Question

Benzene is a resonance hybrid mainly of two Kekule structures. Hence (1) Half the molecules correspond to one structure and half to the structure (2) At low temperatures benzene can be separated into two structures (3) Two structures make equal contribution to resonance hybrid (4) An individual benzene molecule changes back and forth between two structures

Short answer

Statement (3)

Step-by-step solution

Identify the Problem

The exercise focuses on understanding the nature of benzene's structure and its resonance forms.

Analyze Resonance

Resonance in benzene involves the delocalization of electrons across all carbon atoms, creating a stabilized structure.

Evaluate Statements

Review each statement to determine its correctness based on the concept of resonance.

Statement (1)

Statement (1) suggests that half the molecules correspond to one structure and half to the other. This is incorrect because resonance structures are not individual molecules but representations.

Statement (2)

Statement (2) suggests that benzene can be separated into two structures at low temperatures, which is also incorrect. Benzene exists as a resonance hybrid, not an equilibrium between two separable forms.

Statement (3)

Statement (3) suggests that two structures make an equal contribution to the resonance hybrid. This is correct since each Kekule structure equally contributes to the overall resonance hybrid.

Statement (4)

Statement (4) suggests that an individual benzene molecule changes back and forth between two structures. This is incorrect since resonance structures are not actual separate entities but a single hybrid.

Conclusion

Based on the evaluation of the given statements, the correct answer is statement (3).

Key concepts

Kekule structures

In 1865, Friedrich August Kekulé proposed a structure for benzene that revolutionized the understanding of aromatic compounds. According to Kekulé, benzene consists of a six-membered carbon ring with alternating double and single bonds. This means that there are two distinct structures in which the double bonds shift positions around the ring.

These are known as the Kekulé structures of benzene. Despite the elegance of Kekulé's idea, the problem arises when considering the actual behavior of benzene. Experimentally, benzene does not exhibit the distinct characteristics of alternating single and double bonds.

Instead, all carbon-carbon bonds in benzene are equivalent, suggesting something other than the simple Kekulé structures.

The Kekulé structures are useful representations but do not capture the true nature of benzene’s bonding.

In essence:

• Kekulé structures are theoretical constructs.
• They helped advance our understanding of aromatic compounds.
• The actual molecule doesn't correspond to either structure exclusively.

Resonance Hybrid

A resonance hybrid is a more accurate depiction of the structure of benzene. Resonance theory suggests that the true structure of a molecule that cannot be accurately represented by a single Lewis structure is a blend or hybrid of multiple structures.

For benzene, the hybrid is composed of the two Kekulé structures. This means the actual bonding in benzene is an average of the two Kekulé forms.

Since each Kekulé structure equally contributes to the hybrid, benzene’s structure is a superposition of these two forms.

Here's what you need to understand about the resonance hybrid:

• It represents the true, stable structure.
• The real bonding situation in the molecule is an average or blend of the possible structures.
• This model explains why benzene’s carbon-carbon bonds are of equal length.

Thus, the resonance hybrid offers a more accurate picture, explaining the stability and uniform bond lengths found in benzene.

Electron Delocalization

The concept of electron delocalization is key to understanding the resonance hybrid of benzene. In simple terms, electron delocalization means that the electrons are not confined to a single bond or a single pair of atoms.

In the case of benzene, the six pi (π) electrons from the double bonds are shared equally among all six carbon atoms in the ring. This delocalization results in the stabilization of the molecule.

To visualize this:

• Imagine the electrons spreading out evenly over the entire carbon ring, rather than being localized between individual carbon atoms.
• This leads to the uniformity in bond lengths.

The delocalized electrons form what can be thought of as a 'cloud' or 'ring' above and below the plane of carbon atoms. This electron cloud contributes to benzene’s aromatic stability, meaning the molecule is less reactive than one would expect for an unsaturated compound.

This stability helps explain why benzene doesn’t undergo addition reactions typical of alkenes but instead participates in substitution reactions, preserving the aromatic system.