Hydrocarbon

Carbon’s chemical bonding

Aliphatic hydrocarbons

Alkanes

Alkenes

Alkynes

Aromatic hydrocarbons

Petrochemicals

Resources

A hydrocarbon is any chemical compound composed exclusively of carbon and hydrogenatoms.

Carbon atoms have the unique ability to form strong bonds with each other, atom after atom. Every hydrocarbon molecule is built upon a skeleton of carbon atoms bonded to each other either in the form of closed rings or in a continuous rowlike links in a chain. A chain of carbon atoms may be either straight or branched. In every case, whether ring or chain, straight or branched, all the carbon bonds that have not been used in tying carbon atoms together are taken up by hydrogen atoms attached to the carbon skeleton. Because there is no apparent limit to the size and complexity of the carbon skeletons, there is in principle no limit to the number of different hydrocarbons that can exist.

Hydrocarbons are the underlying structures of all organic compounds. All organic molecules can be thought of as being derived from hydrocarbons by substituting other atoms or groups of atoms for some of the hydrogen atoms and occasionally for some of the carbon atoms in the skeleton.

Carbon’s chemical bonding

The carbon atom has four electrons in its outer, or valence, shell. This means that every carbon atom can form four, and only four, covalent (electron pair-sharing) bonds by pairing its four valence electrons with four electrons from other atoms. This includes forming bonds to other carbon atoms, which can form bonds to still other carbon atoms, and so on. Thus, extensive skeleton structures of dozens or hundreds of carbon atoms can be built up.

A carbon atom does not form its four bonds all in the same direction from the nucleus. The bonding electron pairs being all negatively charged tend to repel one another, thereby forcing them as far apart as possible. The bonds will occur in four equally spaced directions. In three-dimensional space, four equally spaced directions from a central location (the carbon atom’s nucleus) point towards the four corners of a tetrahedron.

On two-dimensional paper, the formation of a covalent bond between two carbon atoms can be depicted as follows, where the dots indicate valence electrons and the C’s indicate the rest of the atoms (nucleus plus inner electrons):

The carbon atoms still have unused bonds shown by the unpaired dots, and they can join to third and fourth carbon atoms and so on, building up longer and longer chains:

and

Instead of lining up in straight or normal chains, the carbon atoms may also bond in different directions to form branched chains.

In all of these skeletons, there are still some carbon valence electrons that are not being used for carbon-to-carbon bonding. The remaining bonds can be filled by hydrogen atoms to form hydrocarbon molecules:

Hydrogen is a particularly good candidate for bonding to carbon because each hydrogen atom has only one valence electron; it can pair with one of the carbon atom’s valence electrons to form a bond in one of carbon’s four possible directions without interfering with any of the other three because hydrogen is such a tiny atom. (In addition to its valence electron, a hydrogen atom is nothing but a proton.) Hydrocarbons are divided into two general classes: aromatic hydrocarbons, which contain benzene rings in their structures, and aliphatic hydrocarbons, which are all the rest.

Aliphatic hydrocarbons

The carbon-atom skeletons of aliphatic hydrocarbons may consist of straight or branched chains, or of (non-benzene) rings. In addition, all of the carbon atoms in the skeletons may be joined by sharing single pairs of electrons (a single bond, represented as C:C or C-C), as in the examples above, or there may be some carbon atoms that are joined by sharing two or three pairs of electrons. Such bonds are called double and triple bonds and are represented as C::C or C=C and C:::C or C≡C, respectively.

Thus, there can be three kinds of aliphatic hydrocarbons: those whose carbon skeletons contain only single bonds, those that contain some double bonds, and those that contain some triple bonds. These three series of aliphatic hydrocarbons are called alkanes, alkenes, and alkynes, respectively. (There can also be “hybrid” hydrocarbons that contain bonds of two or three kinds.)

Alkanes

The alkanes are also called the saturated hydrocarbons, because all the bonds that are not used to make the skeleton itself are filled to their capacity—saturated—with hydrogen atoms. They are also known as the paraffin hydrocarbons, from the Latin parum affinis, meaning “little affinity,” because these compounds are not very chemically reactive.

The three smallest alkane molecules, containing one, two, and three carbon atoms, are shown in three ways.

