What is the Difference Between Anthracene and Phenanthrene?

Anthracene and phenanthrene are both polycyclic aromatic hydrocarbons (PAHs) with three fused benzene rings, but they have different structures and stabilities. The key differences between anthracene and phenanthrene are:

1. Structure: Anthracene has three fused benzene rings in a straight chain, while phenanthrene has three fused benzene rings with a fourth benzene ring attached. Anthracene is fused linearly, whereas phenanthrene is fused at an angle.
2. Resonance Structures: Phenanthrene has five resonance structures, which is one more than anthracene. This extra resonance structure contributes to the increased stability of phenanthrene.
3. Stability: Phenanthrene is more stable than anthracene by about 6 kcal/mol due to its additional resonance structure. This increased stability is also reflected in the higher ionization potential of phenanthrene compared to anthracene.
4. Sources: Anthracene is found in coal tar and is a common impurity in phenanthrene, while phenanthrene can be found in cigarette smoke and is used in the production of dyes, plastics, pesticides, explosives, and drugs.

In summary, anthracene and phenanthrene are structural isomers with the same chemical formula but different structures, resonance structures, and stabilities. Phenanthrene is more stable than anthracene due to its additional resonance structure, which results from its non-linear fusion of benzene rings.

Comparative Table: Anthracene vs Phenanthrene

Anthracene and phenanthrene are both polycyclic aromatic hydrocarbons with the same molecular formula, but they differ in structure and reactivity. Here is a comparison table highlighting their differences:

Both anthracene and phenanthrene are structural isomers, meaning they have the same molecular formula but different arrangements of atoms in their molecular structures. Anthracene consists of three linearly fused benzene rings, while phenanthrene's structure includes two outer benzene rings fused with a central five-membered ring, making it non-planar. Anthracene tends to undergo addition reactions at the central bond due to the susceptibility of the conjugated system, whereas phenanthrene is more prone to electrophilic aromatic substitution reactions.