Why is gasoline aromatic
Lexicon> Letter K> Hydrocarbons
Definition: substances that only consist of the chemical elements carbon and hydrogen
More general terms: fuel, fuel
More specific terms: alkanes, alkenes, alkynes, cycloalkanes, aromatics
Molecular formula: CxHy
English: carbon hydrates
Author: Dr. Rüdiger Paschotta
How to quote; suggest additional literature
Original creation: January 11, 2015; last change: 03/26/2020
Hydrocarbons are substances that only consist of the chemical elements carbon and hydrogen. The simplest hydrocarbon is methane (CH4); a molecule of it consists of a single carbon atom (C) to which four hydrogen atoms (H) are connected. Heavier hydrocarbons can be liquid or solid at room temperature.
Some authors use an expanded term for hydrocarbons, which can contain elements other than carbon and hydrogen. For example, the term “oxygen-containing hydrocarbons” is used; these can be, for example, alcohols, ethers, ketones and aldehydes.
Hydrocarbons are used in huge quantities as energy carriers and are mainly obtained from petroleum in petroleum refineries. However, there are also many other areas of application such as B. the production of plastics and many chemicals as well as the use as lubricants, solvents, propellants and refrigerants.
The structurally simplest hydrocarbons are the linear ones Alkanes (n-alkanes), in which a number of carbon atoms form a linear chain and in turn hydrogen atoms are connected to the carbon atoms. Only so-called single bonds occur here; so there are saturated hydrocarbons. The two outermost carbon atoms are each connected to three hydrogen atoms, the others only with two (see Figure 1). Incidentally, methane is also considered an alkane, although it does not contain a carbon chain.
Other variants of alkanes contain branched molecules (see Figure 2); they are called Isoalkanes or i-alkanes designated. For example, a distinction is made between linear n-butane and branched isobutane; both have the same empirical formula (C4H10) and are therefore called Isomers designated.
Otherwise there are also alkanes with cyclic (ring-shaped) molecules; the simplest of these is cyclohexane (C.6H12).
The lightest alkanes (methane, ethane, propane, butane) are gaseous at room temperature. Heavier representatives such as pentane, hexane, heptane and octane (important components of gasoline, diesel fuels and kerosene) are liquid at room temperature, and even heavier ones can also be solid. For example, paraffin consists mainly of longer-chain alkanes with about 20 to 30 carbon atoms per molecule.
Alkenes and alkynes
The substance group of the Alkenes (old name: Olefins) denotes compounds in which so-called double bonds occur between hydrocarbons. In this case, fewer hydrogen atoms are added because fewer bonds are available for this. Such substances are called unsaturated. The simplest alkenes are ethene (ethylene) (C2H4), Propene (propylene) (C.3H6), 1-butene (C.4H8) and isobutene (C4H8). Again there are versions with branched molecules, which is what the name means Isoalkenes leads. In addition, a distinction is made between monounsaturated alkenes and polyunsaturated ones that contain several double bonds. Here, too, there are cyclic variants such as cyclohexene (C.6H10).
At Alkynes there is at least one triple bond. Simple representatives of this group are ethyne (acetylene) (C2H2) and the propyne (C.3H4).
Aromatic hydrocarbons (aromatics)
The form another important group of substances aromatic hydrocarbons. These contain cyclic planar molecules. Its simplest representative is benzene (benzene, C.6H6), which contains a single carbon ring. Structural formulas are often drawn with three double bonds and three single bonds, although in reality the bonding electrons are delocalized, i.e. evenly distributed over the ring. Variants of these molecules can, for example, other groups attached to the ring such as. B. contain alkyl groups (e.g. with toluene).Many polycyclic aromatic hydrocarbons are very problematic environmental pollutants.
Hydrocarbons with multiple such rings are called polycyclic aromatic hydrocarbons (PAK). (The spelling polycyclic also occurs.) The carbon rings can either be fused directly to one another (e.g. in the case of naphthalene) or only e.g. B. be connected to each other via single bonds. Many of the polycyclic aromatics are very toxic and also carcinogenic (even if they come into contact with the skin) and are therefore very problematic environmental pollutants. They are natural components of petroleum, but also of tar and coal. Petroleum products such as B. Gasoline and diesel fuel contain traces of polycyclic aromatics, and these can also arise from incomplete combustion of fuels or other fuels. PAHs are also found in tobacco smoke and in hot roasted or grilled meat. The pollution of the air with polycyclic aromatics occurs mainly through the combustion of fossil fuels.
Non-aromatic hydrocarbons are called aliphatic hydrocarbons designated. For example, cyclohexane is considered to be aliphatic, while benzene is considered to be aromatic.
