How many benzene rings are present in the fused polycyclic compound phenanthrene?

Phenanthrene is a polycyclic aromatic hydrocarbon (PAH) composed of three fused benzene rings arranged in a linear but angular structure. It has the molecular formula C₁₄H₁₀. The rings are fused in such a way that they share carbon–carbon bonds, forming a stable aromatic system. Each ring in phenanthrene contributes to the delocalized π-electron cloud that extends across the entire molecule.

In phenanthrene, the three benzene rings are fused in a specific arrangement that creates a bent structure, unlike anthracene, which has three rings fused in a straight line. The shared carbon atoms between the rings lead to extended conjugation, allowing π-electrons to delocalize over all the carbon atoms. This delocalization provides high stability and aromatic character to the compound.

Phenanthrene is an important component of coal tar and is found in fossil fuels and combustion products. It is less reactive than non-aromatic hydrocarbons because of its strong aromatic stabilization. Phenanthrene undergoes electrophilic substitution reactions, similar to benzene, such as nitration and sulfonation, but the reaction sites depend on the ring positions and resonance distribution.

The compound’s aromatic character is confirmed by its resonance energy and the uniform bond lengths observed through X-ray studies. It obeys Hückel’s rule (4n + 2 π-electrons), where each ring contributes to a conjugated π-system, maintaining aromatic stability throughout the molecule.

Phenanthrene and its derivatives are studied in organic chemistry because they serve as structural models for understanding larger aromatic systems like polyaromatic hydrocarbons, dyes, and pharmaceuticals. Therefore, the fused polycyclic compound phenanthrene contains three benzene rings connected in a continuous conjugated system that provides exceptional chemical stability