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Aromatic Hydrocarbons

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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.
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