Problem 67 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reacts fastest with conc. HCl \(\left(\varphi\right.\) is \(\left.C_{6}-H_{3}\right) ?\) (a) \(\varphi-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH}\) (b) \(\varphi-\mathrm{CHOH}-\mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3}-\mathrm{OH}\) (d) \(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH}\)

Step-by-Step Solution

Answer

Option (b) reacts fastest due to the stable secondary carbocation formation.

Step 1: Analyze the Molecular Structures

Consider the structures of each compound in relation to their ability to donate a leaving group when reacting with HCl. Understanding the structure will help assess stability of possible carbocations or the efficiency of leaving groups.

Step 2: Identify the Type of Reaction

This reaction involves an alcohol reacting with HCl to form an alkyl chloride. Generally, a tertiary alcohol reacts fastest due to the stability of the carbocation formed.

Step 3: Evaluate Carbocation Stability

For the molecule in option (b), the formation of a secondary carbocation is possible, stabilized by the aryl group ( ext{phenyl}). In option (a) and (c), no similarly stable carbocations are formed. Option (d) leads to a primary carbocation which is least stable.

Step 4: Evaluate Leaving Group Efficiency

The phenyl group in option (b) may participate in stability through resonance, making the hydroxyl group a better leaving group when protonated by HCl compared to other alcohols that have less stable intermediates.

Step 5: Conclude which Reacts Fastest

Option (b),  ext{phenyl-CHOH-CH}_3, will react the fastest with conc. HCl, as it can form a more stable carbocation intermediate with the aid of the phenyl group.

Key Concepts

Carbocation Stability

Carbocations are positively charged carbon atoms, formed when an alcohol reacts with a strong acid like HCl. In these reactions, alcohols lose their hydroxyl group, resulting in a carbocation. The stability of this intermediate affects the reaction rate significantly. Stable carbocations enable faster reaction progression because they help lower the activation energy. Stability hierarchy in carbocations is largely based on the degree of substitution: - Tertiary carbocations (3°) are the most stable, as they are surrounded by three alkyl groups that donate electron density to the positively charged carbon. - Secondary carbocations (2°) have two alkyl groups for stabilization. - Primary carbocations (1°) are the least stable, with only one alkyl group aiding in stabilization. The presence of adjacent groups, like a phenyl group (C₆H₅), can also increase stability through resonance, distributing the positive charge over an extended area. Understanding the stability of carbocations helps in predicting which alcohol will react more efficiently with HCl.

Tertiary Alcohol Reaction

In the realm of alcohol reactions, the degree of substitution plays a pivotal role, particularly evident with tertiary alcohols. A tertiary alcohol, having three organic groups attached to the carbon with the hydroxyl group, reacts readily with HCl. This is due to the interference and influence of adjacent electron-rich groups, which facilitate the loss of the hydroxyl group. The reaction pathway generally includes protonation of the hydroxyl group to form water, an excellent leaving group. This step is rapid when the starting alcohol is tertiary, as the resultant carbocation is both formation-friendly and stable, requiring lower energy to form. As such, tertiary alcohols tend to react more quickly with HCl, compared to secondary and primary counterparts. This reaction type is essential when crafting alkyl chlorides from alcohols, with tertiary alcohol conversion occurring almost instantaneously owing to enhanced carbocation stability.

Leaving Group Efficiency

Leaving groups are pivotal in determining reaction pathways, particularly in hydrohalogenation reactions. The efficiency of a leaving group is defined by how readily it departs the parent molecule, allowing subsequent reaction steps to progress. A good leaving group is capable of leaving with minimal energy input, enhancing reaction speed. In the case of alcohols reacting with HCl, the hydroxyl group must first be protonated to become water, a superior leaving group due to its stability and neutrality once it departs. For example, in the question's scenario, option (b) involves a phenyl group substituent that can engage in resonance stabilization of the resultant carbocation after the hydroxyl leaves. Such involvement improves leaving group efficiency, as the molecule can consistently stabilize post-leaving group scenarios. Hence, when assessing reaction speed, it is essential to consider how efficiently a group can leave, contributing to the reactivity of alcohols in transformation into alkyl chlorides.