Question

Question: Treatment of indene with $\mathit{N}\mathit{a}\mathit{N}{\mathit{H}}_{\mathbf{2}}$forms its conjugate base in a Brønsted–Lowry acid-base reaction. Draw all reasonable resonance structures for indene’s conjugate base, and explain why the $\mathit{p}{\mathit{K}}_{\mathbf{a}}$of indene is lower than the $\mathit{p}{\mathit{K}}_{\mathbf{a}}$of most hydrocarbons.

Short answer

Answer

Most of the other hydrocarbons are non-aromatic or antiaromatic and thus have higher $p{K}_a$value than other hydrocarbons.

Step-by-step solution

Indene

There are several polycyclic hydrocarbons, and one among them is indene. Substituted indenes are present in several natural products and biological molecules.

Brønsted–Lowry acid-base reaction

As per Brønsted–Lowry's theory, acids are species that can contribute their protons to others, and bases are substances that can acquire protons.

Resonance structures of indene’s conjugate base

The resonance structures of indene’s conjugate base can be given as:

Resonance structures of indene’s conjugate base

The conjugate base of indene possesses $10\mathrm{\pi }$ electrons making it aromatic and leading them to become stable. Most of the other hydrocarbons are anti-aromatic or non-aromatic and have a high $p{K}_a$value.

Therefore, indene, which is aromatic, is more acidic than other hydrocarbons