Question

Benzene exhibits resonance. Explain this statement in terms of the different Lewis structures that can be drawn for benzene.

Short answer

Benzene exhibits resonance as it can be represented by two resonance contributors with double bonds shifted around the ring. These contributors collectively describe the true structure of benzene as a resonance hybrid, where electrons are delocalized over the entire molecule giving it higher stability and unique properties.

Step-by-step solution

Define Resonance

Resonance is a phenomenon in chemistry where molecules with a conjugated system (alternating single and double bonds) can be described by multiple Lewis structures that differ only in the position of their π electrons. None of these structures individually represent the true nature of the molecule completely, but their superposition gives a more accurate description: the resonance hybrid. The resonance contributors or structures do not switch back and forth between each other but contribute collectively to the overall electronic structure of the molecule.

Draw the Benzene Molecule

First, let's draw the benzene molecule. Benzene is an aromatic hydrocarbon with the molecular formula \(C_6H_6\). Each carbon atom in benzene is bonded to two other carbon atoms and one hydrogen atom. Its structure consists of a hexagonal ring with alternating single and double bonds.

Draw the Resonance Contributors

To represent the resonance in benzene, we can draw two different Lewis structures, known as resonance contributors. In each structure, the double bonds are shifted to different positions in the hexagonal ring. Here are the two resonance contributors for benzene: \[ \begin{array}{cc} \ce{A:} & \ce{ {-}H-C=C-C-H-C=C-C-{-}H\ce{<-} -> H-C-C=C-C=C-H} \end{array} \]

Connect the Contributors with a Resonance Arrow

To show that these two resonance contributors represent the same molecule and differ only in the position of electron pairs, we connect them with a double-headed resonance arrow: \[ \begin{array}{cc} \ce{A:} & \ce{ {-}H-C=C-C-H-C=C-C-{-}H\ce{<->} H-C-C=C-C=C-H} \end{array} \]

Explain the Resonance in Benzene

The two resonance contributors collectively describe the true structure of benzene, which is a resonance hybrid of those structures. The double bonds are distributed evenly throughout the six carbon atoms, creating a system where each carbon-carbon bond is neither a single bond nor a double bond but something in between. This gives benzene higher stability and unique properties compared to other non-aromatic hydrocarbons. In conclusion, benzene exhibits resonance, which is depicted by the two resonance contributors where double bonds are shifted around the ring. These contributors collectively describe the true structure of benzene as a resonance hybrid with electrons delocalized over the entire molecule, leading to its unique stability and properties.