Hydrocarbons (World of Earth Science)
Hydrocarbons are compounds composed solely of carbon and hydrogen. Despite their simple composition, hydrocarbons include a large number of different compounds with a variety of chemical properties. Hydrocarbons are derived from oil deposits, and are the source of gasoline, heating oil, and other "fossil fuels." Found in a variety of geological settings, hydrocarbons provide the carbon skeletons required for the thousands of chemicals produced by the chemical industry.
Hydrocarbons are classified on the basis of their structure and bonding. The three major classes are aliphatics, alicyclics, and aromatics. Aliphatics have carbon backbones that form straight or branched chains, with no rings. Alicyclics are ring compounds that, while they may have one or more double bonds, do not form conjugated sets of double bonds around the ring like benzene. Aromatics are compounds with at least one benzene ring. Aliphatic means fatty, and aromatic refers to odor, but these terms no longer have significance for the compounds they describe.
Because hydrocarbons are nonpolar, they are generally insoluble in water, and dissolve in nonpolar solvents.
The aliphatic hydrocarbons are further divided into alkanes, alkenes, and alkynes. Alkanes (sometimes called paraffins) have only single bonds, while alkenes (sometimes called olefins) have a carbon-carbon double bond, and alkynes have a carbon-carbon triple bond. Compounds with two double bonds are known as dienes. Compounds with double or triple bonds are referred to as "unsaturated," while those without are "saturated," meaning all of their carbons are bonded to the maximum number of hydrogens.
All alkanes have the general formula CnH2n+2, where n is the number of carbon atoms in the compound. Alkenes have the formula CnH2n, while alkynes are CnH2n. Because of these regularities, the members of each group are known as a homologous series.
The simplest alkane (and indeed, the simplest hydrocarbon) is methane, CH4. The four C-H bonds are directed towards the four corners of a tetrahedron, with carbon at its center and hydrogens at each vertex. The C-H bonds are slightly polar, and equivalent in length and strength. The angle formed by any pair of bonds is 109.5 degrees, the tetrahedral angle.
Despite the bond polarity, there is no net dipole because of the symmetry of the molecule, and methane has very weak intermolecular attractions, consisting only of van der Waals attractions. These attractions are proportional to surface area, which is small for the compact methane molecule and, as a result, methane has a very low melting point and boiling point, 97.4°F (83°C) and 58.7°F (61.5°C), respectively.
Methane is found in oil deposits, forming the majority of the "natural gas" fraction. Methane is also formed by certain anaerobic bacteria, especially in swamp bottoms, where it bubbles to the surface as "marsh gas." It is used in large quantities as fuel for heating and cooking because it burns with oxygen to produce carbon dioxide and water. Methane burns cleanly, with very little soot or smoke.
The next alkane is ethane, C2H6. Its higher melting and boiling points (77.6°F [72°C] and 27.3°F [8.5°C], respectively) reflect its larger surface area and consequently greater van der Waals attraction.
Propane, C3H8, is an important fuel because it can be liquified at pressures low enough to easily maintain in commercial and consumer apparatus, allowing easy transport and storage, but vaporizes to burn almost as cleanly as methane.
Higher alkanes continue the trend of increased surface area and higher melting and boiling points. However, as the number of carbon atoms increases, structural isomerism becomes possible, allowing the same molecular formula to describe two or more compounds with different structures and different physical properties. For example, Butane (C4H10) can be either a straight-chain molecule, or a branched one, with three carbons in a straight chain and the fourth branching off from the middle carbon. Compounds with the same molecular formula but with different three-dimensional structures are called isomers. The straight chain isomer is called n-butane (for "normal"), while the branched one is called either iso-butane, or 2-methyl propane. This latter name indicates that the longest straight-chain backbone within the molecule has three carbons (propane), and there is a single-carbon branch (methyl) at the second position in the main chain. The extended structure of n-butane gives it a boiling point of 32°F (0°C), while the more spherical iso-butane boils twelve degrees lower, at 10.4°F (2°C), due to its smaller surface area. As the number of carbons increase, so too does the number of possible isomers.
The simplest alkene is ethene, C2H4, also called ethylene. Ethene is a planar molecule, with the four hydrogens splayed out in a plane and angles between bonds of 120 degrees. Ethene melts at 72.2°F (69°C), and boils at 51.6°F (02°C).
Higher alkenes take their names from the corresponding alkanes: propene, butene, pentene, etc. However, beginning with butene, isomerism is possible based on the position of the double bond. If the bond is between C1 and C2, the compound is 1-butene, while if it lies between C2 and C3, it is 2-butene (the compound with a double bond between C3 and C4 is equivalent to 1-butene). In addition, the non-rotation of the double bond means there are two structural isomers of 2-butene, one with the two terminal carbons on the same side of the C=C long axis, termed cis-2-butene, and one with them on opposite sides, trans-2-butene. As might be expected, the number of possible isomers rises with an increase in number of carbons.
The chemical reactions of the alkenes is more complex and richer than that of the alkanes, due to the presence of the double bond. The most common reaction is of addition across the double bond. For instance, addition of water to ethene creates ethyl alcohol. Alkenes are often prepared by the reverse of this reaction, dehydration across the double bond.
Alkynes have a triple bond. The simplest alkyne is C2H2, athyne, also called acetylene. Acetylene is an important high-temperature fuel used especially for metal cutting.
Cyclic alkenes, such as cyclohexene (C6H10), are common solvents, and also serve as starting points for a number of organic syntheses.
Benzene (C6H6) is a cyclohexane (the carbons are bonded in a ring) with three double bonds alternating around the ring. The electrons in benzene's bonds are highly delocalized, to the point that it is no longer accurate to describe them as belonging to individual carbons. Instead, they form a cloud "ring" above and below the plane of the carbons and, as a result, all the carbon-carbon bonds are of equivalent length and strength.
See also Atomic theory; Fuels and fuel chemistry; Geochemistry; Petroleum detection; Petroleum, economic uses of; Petroleum extraction; Petroleum, history of exploration; Petroleum