The structural formulas are one way in which simple organic molecules can be depicted in two dimensions on paper; each line indicates a single covalent bond-a shared pair of electrons. The three-dimensional ball-and-stick models and space-filling models, in which the balls represent the carbon and hydrogen atoms (roughly to scale) and the sticks represent the bonds, are used by chemists to study the shapes of molecules.

The names and formulas of the first eight normal (not branched) alkanes are: Methane (CH₄); Ethane (C₂H₆); Propane (C₃H₈); Butane (C₄H₁₀); Pentane (C₅H₁₂); Hexane (C₆H₁₄); Heptane (C₇H₁₆); Octane (C₈H₁₈).

While the first four alkanes were named before their structures were known, the rest have been named with Greek roots that tell how many carbon atoms there are in the chain: pent = five, hex = six, and so on, all ending in the “family name,” -ane. The chemical formula of an alkane hydrocarbon can be obtained quickly from the number of carbon atoms, n, in its skeleton: the formula is C_nH_2n+2. This method works because every carbon atom has two hydrogen atoms attached except for the two end carbon atoms, which have two extra ones. As an example, the formula for pentane is C₅H₁₂.

The branched alkanes are named by telling what kinds of branches—methyl, ethyl or propyl groups, etc.—are attached to the main chain and where. For example, (SEE PRINT COPY)

H₃C-CH-CH₂-CH₂-CH₃≡CH₃

is named 2-methyl pentane; the 2 indicates that the methyl group (-CH₃) branches off the second carbon atom from the nearest end of the pentane chain.

The four lightest normal alkanes, having the smallest (lowest molecular weight) molecules, are gases at room temperature and pressure, while the heavier ones are oily liquids, and still heavier ones are waxy solids. Alkanes, which are the major constituents of crude oil, do not mix with water and float on its surface. The wax known as paraffin is a mixture of alkanes containing between 22 and 27 carbon atoms per molecule.

All hydrocarbons burn in air to form carbon dioxide and water. Methane, CH4, as the major constituent of natural gas, is widely used as a heating fuel. Also known as marsh gas, methane occurs naturally in marshes and swamps, being produced by bacteria during the decomposition of plant and animalmatter. It can form explosive mixtures with air, however, and is therefore a hazard when present in coalmines. Bacteria-produced methane may have potential as a commercial fuel source.

Propane, C3H8, and butane, C4H10, are compressed into tanks, where they liquefy and can be used as portable fuels for such applications as barbecue grills, mobile-home cooking, and disposable cigarette lighters. Because these compounds are pure and burn cleanly, they are being explored as fuels for non-polluting automobile engines. They are often referred to as LPG-liquefied petroleum gas.

Cycloalkanes are alkanes whose carbon atoms are joined in a closed loop to form a ring-shaped molecule. Cyclopropane, which contains three carbon atoms per molecule, has molecules that are in the shape of a three-membered ring, or triangle. Cyclohexane, with six carbon atoms, has hexagonal molecules; it is used as a good solvent for many organic compounds.

Alkenes

The alkenes, sometimes called olefins, are hydrocarbons that contain one or more double bonds per molecule. Their names are parallel to the names of the alkanes, except that the family ending is -ene, rather than -ane. Thus, the four smallest molecule alkenes containing two, three, four, and five carbon atoms are ethene (also called ethylene), propene (also called propylene), butene (also called butylene), and pentene. (There can be no “methene,” because there must be at least two carbon atoms to form a double bond.) A number preceding the name indicates the location of the double bond by counting the carbon atoms from the nearest end of the chain. For example, 2-pentene is the five-carbon alkene with the structure

H₃C-CH =CH-CH₂-CH₃

The locations of branches are similarly indicated by numbers. For example, 3-ethyl 2-pentene has the structure (SEE PRINT COPY)

H₃C-CH =C-CH₂-CH₃≡C₂H₅

The lightest three alkenes, ethylene, propylene, and butene, are gases at room temperature; from there on, they are liquids that boil at higher and higher temperatures. The chemical formula of an alkene containing only one double bond per molecule can be obtained from the number of carbon atoms in its molecules: if n is the number of carbon atoms, the formula is C_nH_2n. Thus, the formula for pentene is C₅H₁₀.

