > secondary > primary), the tertiary alcohol, 2-methylpropan-2-ol, will react the fastest with Lucas reagent at room temperature. Tertiary alcohols typically react immediately, providing a cloudy solution shortly after mixing.

Key Concepts

Primary Alcohols

Primary alcohols consist of an -OH group attached to a carbon atom that is also bonded to only one other carbon atom. A common example is butan-1-ol. These alcohols have a simpler and more linear structure compared to other alcohols. Because of their structure, primary alcohols react slowly with the Lucas reagent. This is due to the fact that primary carbocations, which form during the reaction, are quite unstable. Hence, primary alcohols tend to show little to no cloudiness when mixed with Lucas reagent at room temperature, reflecting their lower reactivity.

Secondary Alcohols

Secondary alcohols are characterized by the -OH group being attached to a carbon atom bonded to two other carbon atoms. An example is butan-2-ol. Their intermediate structure influences their reactivity. When secondary alcohols react with Lucas reagent, they form secondary carbocations, which are more stable than primary ones, but less stable than tertiary carbocations. Hence, they show an intermediate rate of reaction. When a secondary alcohol is tested with Lucas reagent, cloudiness forms within a few minutes, indicating the formation of alkyl chlorides.

Tertiary Alcohols

Tertiary alcohols have the -OH group attached to a carbon atom bonded to three other carbon atoms, exemplified by 2-methylpropan-2-ol. These alcohols have the most complex structure, which significantly increases their reactivity. Tertiary alcohols react very rapidly with Lucas reagent, often producing a cloudy solution almost instantaneously. This quick reaction is due to the formation of a highly stable tertiary carbocation, which facilitates the conversion of the alcohol into an alkyl chloride quickly and visibly.

Alcohol Reactivity

The reactivity of alcohols with the Lucas reagent is an essential concept in organic chemistry. The key factors influencing reactivity are the structure and stability of the carbocations formed during the reaction. Tertiary carbocations are the most stable, followed by secondary, and then primary. As a result, tertiary alcohols react the fastest, while primary alcohols are the slowest. This reactivity order enables chemists to distinguish between different types of alcohols effectively. When performing tests, observing the time taken for cloudiness to appear is crucial for identifying the alcohol type.

Alkyl Chlorides

When alcohols react with Lucas reagent, they are converted into alkyl chlorides. This transformation involves replacing the hydroxyl group (-OH) with a chloride ion. Alkyl chlorides are important compounds in organic chemistry, serving as intermediates for synthesizing a wide range of compounds. The ease of formation of alkyl chlorides depends on the alcohol type. Tertiary alcohols quickly form alkyl chlorides, allowing for their rapid identification. Conversely, primary alcohols form them very slowly, indicating a low reactivity, which makes them harder to identify using this method. Understanding this reaction process is vital for efficient manipulation and identification of alcohols in chemical practices.