Problem 145 - A Complete Resource Book in Chemistry for JEE Main

Question

Place the following compounds in the decreasing order of their relative reactivity with electrophile \(\left(\mathrm{E}^{+}\right)\). 1\. p-nitrochlorobenzene 2\. 2, 4 -dinitrochlorobenzene 3\. chlorobenzene (a) \(3>1>2\) (b) \(1>2>3\) (c) \(2>1>3\) (d) \(3>2>1\)

Step-by-Step Solution

Answer

(a) \(3>1>2\), chlorobenzene is most reactive.

Step 1: Understanding Electrophilic Substitution Reactions

In aromatic electrophilic substitution reactions, the presence of electron-withdrawing or electron-donating groups affects the reactivity. Electron-withdrawing groups, like the nitro group (-NO2), deactivate the aromatic ring and make it less reactive towards electrophiles compared to electron-donating groups or unsubstituted rings.

Step 2: Analyze the Effect of Substituents

Consider the structure of each compound: 1. p-nitrochlorobenzene: Contains one nitro group, which is a strong electron-withdrawing group. 2. 2,4-dinitrochlorobenzene: Contains two nitro groups, further deactivating the ring due to increased electron withdrawal. 3. Chlorobenzene: Contains a chlorine substituent, which is weakly deactivating compared to the nitro group.

Step 3: Order by Decreasing Reactivity

Since more nitro groups lead to stronger deactivation: - Chlorobenzene is the most reactive because it has no nitro groups. - p-Nitrochlorobenzene is less reactive than chlorobenzene due to one nitro group. - 2,4-Dinitrochlorobenzene is the least reactive because it has two deactivating nitro groups.

Step 4: Select the Correct Answer

Based on the analysis, the order from most reactive to least reactive is: chlorobenzene (3), p-nitrochlorobenzene (1), and 2,4-dinitrochlorobenzene (2). Therefore, the correct answer is (a) \(3>1>2\).

Key Concepts

Reactivity with Electrophiles

Electrophiles are species that seek electrons. In electrophilic substitution reactions, aromatic compounds react with these electron-seeking entities. The reactivity of an aromatic compound towards electrophiles largely depends on its electronic environment.

Aromatic rings, like benzene, generally possess a cloud of \( \pi \) electrons, making them attractive to electrophiles. However, the reactivity can be influenced significantly by substituents attached to the ring.

• Electron-donating groups increase the electron density, enhancing reactivity with electrophiles.
• Electron-withdrawing groups decrease electron density, reducing reactivity with electrophiles.

Understanding this balance allows you to predict how different compounds will behave in electrophilic substitution reactions based on their structure.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are a key factor in determining the reactivity of aromatic compounds with electrophiles. These groups pull electron density away from the aromatic ring, generally reducing its reactivity with electrophiles.

The nitro group (-NO2) is a classic example of a strong electron-withdrawing group. It contains highly electronegative atoms, like oxygen, which help it withdraw electrons effectively:

• This makes the aromatic ring less nucleophilic.
• The ring becomes less reactive towards electrophilic attack.

The position of the EWG on the ring also plays a crucial role. When there's more than one such group, like in 2,4-dinitrochlorobenzene, the effect of electron-withdrawing increases, leading to further deactivation.

Deactivation of Aromatic Rings

Deactivation refers to the decrease in reactivity of an aromatic ring due to the presence of electron-withdrawing groups. These deactivating substituents make the ring less likely to participate in an electrophilic substitution.

Chlorobenzene serves as a benchmark for understanding this concept. It has a chlorine atom, which is a weak deactivator compared to the nitro groups found in compounds like p-nitrochlorobenzene and 2,4-dinitrochlorobenzene.

• Chlorobenzene is more reactive due to having no strong deactivating groups.
• p-Nitrochlorobenzene is less reactive as it contains one nitro group.
• 2,4-Dinitrochlorobenzene, with two nitro groups, is the least reactive.

The more nitro groups present, the more deactivated the ring becomes, illustrating how these groups collectively reduce the ring's ability to interact with electrophiles.