Q8P - Organic Chemistry (Mcmurry)

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.

Step-by-Step Solution

Answer

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

Step1: 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.

Step 2: 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.