Q15-36E - Organic Chemistry (Mcmurry)

Question

Pentalene is a most elusive molecule that has been isolated only at liquid-nitrogen temperature. The pentalene dianion, however, is well known and quite stable. Explain.

Pentalene

Pentalene dianion

Step-by-Step Solution

Answer

The pentalene dianion (10 electrons)  is aromatic and therefore much more stable whereas Pentalene is antiaromatic.

Step-by-Step Solution Step 1: Aromatic compound

Aromaticity is defined by Huckel’s $(\mathbf{4}\mathbf{n}\mathbf{+}\mathbf{2})$ rule. According to this theory, Molecules with $(\mathbf{4}\mathbf{n}\mathbf{+}\mathbf{2})$pi-electron are aromatic. Molecules with 4n pi electrons are not aromatic systems. These systems are called antiaromatic and are most often very unstable.

Step 2: Pentalene

Pentalene has 8 electrons and is an unstable compound that requires liquid nitrogen temperatures to become relatively stable. The pentalene dianion (10 electrons) on the other hand is aromatic and therefore much more stable whereas Pentalene is antiaromatic.