Alkenes are called unsaturated hydrocarbons; if there is more than one double bond in an alkene molecule it is said to be polyunsaturated. In principle, two more hydrogen atoms could be added to each double bond to saturate the compound, and in fact this does happen quite easily when hydrogen gas is added to an alkene in the presence of a catalyst. This process is called hydrogenation.

Other elements, such as the halogens and hydrogen halides, can also be added easily to the double bonds in alkenes. The resulting halogenated hydrocarbons are very useful but are often toxic or environmentally damaging. Trichloroethylene is a useful solvent, chlorinated hydrocarbons have been used as insecticides, and chlorofluorocarbons (CFCs or Freons) are used as refrigerants but have been shown to damage Earth’s ozone layer.

Alkynes

Alkynes are hydrocarbons that contain one or more triple bonds per molecule. Their names are parallel to the names of the alkanes except that the family ending is -yne. Thus, the four smallest-molecule alkynes are ethyne (more usually called acetylene), propyne, butyne, and pentyne. Alkynes containing one triple bond have chemical formulas given by C_nH_2n-2, where n is the number of carbon atoms in the molecule. Thus, the formula for pentyne is C₅H₈. Acetylene, propyne, and butyne are gases at room temperature; the rest are liquids.

The most famous of the alkynes is the first member of the series: acetylene, C₂H₂. It forms explosive mixtures with air or oxygen, but when mixed with oxygen in a controlled way in an oxyacetylene torch it burns with a very hot flame—up to 6,332ºF (3,500ºC) which is hot enough to cut and weld steel. Because acetylene is explosive when compressed into liquid form, the tanks of acetylene that welders use contain acetylene dissolved in acetone.

Other important alkenes are styrene, C₆H₅-CH =CH₂, from which the plastic polystyrene is made, and isoprene, CH₂ =C(CH₃)CH=CH₂, which is the monomer of natural rubber. (In this shorthand structural formula for isoprene, the parentheses indicate that the CH3 group within them is a branch attached to the preceding carbon atom.)

Aromatic hydrocarbons

An aromatic hydrocarbon is any hydrocarbon that contains one or more benzene rings in its molecule. The name aromatic is historical in origin, and does not at all imply that these compounds have pleasant aromas. Aromatic hydrocarbons are the basis of many aromatic compounds containing other atoms such as oxygen and nitrogen in addition to the carbon and hydrogen that are of extreme biological and industrial importance.

The simplest aromatic hydrocarbon is benzene itself, C₆H₆, whose molecule is a hexagonal ring of six CH groups. Various carbon–and–hydrogen groups can be substituted for any or all of the hydrogen atoms in benzene to form substituted benzenes. Benzene’s own phenyl groups, C6H5, can bond to each other end to end, to form polycyclic (multiple-ring) hydrocarbons, or they can fuse together along the hexagons’ sides to form condensed ring or fused ring hydrocarbons.

In this figure, the bonds leading to all the hydrogen atoms are omitted for simplicity as is the usual practice among chemists. Also, the benzene rings are drawn with alternating double and single bonds

between the carbon atoms. In reality, however, resonance makes all the carbon-carbon bonds equal at an intermediate value between single and double. Chemists therefore usually draw the benzene ring simply as a hexagon with a circle inside:

The hexagon represents the six carbon atoms and their attached hydrogen atoms, while the circle represents all the bonding electrons as if they were everywhere in the molecule at once. In chemists’ shorthand, then, naphthalene would be depicted as shown in the above figure.

Among the important substituted benzenes are methyl benzene, commonly known as toluene, and dimethyl benzene, commonly known as xylene. They are both powerful solvents for organic compounds and are used as starting materials for the synthesis of drugs, dyes, plastics, and explosives. Treatment of toluene with nitric and sulfuric acids produces the explosive trinitrotoluene, or TNT.

Among the important condensed ring aromatic hydrocarbons are naphthalene and anthracene whose molecules consist of two and three hexagonal benzene rings, respectively, fused together along one side.

Both are derived from coal tar and are used as starting materials for the synthesis of many useful compounds. Naphthalene is a crystalline solid with a strong, pungent odor; it is used as a moth repellant and a deodorant-disinfectant.

