Here’s a explanation of how polycyclic aromatic hydrocarbons (PAHs) react:
What PAHs Are
- Polycyclic aromatic hydrocarbons (PAHs) are compounds containing two or more fused benzene rings.
- Examples include naphthalene, anthracene, and phenanthrene.
- Like benzene, they have delocalized π-electrons, making them aromatic and relatively stable.
Reactivity of PAHs
- Electrophilic Substitution Reactions
- PAHs generally undergo electrophilic aromatic substitution (EAS), similar to benzene.
- Common reactions include nitration, sulfonation, halogenation, and Friedel-Crafts reactions.
- Certain positions on the fused rings are more reactive, depending on the electron density and resonance stabilization.
- For example, in naphthalene, the alpha (1-) position is more reactive than the beta (2-) position because the intermediate is more stabilized by resonance.
- Oxidation Reactions
- PAHs can undergo oxidation, especially at the side chains or certain reactive positions on the rings.
- Strong oxidizing agents can break the side chains into carboxylic acids or modify specific rings.
- Addition Reactions (less common)
- PAHs can undergo addition reactions under special conditions, but these disrupt aromaticity and are less favored.
Key Points
- PAHs are more reactive than benzene at certain positions due to the distribution of electron density across fused rings.
- Electrophilic substitution is the most common reaction, similar to benzene, but position selectivity matters.
- Resonance stabilization influences which carbon atoms react.