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

Question

Place the following acid chlorides in the decreasing order of reactivity to hydrolysis. (1) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCl}\) (2) \(\mathrm{p}-\mathrm{O}_{2} \mathrm{NC}_{6} \mathrm{H}_{4} \mathrm{COCl}\) (3) \(\mathrm{p}-\mathrm{CH}_{3} \mathrm{OC}_{6} \mathrm{H}_{4} \mathrm{COCl}\) (a) \(2>1>3\) (b) \(3>1>2\) (c) \(1>2>3\) (d) \(1>3>2\)

Step-by-Step Solution

Answer

Option (a) \(2 > 1 > 3\).

Step 1: Understand Reactivity of Acid Chlorides

Acid chlorides are highly reactive towards nucleophiles due to the electronegativity of the chlorine atom, which makes the carbonyl carbon more susceptible to attack. Reactivity can be influenced by substituents attached to the aromatic ring.

Step 2: Analyze Substituent Effects

To determine the effect of substituents, consider their electron-withdrawing or electron-donating nature. Electron-withdrawing groups (EWG) increase the positive charge on the carbonyl carbon, enhancing reactivity. Electron-donating groups (EDG) do the opposite.

Step 3: Compare the Substituents

Identify the nature of the substituents on the aromatic rings: p-\(\text{O}_2\text{N}\) (strong EWG), p-\(\text{CH}_3\text{O}\) (EDG), and no substituent (neutral). Order of EWGs' influence: p-\(\text{O}_2\text{N}\) > no substituent > p-\(\text{CH}_3\text{O}\).

Step 4: Arrange Based on Reactivity

Base the reactivity order on the substituent effects from Step 3: p-\(\text{O}_2\text{N}\text{C}_6\text{H}_4\text{COCl}\) is the most reactive due to the strongest EWG, followed by \(\text{C}_6\text{H}_5\text{COCl}\) with no substituent for moderate reactivity, and p-\(\text{CH}_3\text{O}\text{C}_6\text{H}_4\text{COCl}\) is least reactive due to the EDG.

Step 5: Choose the Correct Order

The correct order of reactivity to hydrolysis, from most to least reactive, is \(2 > 1 > 3\). Thus, the correct answer is option (a).

Key Concepts

Nucleophilic Attack

A nucleophilic attack is a critical concept in understanding the chemistry of acid chlorides. When we speak of nucleophilic attack, we mean that a nucleophile, which is a particle rich in electrons, seeks out a positively charged site to form a bond. In the case of acid chlorides, this is the carbon atom in the carbonyl group because it is attached to a very electronegative chlorine atom. The chlorine, being more electronegative, pulls electron density away from the carbon, making it partially positive and an attractive spot for electron-rich nucleophiles. During the nucleophilic attack process, the nucleophile forms a bond with the carbonyl carbon, while the lone pair on the oxygen forms a negative charge due to resonance, aiding this attack. As a result, the carbonyl becomes more susceptible to breaking, allowing transformations such as hydrolysis to proceed efficiently.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) play a pivotal role in influencing the reactivity of acid chlorides. These are groups that pull electron density away from the carbon atom they are bonded to, often by way of resonance or induction. By doing this, they increase the positive charge on the carbonyl carbon, making it an even more attractive target for nucleophiles. In the context of the original exercise, there is a strong electron-withdrawing group, namely the p-NO_2 (nitro group), present in compound 2. This group pulls electron density away not only from the carbon it is directly attached to but also from the carbonyl group, amplifying its susceptibility to nucleophilic attacks. This enhancement makes such EWG-containing acid chlorides extremely reactive towards hydrolysis, as they offer the most favorable conditions for nucleophiles to attack.

Electron-Donating Groups

Electron-donating groups (EDGs) have the opposite effect compared to electron-withdrawing groups. Rather than pulling electron density away, they push electron density towards the carbon they are attached to. This effect can reduce the positive charge on the carbonyl carbon in acid chlorides, making it less likely to participate in reactions like nucleophilic attacks. Consider the methoxy group (p-CH_3O) in compound 3 from the exercise. This group is known for its electron-donating effect through resonance. By donating electrons into the aromatic ring and subsequently into the carbonyl group, it effectively reduces the reactivity of the acid chloride. This makes the carbonyl carbon less electrophilic and, therefore, less attractive for nucleophilic attack, resulting in a lower reactivity towards hydrolysis.