Petrochemicals

The primary source of hydrocarbons is petroleum or crude oil, a naturally occurring liquid found primarily in sedimentary rocks. Petroleum consists almost entirely of a mixture of alkanes with some alkenes and smaller amounts of aromatic hydrocarbons. When petroleum is distilled at a series of different temperatures, the lowest molecular-weight hydrocarbons boil off at the lowest temperatures and the higher molecular weight ones boil off at successively higher temperatures. This process, called fractional distillation, is used to separate the complex mixture of compounds. Table 1 shows the various hydrocarbon mixtures (known as fractions) that distill off in various temperature ranges.

In addition to isolating the hydrocarbons that occur naturally in petroleum, oil refineries use a variety of processes to convert some of them into other more desirable hydrocarbons.

A vast number of synthetic (human-made) organic chemicals, including drugs, plastics, paints, adhesives, fibers, detergents, synthetic rubber, and agricultural chemicals, owe their existence to petrochemicals: chemicals derived from petroleum. The top six petrochemicals produced in the United States are ethylene, propylene, benzene, xylene, butadiene (the four-carbon-atom alkene with two double bonds), and toluene. From these, hundreds of other chemicals are manufactured.

Gasoline

Probably the most important product of the fractional distillation of petroleum is gasoline, a mixture of alkanes containing six to ten carbon atoms in their molecules: hexane (C₆H₁₄), heptane (C₇H₁₆), octane (C₈H₁₈), nonane (C₉H₂₀), and decane (C₁₀H₂₂), plus small amounts of higher-molecular weight alkanes. More than six trillion gallons of gasoline are burned each year in the United States.

Gasoline must have certain properties in order to work well in automobile engines. If the gasoline-air mixture does not explode smoothly when ignited by the spark in the cylinder, that is, if it makes a fast, irregular explosion instead of a fast but gentle burn, then the explosive force will hit the piston too soon, while it is still trying to move down into the cylinder. This clash of ill-timed forces jars the engine, producing a metallic clanking noise called a knock, which is especially audible when the engine is laboring to climb a hill. Extensive knocking can lead to serious engine damage, so gasolines are formulated to minimize this effect.

Of all the hydrocarbons that can be in gasoline, normal (straight-chain) heptane, C₇H₁₆, has been found to make auto engines knock worst. It has been assigned a value of zero on a scale of gasoline desirability. The hydrocarbon that knocks least is a branched-chain form of octane, C₈H₁₈, called iso-octane. It has been rated 100. Every gasoline blend is assigned an octane rating between zero and 100, according to how much knocking it produces under standard test conditions. Most automobile fuels sold have octane ratings above 85. High-octane gasolines that are even better than isooctane because of anti-knock additives can have ratings above 100.

The C₆ to C₁₀ hydrocarbons make up only about 20–30% of crude oil, which is far from enough to supply the world’s appetite for gasoline. But even if there were enough of it, the natural mixture has an octane rating of only about 40 to 60—not good enough for modern engines. Refineries therefore modify the natural mixture of molecules by breaking down big molecules into smaller ones (cracking) and by reshaping some of the smaller molecules into forms that knock less (reforming).

Gasoline sold at the pump has been blended with additives. Lead-containing antiknock compounds such as tetraethyl lead, Pb(C₂H₅)₄, are no longer used because lead is a toxic air pollutant; methyl-tertbutyl ether (MBTE) is used instead. Other additives remove harmful engine deposits, prevent gum formation, inhibit rusting, prevent icing, clean the carburetor, lubricate the cylinders, and dye the gasoline distinctive colors for identification purposes.

See also Chemical bond; Formula, structural.

Resources

BOOKS

Anslyn, E.V. and D.A. Dougherty. Modern Physical Organic Chemistry. Herndon, VA: University Science Books, 2005.

Hsu, C.S. and P.R. Robinson, eds. Practical Advances in Petroleum Processing. Berlin: Springer, 2006.

Robert L. Wolke

Hydrocarbon

A hydrocarbon is any chemical compound whose molecules are made up of nothing but carbon and hydrogen atoms .

