Chapter 2, Problem 1PP

PRACTICE PROBLEM

Cyclobutadiene (below) is like benzene in that it has alternating single and double bonds in a ring. However, its bonds are not the same length, the double bonds being shorter than the single bonds; the molecule is rectangular, not square. Explain why it would be incorrect to write resonance structures as shown.

Interpretation Introduction

Interpretation: The given resonance structure written in an incorrect way, is to be explained.

Concept introduction:

A resonance structure is very helpful, as it describes the molecule, and the ions, completely.

While writing a resonance structure, only the delocalization of PI–electrons takes place. The nuclei positions remain the same in all the structures.

All the resonance structures must be proper Lewis structures.

More stable the structure, greater is the contribution to the resonance hybrid.

Answer to Problem 1PP

Solution:

In the two representations, the four carbon atoms occupy different positions.

Explanation of Solution

Given information:

According to the rules of writing resonance structures, in all the structures, the positions of the nuclei remain the same and only the delocalization of PI - electrons occurs and the bonding framework of a molecule must remain intact. In the given structure of cyclobutadiene, it has alternating double and single bonds, but in the representation, the position of the four carbon atoms is different. Thus, it is the incorrect way of writing a resonance structure.

Conclusion

In the two representations, the positions of the four carbons atoms are different.