Question

Question: Pentalene, azulene, and heptalene are conjugated hydrocarbons that do not contain a benzene ring. Which hydrocarbons are especially stable or unstable based on the number of electrons they contain? Explain your choices.

Short answer

Answer

Pentalene: It is anti-aromatic and is highly unstable.

Azulene: It is aromatic and is highly stable.

Heptalene: It is antiaromatic and is unstable.

Step-by-step solution

Aromatic and anti-aromatic compounds

The compounds which are cyclic, planar, and have a total of $\left(4n+2\right)\mathrm{\pi }$ electrons are aromatic in nature. The aromatic compounds are very much stable in nature.

The compounds which are cyclic, planar, and have a total of $4n\mathrm{\pi }$ electrons are anti aromatic in nature. The anti-aromatic compounds are very less stable as compared to the similar acyclic compounds with the same number of $\mathbf{\pi }$electrons.

The stability of the given compounds

All of the given compounds are completely conjugated. The compound which has a total of $\left(4n+2\right)\mathrm{\pi }$electrons are aromatic and are highly stable in nature whereas the compounds with $4n\mathrm{\pi }$ electrons are anti aromatic in nature and are highly unstable.

The compound pentalene has a total of eight $\mathrm{\pi }$electrons. Therefore, it is antiaromatic in nature. Hence, it is unstable.

The compound azulene has a total of $10\mathrm{\pi }$ electrons. Therefore, it is aromatic in nature. As a result, it is a very stable compound.

The compound heptalene has a total of $6\mathrm{\pi }$ electrons. Therefore, it is antiaromatic in nature. As a result, heptalene is an unstable compound.