Hydrocarbon

Of the classes of hydrocarbons, aromatic compounds uniquely (or nearly so) undergo substitution reactions. The chemical process practiced on the largest scale is the reaction of benzene and

The resulting ethylbenzene is dehydrogenated to styrene and then polymerized to manufacture polystyrene, a common thermoplastic material.

Free-radical substitution

Substitution reactions occur also in saturated hydrocarbons (all single carbon–carbon bonds). Such reactions require highly reactive reagents, such as

all the way to CCl₄ (carbon tetrachloride)

C₂H₆ + Cl₂ → C₂H₅Cl + HCl

C₂H₄Cl₂ + Cl₂ → C₂H₃Cl₃ + HCl

all the way to C₂Cl₆ (hexachloroethane)

Addition

Addition reactions apply to alkenes and alkynes. In this reaction a variety of reagents add "across" the pi-bond(s). Chlorine, hydrogen chloride, water, and hydrogen are illustrative reagents.

Addition polymerization

Hydrogenation

Metathesis

Some hydrocarbons undergo metathesis, in which substituents attached by C–C bonds are exchanged between molecules. For a single C–C bond it is alkane metathesis, for a double C–C bond it is alkene metathesis (olefin metathesis), and for a triple C–C bond it is alkyne metathesis.

High-temperature reactions

Cracking

Dehydrogenation

Pyrolysis

Combustion

Combustion of hydrocarbons is currently the main source of the world's energy for

Common properties of hydrocarbons are the facts that they produce steam, carbon dioxide and heat during combustion and that oxygen is required for combustion to take place. The simplest hydrocarbon, methane, burns as follows:

${\displaystyle {\ce {{\underset {methane}{CH4}}+ 2O2 -> CO2 + 2H2O}}}$

In inadequate supply of air, carbon black and water vapour are formed:

${\displaystyle {\ce {{\underset {methane}{CH4}}+ O2 -> C + 2H2O}}}$

And finally, for any

${\displaystyle {\ce {C}}_{n}{\ce {H}}_{2n+2}+\left({{3n+1} \over 2}\right){\ce {O2->}}n{\ce {CO2}}+(n+1){\ce {H2O}}}$

Partial oxidation characterizes the reactions of alkenes and oxygen. This process is the basis of rancidification and paint drying.

Benzene burns with sooty flame when heated in air:

${\displaystyle {\ce {{\underset {benzene}{C6H6}}+{15 \over 2}O2->6CO2{+}3H2O}}}$

Origin

The vast majority of hydrocarbons found on

A seemingly limitless variety of compounds comprise petroleum, hence the necessity of refineries. These hydrocarbons consist of saturated hydrocarbons, aromatic hydrocarbons, or combinations of the two. Missing in petroleum are alkenes and alkynes. Their production requires refineries. Petroleum-derived hydrocarbons are mainly consumed for fuel, but they are also the source of virtually all synthetic organic compounds, including plastics and pharmaceuticals. Natural gas is consumed almost exclusively as fuel. Coal is used as a fuel and as a reducing agent in metallurgy.

A small fraction of hydrocarbon found on earth, and all currently known hydrocarbon found on other planets and moons, is thought to be abiological.^[13]

Hydrocarbons such as ethylene, isoprene, and monoterpenes are emitted by living vegetation.^[14]

Some hydrocarbons also are widespread and abundant in the Solar System. Lakes of liquid methane and ethane have been found on Titan, Saturn's largest moon, as confirmed by the Cassini–Huygens space probe.^[15] Hydrocarbons are also abundant in nebulae forming polycyclic aromatic hydrocarbon compounds.^[16]

Environmental impact

Burning hydrocarbons as fuel, which produces

When soil is contaminated by hydrocarbons, it can have a significant impact on its microbiological, chemical, and physical properties. This can serve to prevent, slow down or even accelerate the growth of vegetation depending on the exact changes that occur. Crude oil and natural gas are the two largest sources of hydrocarbon contamination of soil.^[19]

Bioremediation

Bioremediation of hydrocarbon from soil or water contaminated is a formidable challenge because of the chemical inertness that characterize hydrocarbons (hence they survived millions of years in the source rock). Nonetheless, many strategies have been devised, bioremediation being prominent. The basic problem with bioremediation is the paucity of enzymes that act on them. Nonetheless, the area has received regular attention.^[20] Bacteria in the gabbroic layer of the ocean's crust can degrade hydrocarbons; but the extreme environment makes research difficult.^[21] Other bacteria such as Lutibacterium anuloederans can also degrade hydrocarbons.^[22] Mycoremediation or breaking down of hydrocarbon by mycelium and mushrooms is possible.^[23][24]

Safety

Hydrocarbons are generally of low toxicity, hence the widespread use of gasoline and related volatile products. Aromatic compounds such as benzene and toluene are narcotic and chronic toxins, and benzene in particular is known to be carcinogenic. Certain rare polycyclic aromatic compounds are carcinogenic. Hydrocarbons are highly

See also

References