So-called polymerisation reactions can produce molecules with very long carbon chains from hydrocarbons, which are called Polymers are designated. These are widely used as plastics. Examples are polyethylene (polyethene) and polypropylene. However, plastics often contain additives (e.g. plasticizers, stabilizers, reinforcing agents and dyes) that are often not hydrocarbons themselves. Also, not all polymers are hydrocarbons.
Chemical reactions between hydrocarbons and halogen (or substances containing halogen) can cause so-called halogenated hydrocarbons (Halogenated hydrocarbons), of which the chlorinated and fluorinated variants are the most important. These substances are no longer hydrocarbons per se, as they also contain atoms other than carbon and hydrogen.
Halocarbons are sometimes very toxic and are used, for example, as insecticides and disinfectants. Some of them are also very difficult to biodegrade, for example the extremely poisonous dioxins. Other halogenated hydrocarbons (e.g. CFCs) are non-toxic, but they damage the ozone layer of the earth's atmosphere, which is why their use has been largely banned under the Montreal Protocol since 1989.
Hydrocarbons as an energy source
Fossil fuels consist to a large extent of hydrocarbons. This is especially true for oil and natural gas. Petroleum is mainly a mixture of many different hydrocarbons, some of which are only separated into fractions of different weights in petroleum refineries, but some of which are also chemically converted. Mixtures of substances can also be produced from biomass using various processes, which consist largely of hydrocarbons. In both cases, the energy sources obtained in this way can also contain various toxic substances, for example aromatics.
Compared to alcohols such as ethanol, whose molecules also contain oxygen atoms, the calorific value (i.e. the energy density) of pure hydrocarbons is higher.
Combustion of hydrocarbons
The complete combustion of hydrocarbons (which requires a sufficiently high combustion air ratio) leads to carbon dioxide (CO2) and water vapor. In principle, there are no toxic pollutants at all; only the carbon dioxide produced is harmful to the climate if it enters the atmosphere as usual after combustion and is not disposed of as part of CCS.
However, if the combustion is incomplete, toxic carbon monoxide (CO) can also be produced, and soot in the form of fine dust can also form. Likewise, only partially or not at all converted hydrocarbons (unburned hydrocarbons) can remain in the exhaust gas; the resulting hydrocarbon emissions (HC emissions) are very undesirable and may have to be reduced with the help of a catalytic converter. Unburned hydrocarbons in the air are not only partially toxic, they also contribute to the formation of ozone.
Advantages of liquid and gaseous energy sources
The liquid physical state of many hydrocarbons is often favorable for use as an energy carrier: Liquids can be easily stored and transported through pipelines. They also have a much higher density (and thus a correspondingly higher volumetric energy density) than gases without the need to apply high pressure. For combustion, liquid hydrocarbons must first be vaporized, which is not a problem at least with the lighter hydrocarbons.
The disadvantage is the fact that many hydrocarbons are hazardous to water and must therefore be prevented from penetrating into the soil and the groundwater. It is true that hydrocarbons are relatively difficult to mix with water; they tend to be hydrophobic (water-repellent). However, certain amounts of toxic substances can still be absorbed by the water.
Heavy hydrocarbons tend to be less suitable as energy sources because their clean combustion is more expensive. Heavy oil is still widely used (especially for ship propulsion), but mainly only because of its low price; A certain part of the crude oil can hardly be avoided when it is processed in a crude oil refinery into heavy oil.
Gaseous hydrocarbons such as methane, propane and butane are particularly easy to clean (i.e. with only minimally toxic exhaust gases) to burn. They can be easily transported through pipelines, but are less easy to carry in vehicles. Unburned methane released into the atmosphere is extremely harmful to the climate, while propane and butane are converted into carbon dioxide and water in the atmosphere in a relatively short time.
With the help of electrical energy, hydrogen can be obtained through electrolysis, which can also serve as an energy carrier (→RE gas). However, since hydrocarbons as energy carriers mainly have more favorable properties, it is an interesting option to produce them from hydrogen and carbon dioxide (→Power to gas, Power to X).
So-called synthetic fuels, which are obtained for example through biomass gasification or from natural gas, also consist mainly of hydrocarbons. The composition of these mixtures can be optimized through targeted control of the processes; their quality as an energy source is often particularly high.
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See also: methane, propane, butane, benzene, crude oil, natural gas, gasoline, diesel fuel, liquefied petroleum gas, heavy oil, synthetic fuel, renewable energy gas, power to gas, measurement methods for fuel consumption and exhaust gas values, evaporative emissions
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