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Alkanes: Structure and Constitution

Alkanes: Usage

Alkanes are very versatile and are being used as solvents, heating oils, fuels, in fat synthesis, in the synthesis of fatty acids by air oxidation, in the manufacture of albumen, in the transformation to olefins, etc. The latter are starting materials for the preparation of alkyl benzenes which play an important role in the synthesis of degradable detergents.

Methane

The largest amount of methane produced is used for heating purposes. Only a small part is further processed to yield, for example, hydrogen cyanide, acetylene, chloroalkanes, carbon disulfide, and carbon black. Methane plays an important role in the generation of a carbon dioxide-hydrogen mixture either by incomplete combustion of methane and/or by reaction of the alkane with water vapor. The mixture known as syngas is being used in the synthesis of methanol, for the preparation of aldehydes and alcohols by oxosynthesis (hydroformylation) and for the generation of hydrogen, e.g. to be used in the synthesis of ammonia. Methane is an important raw material for large-scale industrial syntheses, for example, in the syntheses of halogenated hydrocarbons, such as methyl chloride, methylene chloride, chloroform, carbontetrachloride, etc. , fluorocarbons, carbon disulfide, acetylene, hydrogen cyanide, carbon black, etc.. Reaction with H2O yields syngas and oxidative coupling or reaction in methane plasma leads to C2 compounds. Increasingly, microbial use gains importance in the synthesis of proteins using methane-oxidizing bacteria for example. However, only a small amount of generated methane is used in the chemical industry, most of it is being used for fuel either in the form of natural gas or in metal cylinders compressed under ca. 150 bar of pressure. Liquid methane is transported in tankers at temperatures below -161°C.

Ethane

Ethane is being used as heating fuel. Additionally, ethane is utilized to prepare ethylene by pyrrolysis or acetaldehyde and acetic acid by catalytical oxidation.

Propane

Propane is an important raw material in petrochemistry. In pressurized gas cylinders, it finds use as liquid gas (LPG) in laboratories and households for heating and combustion purposes [medium calorific value (Hu) 93 MJ/ m3], as a coolant in industry, as a selective solvent for higher-boiling crude oil fractions and for the removal of asphalt, and in organic syntheses (chlorination, oxidation, ammonia oxidation, nitration). However, its main use is as starting material for the generation of ethylene and propylene. A dehydrocyclo-oligomerization process using zeolite catalysts is being used to prepare BTX-compounds (benzene-toluene-xylene) from propane as feeedstock. Propane, generally in combination with butane, is used today in many cases as propellant in aerosol sprays instead of chlorofluoro hydrocarbons. However, country-based regulations regarding flammability and the presence of scent-intensive impurities have to be observed.

Butane

Butane is an important feedstock for the petrochemical industry. In pressurized gas cylinders, it finds use as liquid gas (LPG) in laboratories and households for heating and combustion purposes (calorific value 124 MJ/ m3), as low-temperature solvent and as extraction medium. Mostly, it is being used for the technical syntheses of 1,3-butadiene, maleic anhydride and so-called polymer gasoline.

Pentane

In addition to being used as a solvent, n-pentane is utilized as a foaming agent in the preparation of phenolic resins and polystyrene, as a propellant for aerosol sprays, as a filling of low-temperature thermometers, and as reference compound in gas chromatography.

Hexane

Hexane is used for the extraction of oils and fats, as solvent and reaction medium in the preparation of plastics and synthetic rubber during the polymerization process and as dilutent for fast-drying laquers, printing inks and glues. Furthermore, it is used as eluent and solvent in thin layer chromatography and spectroscopy and as a substitute for mercury in thermometers.

Heptane

Heptane serves as a solvent in the laboratory and for fast-drying lacquers and glues.

Octane

Octane is used for the aromatization to xylenes and ethylbenzene but mainly as a solvent and in azeotropic distillations. Determination of the octane number is not carried out by using octane but isooctane, one of the 18 isomers of octane.

Nonane

Nonane is used for the preparation of tensides and as a carrier in distillation processes.

Undecane / dodecane

Both of these alkanes are used as references in gas chromatography. Chlorinated dodecane serves as starting material in the synthesis of alkylbenzene sulfonates.

Synthesis

Alkanes can be synthesized either by coal hydrogenation, Fischer-Tropsch synthesis (see benzene), cracking, catalytical hydrogenation of alkenes, reduction of halogenated alkanes, hydrolysis of Grignard compounds, Wurtz coupling, hydrolysis of carbides, Kolbe electrolysis, or by polymerization from ethylene.

Since the availability of crude oil and coal is limited, sources of fossil energy will be substituted gradually by environment-friendly sources (sun, wind and water). For the chemical industry, however, crude oil and coal remain important sources of raw materials. Main research efforts are therefore applied to the development of processes and applications for the use of renewable biomass as a source of raw materials for the chemical industry.

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