Question

The replacement of chlorine from chlorobenzene to give phenol requires a drastic condition, but the chlorine of 2,4 -dichloro-nitro-benzene is readily replaced since (1) nitro groups make the aromatic ring electron rich at ortho/para-positions (2) nitro groups withdraw electrons from the metaposition of the aromatic ring (3) nitro groups donate electrons at meta-position (4) nitro groups withdraw electrons from orthol para-positions of the aromatic ring

Short answer

Option (4): Nitro groups withdraw electrons from ortho and para positions of the aromatic ring.

Step-by-step solution

Understand the question

Determine why the replacement of chlorine from 2,4-dichloro-nitrobenzene is easier compared to chlorobenzene. Evaluate the effects of the nitro group on the aromatic ring.

Effect of nitro groups on aromatic rings

Nitro groups are electron-withdrawing groups. They pull electron density away from the aromatic ring through the -I (inductive) effect and through the -M (resonance) effect, especially from the positions ortho and para to the nitro group.

Analyze substitution positions

Identify the positions of chlorine and nitro groups in 2,4-dichloro-nitrobenzene. The nitro group at the 1-position withdraws electrons from the ortho (2) and para (4) positions, where the chlorine atoms are also located.

Relate to ease of substitution

Since nitro groups withdraw electrons from the ortho/para positions, they make these positions electron-deficient. Chlorine atoms attached to these electron-deficient positions can be more readily replaced due to the increased reactivity of the carbon-chlorine bond.

Determine the correct answer

Evaluate the given options and choose the one that correctly describes the effect of nitro groups on the substitution positions. The correct option is that nitro groups withdraw electrons from ortho and para positions of the aromatic ring.

Key concepts

Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS) is a fundamental reaction in organic chemistry. It involves the replacement of a hydrogen atom on an aromatic ring with an electrophile. This reaction is common in benzene and its derivatives. There are several steps involved in an EAS reaction. Firstly, the aromatic ring acts as a nucleophile and interacts with the electrophile to form a carbocation intermediate. This intermediate is stabilized by resonance. Finally, a base removes a proton from the intermediate, restoring the aromaticity of the ring.
Some key points to remember about EAS include:

• It generally proceeds via a carbocation intermediate.
• Electron-rich aromatic systems react faster in EAS.
• The nature and position of substituents on the ring significantly affect the reactivity and orientation of the electrophile.

Understanding these basics helps explain why certain substituents can make benzene rings either more or less reactive in EAS.

Electron Withdrawing Groups

Electron Withdrawing Groups (EWGs) are substituents on an aromatic ring that pull electron density away from the ring through inductive or resonance effects. Common EWGs include nitro groups (-NO₂), carbonyl groups (like -COOH and -CHO), and halogens (like -Cl and -Br).

• EWGs decrease the electron density on the ring, making it less nucleophilic and thus less reactive towards electrophiles.
• They often result in the deactivation of the ring towards EAS reactions.
• EWGs also influence the distribution of electron density, making certain positions on the ring more electron-deficient and reactive.

For example, nitro groups are strong EWGs. They can withdraw electrons both inductively (-I effect) and through resonance (-M effect). This makes ortho and para positions electron-deficient and more reactive towards substitution reactions.

Chlorobenzene

Chlorobenzene is a derivative of benzene where one hydrogen atom is replaced by a chlorine atom. This simple substitution has a profound effect on the chemical properties of benzene. Chlorine is an electron-withdrawing group, primarily through its inductive effect.

• Due to the chlorine atom, chlorobenzene is less reactive towards electrophilic aromatic substitution compared to benzene.
• The electron-withdrawing effect of chlorine makes the ring less nucleophilic, although chlorine also has the capacity to donate electrons through resonance.
• This makes the ortho and para positions slightly more reactive relative to the meta position in chlorobenzene.

However, the reactivity is still significantly less than unsubstituted benzene. In more substituted systems like 2,4-dichloro-nitrobenzene, the combined electron-withdrawing effects make certain positions extremely reactive, making substituent replacement easier.