Q.47. - Organic Chemistry

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.

Step-by-Step Solution

Answer

Answer

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

Step 1: Indene

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

Step 2: 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.

Step 3: 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