Concept: Resonance Structures

Resonance structures are different Lewis structures that represent the same molecule or ion, where the actual structure is a hybrid of all possible resonance forms. They differ only in the placement of electrons, not atoms.

Step 1: Analyze the Given Structure

The given structure is a nitro group (-NO₂) attached to a benzene ring. The nitro group itself can have resonance due to the delocalization of electrons between nitrogen and oxygen atoms.

Step 2: Draw the Lewis Structure of Nitro Group

The nitro group (NO₂) has a nitrogen atom bonded to two oxygen atoms. Nitrogen has 5 valence electrons, each oxygen has 6. Total valence electrons = 5 + 6 + 6 = 17, but since it is often in a charged form (e.g., NO₂⁻ has 18 electrons), we consider the common structure.

For NO₂ (neutral), it is a radical, but in organic contexts like attached to benzene, it is usually NO₂ group without charge, but actually in many cases it is represented with resonance.

Actually, when nitro group is attached to benzene, it is -NO₂, which is neutral, but the nitro group itself has two major resonance structures:

Structure A: N=O double bond with one O and N-O single bond with other O, where the single-bonded O has a negative charge and N has positive charge.

Structure B: The double bond is with the other O, and single with the first, with charges similar.

So, for the nitro group alone, there are 2 resonance structures.

Step 3: Consider the Benzene Ring

Benzene itself has 2 resonance structures (Kekulé structures), but actually it is a ring with delocalized electrons, often represented by two alternating double bond structures.

When a substituent like nitro group is attached, it can participate in resonance with the ring. The nitro group is electron-withdrawing and can accept electrons from the ring.

Step 4: Resonance in Nitrobenzene

In nitrobenzene, the nitro group can have its own resonance (2 structures), and it also interacts with the benzene ring. The benzene ring has 2 main Kekulé structures, but with substituent, there are additional resonance structures where the double bonds shift.

Specifically, for nitrobenzene, there are resonance structures where the electron density from benzene is donated to the nitro group. This involves structures where the positive charge is on the ortho and para positions relative to nitro group.

Typically, nitrobenzene has multiple resonance structures: the two for nitro group itself, and several involving the ring. In total, for nitrobenzene, there are 4 significant resonance structures:

The primary structure with nitro group having one double bond to O.
The other nitro resonance with double bond to other O.
A structure where benzene donates electrons to nitro, putting positive charge on ortho position.
Another where positive charge is on other ortho or para position.

Actually, standard teaching often states that nitrobenzene has 3 resonance structures: two from the nitro group and one from the benzene ring interaction, but actually there are more due to symmetry.

Step 5: Count the Resonance Structures

For the nitro group attached to benzene (nitrobenzene), the number of resonance structures is 4. This includes:

Two structures where the nitro group has the double bond to each oxygen (with charges).
Two structures where the benzene ring donates electron density to the nitro group (with positive charge on ring at ortho and para positions; due to symmetry, ortho positions are equivalent, so often considered as one, but actually distinct in drawing).

However, commonly, it is accepted that there are 3 major resonance structures for nitrobenzene: the two of nitro group and one from ring donation (with positive charge distributed). But actually, for accuracy, there are 4 significant resonance contributors.

Upon recall, many textbooks state that the nitro group has 2 resonance structures, and when attached to benzene, it gives additional structures, making total for nitrobenzene as 4.

Final Answer

The number of resonance structures for nitrobenzene (the given molecule) is 4.

Related Topics

Lewis Structures
Electron Delocalization
Aromaticity
Electrophilic Substitution in Benzene

Formulae and Theory

Resonance energy: The stability gained due to resonance. For benzene, resonance energy is about 36 kcal/mol.

Rules for resonance structures:

Atoms must not be moved.
Only electrons are redistributed.
All structures must be valid Lewis structures.
The actual molecule is a hybrid of all contributors.

Formula for calculating formal charge: $F = V - L - \frac{B}{2}$ , where V is valence electrons, L is lone pair electrons, B is bonding electrons.