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

Question

\(\mathrm{H}\) Which of the following reacts fastest with conc. HCl \(\left(\varphi\right.\) is \(\left.\mathrm{C}_{6}-\mathrm{H}_{5}\right) ?\) (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

(b) \( \varphi-\mathrm{CHOH}-\mathrm{CH}_{3} \) reacts fastest.

Step 1: Understanding the Reaction Type

The reaction involved is with concentrated HCl, which indicates that we are dealing with an acid-catalyzed nucleophilic substitution reaction where HCl donates a proton to form a better leaving group such as water from the alcohol.

Step 2: Analyze Each Compound's Reactivity

Evaluate each option to see which will form the most stable carbocation after the leaving group departs.- (a) \( \varphi-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{OH} \): The carbocation formed is primary, not very stable.- (b) \( \varphi-\mathrm{CHOH}-\mathrm{CH}_{3} \): The carbocation formed is secondary and resonates with benzene ring, enhancing stability.- (c) \( \mathrm{CH}_{3}-\mathrm{OH} \): The carbocation is primary, very unstable.- (d) \( \mathrm{CH}_{2} = \mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH} \): The carbocation formed on hydration is allylic, stable due to resonance.

Step 3: Select Compound with Most Stable Intermediate

The benzyl carbocation formed from compound (b) is the most stable due to resonance stabilization with the aromatic ring, making it react fastest with \( \text{HCl} \).

Key Concepts

Nucleophilic Substitution Reaction

In organic chemistry, a nucleophilic substitution reaction is a type of reaction where a nucleophile replaces a leaving group in a molecule. This type of reaction is common during interactions between alcohols and strong acids like hydrochloric acid. Here’s how it works:

• The alcohol in the molecule initially has a hydroxyl group (-OH) which isn’t a great leaving group on its own.
• When alcohol reacts with concentrated HCl, the HCl provides a proton (H⁺) that transforms the -OH into water (a much better leaving group).
• As the water group detaches, it makes way for the chloride ion (Cl^-) to replace it, completing the substitution reaction.

This type of reaction is crucial because it defines how alcohols convert to alkyl halides and involves key intermediates which we can better understand by studying factors like the carbocation's stability.

Carbocation Stability

In some reactions, like nucleophilic substitution, the stability of the temporary carbocation plays a significant role in determining the reaction’s speed. A carbocation is a positively charged carbon atom bound to only three other atoms, making it very reactive. Here's what affects that stability:

• **Primary, Secondary, and Tertiary Carbocations:** A primary carbocation is the least stable because it is only bonded to one carbon; secondary is more stable because it is bonded to two; tertiary is the most stable as it is connected to three carbons.
• **Resonance and Inductive Effects:** Stability is greatly enhanced if the carbocation's charge can be delocalized over a larger area or shared with adjacent atoms, often seen in resonance.

In the given exercise, compound (b) demonstrates enhanced stability due to a secondary carbocation that can resonate with the adjacent benzene ring, making it more stable compared to the primary carbocations from other compounds.

Resonance Effect

The resonance effect describes how a molecule’s electron cloud can be spread over multiple atoms, creating a larger area to dissipate charge, thus stabilizing molecular structures. Here’s what happens in relation to carbocations:

• A carbocation adjacent to a benzene ring can share or delocalize its positive charge into the aromatic system. This removes some of the electron deficiency from the carbocation, stabilizing it significantly more than if the charge were confined to one atom.
• This is why in our example, the benzyl carbocation ( \(\varphi- \text{CHOH} - \text{CH}_3\) ) is stabilized because the charge is delocalized into the benzene ring.Making it react faster when exposed to HCl.

Acid-Base Reaction

In organic reactions, knowing about acid-base interactions is important because they often define the initial reaction steps, like protonation. Here's a simplified explanation:

• When HCl is used, it supplies protons (H⁺) to the mixture, being a strong acid. This protonation turns -OH into a better leaving group, water.
• The basic site, -OH, accepts the proton, converting the alcohol part of the molecule into a water molecule, which is stable and easily leaves the reaction site.

These basic principles show how reactive molecules adjust by gaining or losing protons, thus facilitating the entire nucleophilic substitution process. Recognizing these reactions is fundamental in understanding how molecular transformations occur.