Carbon atoms have the unique ability to form strong bonds to each other, atom after atom. Every hydrocarbon molecule is built upon a skeleton of carbon atoms bonded to each other either in the form of closed rings or in a continuous row like links in a chain. A chain of carbon atoms may be either straight or branched. In every case, whether ring or chain, straight or branched, all the carbon bonds that have not been used in tying carbon atoms together are taken up by hydrogen atoms attached to the carbon skeleton. Because there is no apparent limit to the size and complexity of the carbon skeletons, there is in principle no limit to the number of different hydrocarbons that can exist.

Hydrocarbons are the underlying structures of all organic compounds. All organic molecules can be thought of as being derived from hydrocarbons by substituting other atoms or groups of atoms for some of the hydrogen atoms and occasionally for some of the carbon atoms in the skeleton.

Carbon's chemical bonding

The carbon atom has four electrons in its outer, or valence, shell. This means that every carbon atom can form four, and only four, covalent (electron-pair-sharing) bonds by pairing its four valence electrons with four electrons from other atoms. This includes forming bonds to other carbon atoms, which can form bonds to still other carbon atoms, and so on. Thus, extensive skeleton structures of dozens or hundreds of carbon atoms can be built up.

A carbon atom does not form its four bonds all in the same direction from the nucleus. The bonding electron pairs being all negatively charged tend to repel one another, and they will try to get as far apart as possible. The bonds will therefore stick out in four equally spaced directions. In two dimensions, four equally spaced directions from a point would aim at the four corners of a square . But in three-dimensional space , four equally spaced directions from a point (the carbon atom's nucleus) aim at the four corners of a tetrahedron.

On two-dimensional paper , the formation of a covalent bond between two carbon atoms can be depicted as follows, where the dots indicate valence electrons and the C's indicate the rest of the atoms (nucleus plus inner electrons):

The carbon atoms still have unused bonds shown by the unpaired dots, and they can join to third and fourth carbon atoms and so on, building up longer and longer chains:

and

Instead of lining up in straight or normal chains, the carbon atoms may also bond in different directions to form branched chains.

In all of these skeletons, there are still some carbon valence electrons that are not being used for carbon-to-carbon bonding. The remaining bonds can be filled by hydrogen atoms to form hydrocarbon molecules:

Hydrogen is a particularly good candidate for bonding to carbon because each hydrogen atom has only one valence electron; it can pair up with one of the carbon atom's valence electrons to form a bond in one of carbon's four possible directions without interfering with any of the other three because hydrogen is such a tiny atom. (In addition to its valence electron, a hydrogen atom is nothing but a proton.) Hydrocarbons are divided into two general classes: aromatic hydrocarbons, which contain benzene rings in their structures, and aliphatic hydrocarbons, which are all the rest.

Aliphatic hydrocarbons

The carbon-atom skeletons of aliphatic hydrocarbons may consist of straight or branched chains, or of (non-benzene) rings. In addition, all of the carbon atoms in the skeletons may be joined by sharing single pairs of electrons (a single bond, represented as C:C or C-C), as in the examples above, or there may be some carbon atoms that are joined by sharing two or three pairs of electrons. Such bonds are called double and triple bonds and are represented as C::C or C=C and C:::C or C ≡ C, respectively.

Thus, there can be three kinds of aliphatic hydrocarbons: those whose carbon skeletons contain only single bonds, those that contain some double bonds, and those that contain some triple bonds. These three series of aliphatic hydrocarbons are called alkanes, alkenes, and alkynes, respectively. (There can also be " hybrid" hydrocarbons that contain bonds of two or three kinds.)

Alkanes

The alkanes are also called the saturated hydrocarbons, because all the bonds that are not used to make the skeleton itself are filled to their capacity—saturated—with hydrogen atoms. They are also known as the paraffin hydrocarbons, from the Latin parum affinis, meaning "little affinity," because these compounds are not very chemically reactive.

The three smallest alkane molecules, containing one, two, and three carbon atoms, are shown in three ways.

The structural formulas are one way in which simple organic molecules can be depicted in two dimensions on paper; each line indicates a single covalent bond-a shared pair of electrons. The three-dimensional ball-and-stick models and space-filling models, in which the balls represent the carbon and hydrogen atoms (roughly to scale) and the sticks represent the bonds, are used by chemists to study the shapes of molecules.

