Problem 83 - A Complete Resource Book in Chemistry for JEE Main

Question

The alcohol, \(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{OH}\), when shaken with a mixture of anhydrous \(\mathrm{ZnCl}_{2}\) and concentrated HCl gave an immediate oil layer product. The alcohol is (a) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2}-\mathrm{OH}\) (b) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{OH}\) (c) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{CH}_{3}\) (d) \(\mathrm{H}_{3} \mathrm{C}-\left(\mathrm{CH}_{2}\right)_{3}-\mathrm{OH}\)

Step-by-Step Solution

Answer

The alcohol is (b) \((\mathrm{CH}_{3})_{3} \mathrm{C}-\mathrm{OH}\), a tertiary alcohol.

Step 1: Understanding the Reaction

The reaction described in the problem is known as the Lucas test, which is used to classify alcohols based on their reactivity with Lucas reagent (a mixture of anhydrous ZnCl2 and concentrated HCl). Primary alcohols react very slowly, secondary alcohols react within a few minutes, and tertiary alcohols react immediately to form an oily layer due to the formation of an alkyl chloride.

Step 2: Analyze Each Option

Let's analyze each alcohol option:- (a) \((\mathrm{CH}_{3})_{2} \mathrm{CHCH}_{2}-\mathrm{OH}\): This is a secondary alcohol.- (b) \((\mathrm{CH}_{3})_{3} \mathrm{C}-\mathrm{OH}\): This is a tertiary alcohol.- (c) \(\mathrm{H}_{3} \mathrm{C}-\mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{CH}_{3}\): This is a secondary alcohol.- (d) \(\mathrm{H}_{3} \mathrm{C}-(\mathrm{CH}_{2})_{3}-\mathrm{OH}\): This is a primary alcohol.

Step 3: Matching Reaction Conditions to Alcohol Type

According to the Lucas test, a tertiary alcohol will react immediately to form the oil layer. Option (b) \((\mathrm{CH}_{3})_{3} \mathrm{C}-\mathrm{OH}\) being a tertiary alcohol, fits these criteria.

Step 4: Conclusion

Based on the immediate formation of an oil layer, the alcohol in question is a tertiary alcohol, which corresponds to option (b) \((\mathrm{CH}_{3})_{3} \mathrm{C}-\mathrm{OH}\).

Key Concepts

Alcohol Classification

Alcohols are classified based on the connectivity of the carbon atom that carries the hydroxyl (-OH) group. This classification informs us about their chemical behavior, especially when they participate in reactions.

• Primary Alcohols: The carbon atom attached to the -OH group is connected to only one other carbon atom.
• Secondary Alcohols: Here, the carbon carrying the -OH is connected to two other carbon atoms.
• Tertiary Alcohols: In tertiary alcohols, the carbon attached to the -OH group is connected to three other carbon atoms.

Understanding these categories is crucial, as they react differently under the same conditions due to their structural differences. Their reactivity pattern helps in determining how quickly they react with other substances, like acids or alkyl halides.

Tertiary Alcohols

Tertiary alcohols are particularly interesting due to their unique reaction profile compared to primary and secondary alcohols.
These alcohols have their hydroxyl-bearing carbon atom connected to three carbon chains, making them highly stable and often less reactive under many conditions.
However, when it comes to the Lucas test, tertiary alcohols stand out due to their rapid reaction.

• Instantaneous Reaction: Tertiary alcohols quickly turn into an alkyl chloride, resulting in an oily second layer visible in the reaction mixture.
• Reaction Efficiency: This fast reaction is due to the formation of a stable carbocation, which drives the reaction with the Lucas reagent.

The example given, \((\mathrm{CH}_{3})_{3}\mathrm{C}-\mathrm{OH}\), demonstrates such a tertiary structure and efficiently navigates through the Lucas reagent, making it a prime candidate for this type of reaction test.

Reaction with ZnCl2 and HCl

The Lucas test employs a mixture of anhydrous zinc chloride (\(\mathrm{ZnCl}_{2}\)) and hydrochloric acid (\(\mathrm{HCl}\)], commonly known as the Lucas reagent, to distinguish alcohols of varying structures.
The reaction occurs as follows:

• Reaction Basis: The \(\mathrm{ZnCl}_{2}\) acts as a Lewis acid, polarizing the hydroxyl group and facilitating its replacement by a chloride ion from \(\mathrm{HCl}\).
• Primary vs. Tertiary: Where primary alcohols may react slowly, if at all, tertiary alcohols react instantly, forming an alkyl chloride that separates as an oily layer.
• Testing Process: By monitoring the speed and presence of this layer, chemists can determine the structural class of the alcohol.

Recognizing how these reagents act upon contact with alcohols is fundamental for identifying unknown compounds and understanding organic reaction mechanisms.