Question

The ${\mathbf{pK}}_{\mathbf{a}}$ of p-cyclopropylbenzoic acid is 4.45. Is cyclopropylbenzene likely to be more reactive or less reactive than benzene toward electrophilic bromination? Explain.

Short answer

Cyclopropylbenzene is more reactive than benzene toward electrophilic bromination due to electron donating present in the benzene ring.

Step-by-step solution

Identify whether p-cyclopropylbenzoic acid is stronger or weaker.

The ${\mathrm{pK}}_{\mathrm{a}}$of p-cyclopropylbenzoic acid is given as 4.45.

The ${\mathrm{pK}}_{\mathrm{a}}$of benzoic acid is known as 4.19.

The structures of these compounds are shown below:

The $\mathit{p}{\mathit{K}}_{\mathbf{a}}$of p-cyclopropylbenzoic acid is greater than that of benzoic acid so that p-cyclopropylbenzoic acid is a weaker acid.

The substituent, cyclopropyl group destabilizes the negative charge on the $\left(-CO{O}^{-}\right)$carboxylate anion.

Hence, cyclopropyl group is an electron-donating, activating group.

Decide whether cyclopropylbenzene is activator or deactivator.

The structures of both cyclopropylbenzene and benzene are drawn as follows:

We know that the electron-donating group enhances the reactivity toward electrophilic brominating reaction. Cyclopropyl group bonded to the benzene ring is identified as an electron-donating group and activating the electrophilic aromatic substitutions.

In the case of benzene, hydrogen substituent is neither electron-donating group nor electron-withdrawing group.

Therefore, cyclopropylbenzene is more reactive than benzene toward electrophilic bromination reactions.