Problem 152 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following alcohols 1\. 1-phenyl-1-propanol 2\. 3-phenyl-1-propanol 3\. 1-phenyl-2-propanol The correct sequence of the increasing order of reactivity of these alcohols in their reaction with \(\mathrm{HBr}\) is (a) \(1,3,2\) (b) \(2,3,1\) (c) \(2,1,3\) (d) \(1,2,3\)

Step-by-Step Solution

Answer

The correct sequence is (b) 2, 3, 1.

Step 1: Analyze the type of alcohols

To determine the reactivity of alcohols with HBr, it's crucial to identify whether the alcohols are primary, secondary, or tertiary. Tertiary alcohols react fastest with HBr because they form more stable carbocations. Among the given alcohols, identify the position of the hydroxyl group relative to the phenyl group and the carbon chain.

Step 2: Determine the reactivity

1-phenyl-1-propanol (structure: PhC(OH)(CH3)CH2) is a tertiary alcohol. 3-phenyl-1-propanol (structure: Ph(CH2)2CH2OH) is a primary alcohol. 1-phenyl-2-propanol (structure: PhCH(OH)CH3) is a secondary alcohol. The order of reactivity is tertiary > secondary > primary, based on carbocation stability.

Step 3: Arrange in order of increasing reactivity

Given the carbocation stability rule, arrange the alcohols: - The least reactive is the primary alcohol (3-phenyl-1-propanol). - Next is the secondary alcohol (1-phenyl-2-propanol). - The most reactive is the tertiary alcohol (1-phenyl-1-propanol). Thus, the increasing order of reactivity is 3, 2, 1.

Key Concepts

Carbocation Stability

Carbocations are positively charged ions that contain a carbon atom with a positive charge. When alcohols react with hydrogen bromide (HBr), they typically go through a carbocation intermediate, which impacts the reaction rate. The stability of a carbocation plays a critical role here.
More stable carbocations are formed more quickly and lead to faster reactions. Stability is generally enhanced by the introduction of substituents that can stabilize the positive charge, either via hyperconjugation or resonance. Understanding the different types of alkyl carbocations can clarify this concept:

• Primary Carbocations: These have the positive charge on a carbon atom bonded to one other carbon atom. They're the least stable due to less hyperconjugative support.
• Secondary Carbocations: The carbon atom with the positive charge is bonded to two other carbon atoms. They are more stable than primary carbocations.
• Tertiary Carbocations: The positively charged carbon is bonded to three other carbon atoms, offering the greatest number of hyperconjugative structures and maximal stability.

This stability trend explains why tertiary alcohols react faster with HBr, as they form stable carbocations more readily.

Primary, Secondary, Tertiary Alcohols

Classifying alcohols as primary, secondary, or tertiary depends on the connectivity of the hydroxyl group (OH) bearing carbon to other carbon atoms. This classification influences their reactivity, especially in reactions involving protonation and carbocation formation.

• Primary Alcohols: The OH group is attached to a carbon bonded to only one other carbon atom. These alcohols generally form the least stable carbocations.
• Secondary Alcohols: These have the OH group on a carbon atom bonded to two other carbon atoms. These are more reactive than primary alcohols due to higher carbocation stability.
• Tertiary Alcohols: The OH is bonded to a carbon attached to three other carbons. They react the fastest with HBr, given the stable carbocations they can form.

Identifying the correct classification of an alcohol is crucial for predicting its behavior in reactions like the one with HBr, and understanding their reactivity trends can immensely help in solving problems related to organic reactions.

Reaction with HBr

The reaction of alcohols with HBr is a classic demonstration of chemical reactivity driven by carbocation formation. In this process, the hydroxyl group in the alcohol is first protonated, making it a good leaving group as water, which then creates a carbocation. Here's a simplified sequence of this mechanism:

• Protonation of Alcohol: The hydroxyl group picks up a proton from HBr, forming water, which is a good leaving group.
• Carbocation Formation: The subsequent loss of the water molecule leads to the formation of a carbocation intermediate.
• Nucleophilic Attack: Bromide ions, generated from HBr, quickly attack this carbocation to form an alkyl bromide.

The faster and more stable the carbocation formation, the quicker the reaction proceeds. This is why tertiary alcohols, which form the most stable carbocations, are particularly fast to react compared to primary and secondary alcohols. Recognizing this mechanism is essential to predict the reactivity sequence when alcohols are treated with HBr.