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) \(3>2>1\)

Short answer

(a) 3 > 1 > 2

Step-by-step solution

Understand Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a reaction where an electrophile \(E^+\) replaces a hydrogen atom on the benzene ring. Electron-donating groups enhance the reactivity of the benzene towards electrophiles, while electron-withdrawing groups decrease the reactivity.

Analyze Substituents on Compounds

Each compound has different substituents that affect reactivity: 1. **p-nitrochlorobenzene**: Contains a nitro group which is a strong electron-withdrawing group. 2. **2,4-dinitrochlorobenzene**: Contains two nitro groups, making it the strongest electron-withdrawing. 3. **chlorobenzene**: Contains only a chlorine, which is slightly deactivating compared to no substituent.

Relate Substituents to Reactivity

Chlorobenzene (3) is least deactivated because it only has a weakly deactivating chlorine. p-nitrochlorobenzene (1) is more deactivated than chlorobenzene because of one nitro group. 2,4-dinitrochlorobenzene (2) is the most deactivated due to two nitro groups.

Arrange in Decreasing Order of Reactivity

Therefore, the order from most reactive to least reactive with electrophiles is: Chlorobenzene (3) > p-nitrochlorobenzene (1) > 2,4-dinitrochlorobenzene (2).

Select Correct Option

The correct sequence is given by option (a) \(3 > 1 > 2\).

Key concepts

Reactivity Order

Determining the reactivity order of aromatic compounds during electrophilic aromatic substitution involves understanding how different substituents attached to the benzene ring affect its reactivity towards an electrophile, which is a species that accepts electrons. Reactivity is primarily influenced by the nature of the substituents on the benzene, which can either donate electrons and activate the ring or withdraw electrons and deactivate it.
In the given problem, three compounds are compared based on their reactivity. The order being determined is from the most reactive to the least reactive: chlorobenzene, p-nitrochlorobenzene, and 2,4-dinitrochlorobenzene.

• Chlorobenzene, with only a chlorine atom, is less deactivated compared to the others, making it more reactive.
• p-Nitrochlorobenzene has a nitro group, which makes it more deactivated than chlorobenzene.
• 2,4-Dinitrochlorobenzene, with two nitro groups, is most deactivated and thus least reactive.

This order reflects option (a) \(3 > 1 > 2\) as the sequence of decreasing reactivity.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) play a crucial role in electrophilic aromatic substitution reactions by reducing the electron density of the benzene ring. This reduction in electron density leads to decreased reactivity towards electrophiles because the ring becomes less attractive to species that accept electrons.
Nitro groups \((-\text{NO}_2)\) are potent EWGs due to their ability to pull electron density from the ring through both resonance and inductive effects.

• In p-nitrochlorobenzene, the nitro group makes the ring less reactive than chlorobenzene.
• In 2,4-dinitrochlorobenzene, the presence of two nitro groups greatly decreases the ring's reactivity.

As more nitro groups are present, the ring becomes increasingly deactivated, leading to a significant decrease in its reactivity with electrophiles.

Chlorobenzene

Chlorobenzene is a benzene ring with a chlorine substituent. Chlorine is a unique substituent because it is an electron-withdrawing group, but it does not deactivate the benzene ring as strongly as nitro groups. This is because of the opposing effects in play.
Chlorine withdraws electrons through its inductive effect, making the ring less attractive to electrophiles. However, chlorine can also donate its lone pair of electrons through resonance, partially counteracting its deactivating influence.

• It is more reactive than both p-nitrochlorobenzene and 2,4-dinitrochlorobenzene because those compounds have strong electron-withdrawing nitro groups.
• This dual behavior of chlorine marks chlorobenzene as the most reactive of the three compounds in electrophilic aromatic substitution.

Electrophiles

Electrophiles are crucial participants in electrophilic aromatic substitution reactions. They are species that are deficient in electrons and are attracted to electron-rich areas, which in the context of these reactions, is the benzene ring.
Electrophiles love to "attack" the benzene ring and replace one of the hydrogen atoms, leading to the formation of a substituted aromatic compound.
The ease with which this substitution occurs is heavily influenced by the substituents on the benzene:

• Electron-donating groups enhance the ring's attractiveness to electrophiles, increasing reactivity.
• Conversely, electron-withdrawing groups make the ring less attractive for electrophiles, decreasing reactivity.

Understanding the behavior of electrophiles and the role of substituents helps predict the speed and extent of the chemical reaction.