Halogenation can indeed occur on polycyclic aromatic hydrocarbons (PAHs), but the process and outcomes differ from those in monocyclic arenes due to the more complex structure of PAHs. PAHs contain multiple aromatic rings, which can influence the reactivity and selectivity of the halogenation reaction. In PAHs, the electron density is not uniformly distributed across all the aromatic rings, leading to differences in reactivity. Some rings in a PAH molecule might be more reactive towards electrophilic attack due to the influence of adjacent rings or substituents. This variation in reactivity can result in regioselectivity, where certain positions within the PAH are more likely to undergo halogenation. Additionally, the steric hindrance in PAHs can affect the approach of the electrophile to certain positions. As a result, halogenation of PAHs often requires more careful consideration of the specific structure of the compound and the reaction conditions to achieve the desired selectivity and yield.

Side-chain halogenation of arenes tends to produce a mixture of products due to the nature of the radical chain mechanism involved. The process initiates with the generation of halogen radicals, which abstract a hydrogen atom from the arene's side-chain, creating a carbon radical. This carbon radical can react with halogen molecules to form a mono-halogenated product. However, the carbon radical can also further react with additional halogen radicals, leading to multi-halogenated products. Moreover, if the side-chain has multiple hydrogen atoms, the initial radical formation can occur at different positions, resulting in isomeric products. The lack of regioselectivity and the possibility of multiple halogenation under radical conditions contribute to the formation of a mixture of products. The extent and variety of these products depend on the reaction conditions, such as temperature, concentration of the halogen, and the presence of other substituents on the arene.

The presence of substituent groups on the aromatic ring significantly influences the positional selectivity during halogenation. Substituents can be classified as either electron-donating or electron-withdrawing. Electron-donating groups, such as alkyl groups or methoxy groups, increase the electron density on the ring, particularly at the ortho and para positions, thus making these positions more reactive towards electrophilic attack. As a result, halogenation in the presence of such groups typically occurs at these positions. Conversely, electron-withdrawing groups, like nitro groups, decrease electron density on the ring and are meta-directing. They make the ortho and para positions less susceptible to electrophilic attack, hence directing the halogenation to the meta position. These directing effects are a result of the resonance and inductive effects exerted by the substituents, which alter the electron density distribution on the aromatic ring, thereby influencing the site of electrophilic attack during halogenation.

The environmental implications of halogenating arenes are significant, especially considering the persistence and toxicity of many halogenated aromatic compounds. Halogenated arenes, such as polychlorinated biphenyls (PCBs) and dioxins, are known for their stability and resistance to degradation, leading to their accumulation in the environment. These compounds can be bioaccumulative and have been associated with various adverse health effects, including endocrine disruption, carcinogenicity, and toxicity to aquatic life. The production and disposal of halogenated arenes require stringent controls to prevent environmental contamination. Additionally, the use of certain halogens, particularly chlorine and bromine, in these reactions can lead to the formation of harmful by-products. The environmental impact of halogenation processes necessitates the development of greener and more sustainable chemical practices, such as using less toxic halogens or alternative methods for introducing functional groups into arenes.

The nature of the halogen used in the halogenation of arenes significantly affects the reaction due to differences in reactivity and stability of the halogen species. Chlorine and bromine are commonly used in halogenation reactions due to their suitable reactivity. Chlorine is more reactive than bromine, leading to faster reaction rates but potentially lower selectivity. Bromination tends to be more selective, allowing for greater control over the reaction outcome. Iodination of arenes is less common because iodine is less reactive due to its larger size and lower electronegativity. However, iodination can be achieved under certain conditions, often requiring oxidizing agents to facilitate the reaction. The choice of halogen also affects the reaction conditions required, such as the need for catalysts and the reaction temperature. For example, bromination usually requires a stronger Lewis acid catalyst compared to chlorination. Each halogen brings distinct characteristics to the reaction, influencing the mechanism, rate, and product distribution in the halogenation of arenes.