The names and formulas of the first eight normal (not branched) alkanes are: Methane (CH4); Ethane (C2H6); Propane (C3H8); Butane (C4H10); Pentane (C5H12); Hexane (C6H14); Heptane (C7H16); Octane (C8H18).

While the first four alkanes were named before their structures were known, the rest have been named with Greek roots that tell how many carbon atoms there are in the chain: pent = five, hex = six, and so on, all ending in the "family name," -ane. The chemical formula of an alkane hydrocarbon can be obtained quickly from the number of carbon atoms, n, in its skeleton: the formula is CnH2n+2 . This method works because every carbon atom has two hydrogen atoms attached except for the two end carbon atoms which have two extra ones. As an example, the formula for pentane is C5H12.

The branched alkanes are named by telling what kinds of branches—methyl, ethyl or propyl groups, etc.—are attached to the main chain and where. For example,

is named 2-methyl pentane; the 2 indicates that the methyl group (-CH3) branches off the second carbon atom from the nearest end of the pentane chain.

The four lightest normal alkanes, having the smallest (lowest molecular weight ) molecules, are gases at room temperature and pressure , while the heavier ones are oily liquids, and still heavier ones are waxy solids. Alkanes, which are the major constituents of crude oil, do not mix with water and float on its surface. The wax that we call paraffin and make candles from is a mixture of alkanes containing between 22 and 27 carbon atoms per molecule.

All hydrocarbons burn in air to form carbon dioxide and water. Methane, CH4, as the major constituent of natural gas , is widely used as a heating fuel. Also known as marsh gas, methane occurs naturally in marshes and swamps, being produced by bacteria during the decomposition of plant and animal matter . It can form explosive mixtures with air, however, and is therefore a hazard when present in coal mines. On the positive side, bacteria-produced methane has prospects for being developed as a commercial source of fuel.

Propane, C3H8, and butane, C4H10, are compressed into tanks, where they liquefy and can be used as portable fuels for such applications as barbecue grills, mobile-home cooking, and disposable cigarette lighters. Because these compounds are pure and burn cleanly, they are being explored as fuels for non-polluting automobile engines. They are often referred to as LPG-liquefied petroleum gas.

Cycloalkanes are alkanes whose carbon atoms are joined in a closed loop to form a ring-shaped molecule. Cyclopropane which contains three carbon atoms per molecule has molecules that are in the shape of a three-membered ring, or triangle. Cyclohexane, with six carbon atoms, has hexagonal molecules; it is used as a good solvent for many organic compounds.

Alkenes

The alkenes, sometimes called olefins, are hydrocarbons that contain one or more double bonds per molecule. Their names are parallel to the names of the alkanes, except that the family ending is -ene, rather than -ane. Thus, the four smallest molecule alkenes containing two, three, four, and five carbon atoms are ethene (also called ethylene), propene (also called propylene), butene (also called butylene), and pentene. (There can be no "methene," because there must be at least two carbon atoms to form a double bond.) A number preceding the name indicates the location of the double bond by counting the carbon atoms from the nearest end of the chain. For example, 2-pentene is the five-carbon alkene with the structure

The locations of branches are similarly indicated by numbers. For example, 3-ethyl 2-pentene has the structure

The lightest three alkenes, ethylene, propylene, and butene, are gases at room temperature; from there on, they are liquids that boil at higher and higher temperatures. The chemical formula of an alkene containing only one double bond per molecule can be obtained from the number of carbon atoms in its molecules: if n is the number of carbon atoms, the formula is CnH2n. Thus, the formula for pentene is C5H10.

Alkenes are called unsaturated hydrocarbons; if there is more than one double bond in an alkene molecule it is said to be polyunsaturated. In principle, two more hydrogen atoms could be added to each double bond to "saturate" the compound, and in fact this does happen quite easily when hydrogen gas is added to an alkene in the presence of a catalyst. This process is called hydrogenation .

Other elements, such as the halogens and hydrogen halides, can also be added easily to the double bonds in alkenes. The resulting halogenated hydrocarbons are very useful but are often toxic or environmentally damaging. Trichloroethylene is a useful solvent, chlorinated hydrocarbons have been used as insecticides , and chlorofluorocarbons (CFCs or Freons) are used as refrigerants but have been shown to damage the earth's ozone layer.

