Nitrobenzenes are aromatic compounds in which a nitro group (–NO₂) is directly attached to a benzene ring (Ar–NO₂). The nitro substituent is strongly electron-withdrawing, making the ring less reactive toward electrophilic substitution and directing new substituents to the meta position.

In nitrobenzenes, the nitro group is represented by resonance structures featuring N⁺(=O)–O⁻, giving it a powerful −I and −M effect. This withdraws electron density from the aromatic π system, destabilizing the σ-complex in electrophilic aromatic substitution (EAS). As a result, nitrobenzene is markedly deactivated compared with benzene, and incoming electrophiles are directed predominantly to the meta position: ortho/para σ-complexes place positive charge adjacent to the nitro group, which is strongly destabilizing, whereas the meta σ-complex avoids that interaction.

Synthesis.
Nitrobenzenes are typically prepared by nitration of benzene or substituted benzenes using mixed acid (HNO₃/H₂SO₄), which generates the nitronium ion (NO₂⁺) as the electrophile. Reaction conditions must be controlled because multiple nitrations can occur, especially for rings already bearing activating groups.

Reactivity.

• Electrophilic substitution: slow and meta-directed (e.g., meta-halogenation, meta-sulfonation, meta-nitration for further substitution).

• Nucleophilic aromatic substitution (S_NAr): nitro groups strongly activate the ring toward nucleophilic attack when positioned ortho/para to a leaving group, stabilizing the Meisenheimer intermediate. Thus, nitrobenzenes are key platforms for S_NAr in synthesis.

• Reduction: the nitro group can be reduced to anilines (Ar–NH₂) via catalytic hydrogenation (H₂/Pd, Pt, Ni) or metal/acid systems (Fe/HCl, Sn/HCl), often through nitroso and hydroxylamine intermediates. This transformation is one of the main industrial routes to aromatic amines.

• Further transformations: nitrobenzene derivatives can undergo partial reductions, coupling, or rearrangements depending on conditions, making –NO₂ a versatile synthetic handle.

Physical/structural notes.
Nitrobenzenes are typically polar, relatively high-boiling liquids or solids due to the strong dipole of the nitro group. Spectroscopically, they show characteristic IR N–O stretches (asymmetric ~1520–1560 cm⁻¹, symmetric ~1340–1380 cm⁻¹), and the nitro group deshields nearby aromatic protons in NMR.

Applications and hazards.
Nitrobenzenes are widely used as intermediates in dyes, agrochemicals, and pharmaceuticals, mainly because they can be efficiently reduced to anilines. Many nitrobenzene derivatives are toxic and can cause methemoglobinemia; they should be handled with good ventilation and protective equipment.