Aromatic Hydrocarbons Arene Aromatic Hydrocarbons Aromatic hydrocarbons are an important series of hydrocarbons found in almost every petroleum mixture from any part of the world. Aromatics are cyclic but unsaturated hydrocarbons with alternating double bonds (Figure 1.12). The simplest aromatic hydrocarbon is benzene (C6H6 ). The name “aromatic” refers to the fact that such hydrocarbons are commonly fragrant compounds. Although benzene has three carbon-carbon double bonds, it has a unique arrangement of electrons with resonance structures of the double bonds (aromaticity) that allow benzene to be relatively stable. However, benzene is known to be a cancer-inducing compound. For this reason, the amount of benzene allowed in petroleum products such as gasoline or fuel oil is limited by government regulations in many countries. Under standard conditions, benzene, toluene, and xylene are in liquid form whereas higher aromatics such as naphthalene occur as solids in isolation, but dissolve to form a liquid solution with simple aromatics. Benzene 12 σ Sigma Bond 3 π Pi Bond σ σ σ σ σ π σ σ π π σ σ σ σ σ Alkylaromatics Arene substitution patterns •In ortho-substitution, two substituents occupy positions next to each other, which may be numbered 1 and 2. In the diagram, these positions are marked R and ortho. •In meta-substitution the substituents occupy positions 1 and 3 (corresponding to R and meta in the diagram). •In para-substitution, the substituents occupy the opposite ends (positions 1 and 4, corresponding to R and para in the diagram). Polyaromatic Hydrocarbons (PAH) or Polynuclear Aromatic (PNA) • Aromatic hydrocarbons containing more than one ring Polyaromatic Hydrocarbons (PAH) or Polynuclear Aromatic (PNA) Normally, high-molecular-weight polyaromatics contain several heteroatoms such as sulfur, nitrogen, or oxygen, but these compounds are still called aromatic compounds because their electronic configurations maintain the aromatic character. Sulfur is the most important heteroatom found in crude oil and refinery products petroleum and it can be found in cyclic (e.g., thiophenes) and noncyclic compounds such as mercaptans (R-S-H) and sulfides (R-S- R′), where R and R′ are alkyl groups. Sulfur in natural gas is usually found in the form of hydrogen sulfide (H2S). Figure 1.14 shows the types of sulfur compounds in crude oils. The amount of sulfur in a crude oil may vary from 0.05 to 6 % by weight. The presence of sulfur in finished petroleum products is not desirable. For example, the presence of sulfur in gasoline can promote corrosion of engine parts and produce sulfur oxides upon combustion, contributing to air pollution.