Alkynes

Alkynes are hydrocarbons that contain one or more triple bonds per molecule. Their names are parallel to the names of the alkanes except that the family ending is -yne. Thus, the four smallest-molecule alkynes are ethyne (more usually called acetylene), propyne, butyne, and pentyne. Alkynes containing one triple bond have chemical formulas given by CnH2n-2, where n is the number of carbon atoms in the molecule. Thus, the formula for pentyne is C5H8. Acetylene, propyne, and butyne are gases at room temperature; the rest are liquids.

The most famous of the alkynes is the first member of the series: acetylene, C2H2. It forms explosive mixtures with air or oxygen , but when mixed with oxygen in a controlled way in an oxyacetylene torch it burns with a very hot flame—up to 6,332°F (3,500°C) which is hot enough to cut and weld steel . Because acetylene is explosive when compressed into liquid form, the tanks of acetylene that welders use contain acetylene dissolved in acetone .

Other important alkenes are styrene, C6H5-CH=CH2, from which the plastic polystyrene is made, and isoprene, CH2=C(CH3)CH=CH2, which is the monomer of natural rubber. (In this shorthand structural formula for isoprene, the parentheses indicate that the CH3 group within them is a branch attached to the preceding carbon atom.)

Aromatic hydrocarbons

An aromatic hydrocarbon is any hydrocarbon that contains one or more benzene rings in its molecule. The name "aromatic" is historical in origin, and does not at all imply that these compounds have pleasant aromas. Aromatic hydrocarbons are the basis of many aromatic compounds containing other atoms such as oxygen and nitrogen in addition to the carbon and hydrogen that are of extreme biological and industrial importance.

The simplest aromatic hydrocarbon is benzene itself, C6H6, whose molecule is a hexagonal ring of six CH groups. Various carbon-and-hydrogen groups can be substituted for any or all of the hydrogen atoms in benzene to form substituted benzenes. Benzene's own phenyl groups, C6H5, can bond to each other end to end, to form polycyclic (multiple-ring) hydrocarbons, or they can fuse together along the hexagons' sides to form condensed ring or fused ring hydrocarbons.

In this figure, the bonds leading to all the hydrogen atoms are omitted for simplicity as is the usual practice among chemists. Also, the benzene rings are drawn with alternating double and single bonds between the carbon atoms. In reality, however, resonance makes all the carbon-carbon bonds equal at an intermediate value between single and double. Chemists therefore usually draw the benzene ring simply as a hexagon with a circle inside:

The hexagon represents the six carbon atoms and their attached hydrogen atoms, while the circle represents all the bonding electrons as if they were everywhere in the molecule at once. In chemists' shorthand, then, naphthalene would be depicted as shown in the above figure.

Among the important substituted benzenes are methyl benzene, commonly known as toluene, and dimethyl benzene, commonly known as xylene. They are both powerful solvents for organic compounds and are used as starting materials for the synthesis of drugs, dyes, plastics , and explosives . Treatment of toluene with nitric and sulfuric acids produces the explosive trinitro-toluene, or TNT.

Among the important condensed ring aromatic hydrocarbons are naphthalene and anthracene whose molecules consist of two and three hexagonal benzene rings, respectively, fused together along one side.

Both are derived from coal tar and are used as starting materials for the synthesis of many useful compounds. Naphthalene is a crystalline solid with a strong, pungent odor; it is used as a moth repellant and a deodorant-disinfectant.

Petrochemicals

Our primary source of hydrocarbons is petroleum or crude oil, that thick, black liquid that we find in the earth . Petroleum consists almost entirely of a mixture of alkanes with some alkenes and smaller amounts of aromatic hydrocarbons. When petroleum is distilled at a series of different temperatures, the lowest molecular-weight hydrocarbons boil off at the lowest temperatures and the higher-molecular-weight ones boil off at successively higher temperatures. This process, called fractional distillation, is used to separate the complex mixture of compounds. The table shows the various hydrocarbon mixtures ("fractions") that distill off in various temperature ranges.

In addition to harvesting the hydrocarbons that occur naturally in petroleum, oil refineries use a variety of processes to convert some of them into other more desirable hydrocarbons.

