Chapter 15, Problem 1PP

PRACTICE PROBLEM 15.1

Show how loss of a proton can be represented using each of the three resonance structures for the arenium ion and show how each representation leads to the formation of a benzene ring with three alternating double bonds (i.e., six fully delocalized $\pi$ electrons).

Interpretation Introduction

Interpretation:

The loss of protons from three resonating structures of the areniumion, are to be shown.

Concept Introduction:

Arrows present in the mechanism of the reaction indicate the electron transfer, starting from negatively charged species and ending at the positively charged species.

Electrophiles attract negatively charged species and nucleophiles attract positively charged species.

Answer to Problem 1PP

Solution: The loss of protons from the three resonating structures of the arenium ion are shown as follows:

Explanation of Solution

The resonating structures of the arenium ion, formed in the electrophilic substitution reaction of benzene, are shown below:

The removal of proton occurs from the carbon atom where the electrophile is attached and the bond pairs of electrons of $\text{C}-\text{H}$ bond get transferred to the ring, resulting in the formation of three double bonds, which implies that the aromaticity is regained.

The removal of proton from (I) resonating structure is shown as follows:

The removal of proton from (II) resonating structure is shown as follows:

The removal of proton from (III) resonating structure is shown as follows:

Conclusion

The removal of protons from the arenium ion leads to the transfer of bond pairs to the benzene ring and the delocalization of these electrons on the ring occurs, to form the final product.