Question

In electrophilic aromatic substitution reaction, the nitro group is meta- directing because it (1) decreases electron density at meta-position (2) increase electron density at meta-position (3) increase electron density at ortho- and parapositions (4) decreases electron density at ortho- and para-positions

Short answer

The nitro group is meta-directing because it decreases electron density at ortho- and para-positions, making electrophilic substitution more favorable at the meta-position.

Step-by-step solution

Understanding the Nitro Group

The nitro group (\(-NO_2\)) is an electron-withdrawing group due to the strongly electronegative nature of nitrogen and the resonance structures that delocalize the electron density.

Effect on Electron Density

Electron-withdrawing groups decrease electron density through both inductive and resonance effects. Specifically, they pull electron density away from the aromatic ring.

Position-Specific Impact

To determine the directing nature, analyze the resonance structures of the ring with the nitro group. The resonance structures show decreased electron density at the ortho- and para-positions.

Conclusion

Due to the decreased electron density at ortho- and para-positions caused by the nitro group’s electron-withdrawing property, electrophilic substitution is more favorable at the meta-position.

Key concepts

Nitro Group

The nitro group (-NO_2) is a functional group commonly found in organic chemistry. It consists of one nitrogen atom connected to two oxygen atoms. The electronegativity of these atoms makes the nitro group a strong electron-withdrawing group, meaning it pulls electrons towards itself. This property significantly affects the reactivity of aromatic rings, such as benzene, when the nitro group is attached to them.

Meta-Directing Group

In electrophilic aromatic substitution reactions, where new groups replace a hydrogen atom on an aromatic ring, the position at which the substitution happens can be affected by existing substituents. A meta-directing group is a substituent that directs incoming electrophiles to the meta position relative to itself. For the nitro group, it directs new groups to the meta position by reducing electron density at the ortho and para positions due to its electron-withdrawing nature.

Electron Density

Electron density refers to the probability of finding electrons in an area around the nucleus of an atom. In an aromatic ring, electron density distribution is crucial for understanding reactivity patterns. Electron-withdrawing groups, like the nitro group, pull electron density away from the ring. This makes the ring less reactive to electrophiles at positions where the electron density is lower, particularly affecting electrophilic aromatic substitution reactions.

Resonance Structures

Resonance structures are different forms of a molecule where the electron distribution varies while the actual positions of the atoms remain the same. For an aromatic ring with a nitro group, resonance structures can represent how the electron density is delocalized. The nitro group, through resonance, decreases electron density at the ortho and para positions. This helps to understand why the electrophilic substitution is directed to the meta position, which maintains relatively higher electron density.

Electron-Withdrawing Group

An electron-withdrawing group is a substituent that pulls electron density from the rest of the molecule, often through inductive or resonance effects. The nitro group is a classic example, withdrawing electrons due to the high electronegativity of nitrogen and oxygen atoms. This withdrawal of electrons decreases the overall electron density on an aromatic ring, making certain positions less reactive towards electrophiles and consequently affecting where substitutions occur.