Problem 212 - A Complete Resource Book in Chemistry for JEE Main

Question

From amongst the following alcohols the one that would react fastest with conc. HCl and anhydrous \(\mathrm{ZnCl}_{2}\), is [2010] (a) 2-Butanol (b) 2-Methylpropan -2-ol (c) 2-Methylpropanol (d) 1-Butanol

Step-by-Step Solution

Answer

The fastest reacting alcohol is 2-Methylpropan-2-ol (option b).

Step 1: Identify the Lucas Reagent

The combination of conc. HCl and anhydrous ZnCl_2 forms the Lucas reagent, which is used to test alcohol reactivity based on classification (primary, secondary, tertiary). Tertiary alcohols react fastest with this reagent due to the stability of the carbocation formed.

Step 2: Classify the Given Alcohols

Classify each alcohol: - (a) 2-Butanol is a secondary alcohol. - (b) 2-Methylpropan-2-ol is a tertiary alcohol. - (c) 2-Methylpropanol is a primary alcohol. - (d) 1-Butanol is a primary alcohol.

Step 3: Determine Reactivity Based on Classification

Tertiary alcohols react the fastest with the Lucas reagent, followed by secondary and then primary alcohols. The more stable the carbocation formed during the reaction, the faster the reaction.

Step 4: Identify the Fastest Reacting Alcohol

Since 2-Methylpropan-2-ol is a tertiary alcohol, it will form the most stable carbocation, making it react the fastest with conc. HCl and anhydrous ZnCl_2.

Key Concepts

Alcohol Reactivity

The Lucas test is a well-known method for gauging the reactivity of alcohols. Specifically, it determines how different types of alcohols react in the presence of a mixture of concentrated hydrochloric acid (HCl) and anhydrous zinc chloride ( ZnCl_2). This reagent is known as the Lucas reagent. The speed of the reaction depends on the classification of the alcohol involved.

Understanding the hierarchy in alcohol reactivity is key:

• **Tertiary alcohols** react the fastest.
• **Secondary alcohols** take a moderate amount of time to react.
• **Primary alcohols** react the slowest, if at all within a reasonable timeframe.

This order is due to the stability of the carbocations formed during the reaction. Tertiary alcohols form the most stable carbocations, making them react more quickly when tested with the Lucas reagent.

Carbocation Stability

The stability of carbocations plays a crucial role in determining how fast an alcohol will react with the Lucas reagent. Carbocations are positively charged ions that have a carbon atom with only six electrons in its outer shell. This makes them highly reactive intermediates in many organic reactions.

Carbocation stability can be understood as follows:

• **Tertiary carbocations** are the most stable because they are surrounded by three alkyl groups. These groups donate electron density through a phenomenon called hyperconjugation, stabilizing the positive charge.
• **Secondary carbocations** are less stable than tertiary ones because they have fewer alkyl groups donating electron density.
• **Primary carbocations** are the least stable with only one alkyl group for electron donation, making them rare and largely unreactive.

This order of stability directly influences how quickly an alcohol can convert to a corresponding halo alkane in the Lucas test, favoring the formation of stable carbocations.

Tertiary Alcohol

Tertiary alcohols are a specific class of alcohol where the hydroxyl group (—OH) is bonded to a carbon atom that is connected to three other carbon atoms (i.e., a tertiary carbon). This structural feature provides them unique chemical properties, particularly in their reactivity.

Some key points about tertiary alcohols:

• **Increased Reactivity:** Tertiary alcohols exhibit the highest reactivity in the Lucas test due to the stable tertiary carbocation they form upon reaction.
• **Structural Robustness:** The additional alkyl groups not only provide stability but also push hydrogen atoms away, minimizing steric hindrance in some reactions.
• **Common Example:** An example of a tertiary alcohol is 2-Methylpropan-2-ol, which reacts incredibly fast due to its ability to rapidly form a stable tertiary carbocation when using the Lucas reagent.

These characteristics make tertiary alcohols a vital topic when studying alcohol chemistry, particularly in the context of reaction mechanisms and industrial applications.