Which of the following statements is false about resonance contribution structures?

Contributing structures contribute to the resonance hybrid in proportion of their relative energies

The correct answer is option 1. Contributing structures contribute to the resonance hybrid in proportion of their relative energies.

Let us delve into each statement in detail to understand why statement 1 is considered false and why the others are true.

1. Contributing structures contribute to the resonance hybrid in proportion to their relative energies.

This statement is false. The reason is that while more stable (lower energy) contributing structures do have a larger impact on the resonance hybrid, their contributions are not strictly proportional to their energies.

Resonance Hybrid: The resonance hybrid is a weighted average of all possible contributing structures. These structures don't contribute in direct proportion to their energies. Instead, the resonance hybrid is influenced by the overall stability imparted by the delocalization of electrons across the contributing structures.

Weighting Factors: The weights are determined by various factors, including:

Relative Stability: More stable structures contribute more, but not linearly proportional to their energies.

Number of Contributing Structures: More equivalent structures enhance resonance.

Delocalization: The extent of electron delocalization and overlap of p-orbitals affects the contribution.

2. Equivalent contributing structures make the resonance very important.

This statement is true. Equivalent contributing structures indicate that the electrons are delocalized equally among all possible arrangements, which significantly enhances resonance stabilization.

Example: Benzene is a classic example. It has two equivalent Kekulé structures. The actual benzene molecule is a resonance hybrid of these structures, making it exceptionally stable due to equal delocalization of π-electrons.

Impact: Equivalent structures result in a significant lowering of the overall energy of the molecule compared to any individual structure.

3.  Contributing structures represent molecules having no real existence.

This statement is true. Contributing structures are hypothetical and are used to illustrate the concept of resonance.

Actual Molecule: The real molecule is better represented by the resonance hybrid, which is an averaged depiction of all possible contributing structures.

Non-Existence: No single contributing structure exists on its own in reality; they are merely representations of extremes within the resonance hybrid.

4. Contributing structures are less stable than the resonance hybrid.

This statement is true. The resonance hybrid is more stable than any individual contributing structure because the hybrid represents a delocalized system where the electron density is spread out, leading to a lower overall energy.

Stability: The stability arises from the delocalization of electrons, which distributes the electron density more evenly and reduces the energy of the molecule.

Energy Comparison: Each contributing structure is an extreme form that does not capture the full delocalization, hence they are individually less stable than the hybrid.

Conclusion

The falsehood in statement 1 lies in the misunderstanding of how contributing structures affect the resonance hybrid. Their contributions are not directly proportional to their energies, but rather a complex combination of stability, the number of equivalent structures, and the extent of delocalization. Statements 2, 3, and 4 correctly describe the nature of resonance and contributing structures.