A vast number of synthetic (man-made) organic chemicals, including drugs, plastics, paints, adhesives, fibers, detergents, synthetic rubber, and agricultural chemicals, owe their existence to petrochemicals: chemicals derived from petroleum. The top six petrochemicals produced in the United States are ethylene, propylene, benzene, xylene, butadiene (the four-carbon-atom alkene with two double bonds), and toluene. From these, hundreds of other chemicals are manufactured.

Gasoline

Probably the most important product of the fractional distillation of petroleum is gasoline, a mixture of alkanes containing six to ten carbon atoms in their molecules: hexane (C6H14), heptane (C7H16), octane (C8H18), nonane (C9H20), and decane (C10H22), plus small amounts of higher-molecular weight alkanes. More than six trillion gallons of gasoline are burned each year in the United States.

Gasoline must have certain properties in order to work well in automobile engines. If the gasoline-air mixture does not explode smoothly when ignited by the spark in the cylinder, that is, if it makes a fast, irregular explosion instead of a fast but gentle burn, then the explosive force will hit the piston too soon, while it is still trying to move down into the cylinder. This clash of illtimed forces jars the engine, producing a metallic clanking noise called a knock, which is especially audible when the engine is laboring to climb a hill. Extensive knocking can lead to serious engine damage, so gasolines are formulated to minimize this effect.

Of all the hydrocarbons that can be in gasoline, normal (straight-chain) heptane, C7H16, has been found to make auto engines knock worst. It has been assigned a value of zero on a scale of gasoline desirability. The hydrocarbon that knocks least is a branched-chain form of octane, C8H18, called iso-octane. It has been rated 100. Every gasoline blend is assigned an octane rating between zero and 100, according to how much knocking it produces under standard test conditions. Most automobile fuels sold have octane ratings above 85. High-octane gasolines that are even better than iso-octane because of anti-knock additives can have ratings above 100.

The C6 to C10 hydrocarbons make up only about 20-30% of crude oil, which is far from enough to supply the world's appetite for gasoline. But even if there were enough of it, the natural mixture has an octane rating of only about 40 to 60—not good enough for modern engines. Refineries therefore modify the natural mixture of molecules by breaking down big molecules into smaller ones (cracking) and by reshaping some of the smaller molecules into forms that knock less (reforming).

By the time gasolines get to the pump, they are no longer pure hydrocarbon mixtures; they have been blended with additives. Lead-containing antiknock compounds such as tetraethyl lead, Pb(C2H5)4, are no longer used because lead is a toxic air pollutant; methyl-tert-butyl ether (MBTE) is used instead. Other additives remove harmful engine deposits, prevent gum formation, inhibit rusting, prevent icing, clean the carburetor, lubricate the cylinders, and dye the gasoline distinctive colors for identification purposes.

See also Chemical bond; Formula, structural

Resources

books

Amend, John R., Bradford P. Mundy, and Melvin T. Armold. General, Organic and Biological Chemistry. Philadelphia: Saunders, 1990.

Jahn, F., M. Cook, and M. Graham. Hydrocarbon Exploration and Production. Developments in Petroleum Science. Vol. 46. The Netherlands: Elsevier Science, 2000.

Loudon, G. Mark. Organic Chemistry. Oxford: Oxford University Press, 2002.

Schobert, Harold H. The Chemistry of Hydrocarbon Fuels. Boston: Butterworth's, 1990.

Sherwood, Martin, and Christine Sutton. The Physical World. New York: Oxford University Press, 1991.

Robert L. Wolke

hydrocarbon Organic compound containing only carbon and hydrogen. There are thousands of different hydrocarbons, including open-chain compounds, such as the alkanes (paraffins), alkenes (olefins) and acetylenes. Petroleum, natural gas and coal tar are sources of hydrocarbons.

hy·dro·car·bon / ˈhīdrəˌkärbən/ • n. Chem. a compound of hydrogen and carbon, such as any of those that are the chief components of petroleum and natural gas.

hydrocarbon Naturally occurring organic compound containing carbon and hydrogen. Hydrocarbons may be gaseous, solid, or liquid, and include natural gas, bitumen, and petroleum.