Hydrocarbons

Type of physical science: Chemistry

Field of study: Chemical compounds

Chemicals consisting of carbon chains of various lengths with hydrogens attached are called hydrocarbons. Hydrocarbons are sources of chemical energy and include many compounds.

Overview

Carbon is unique because it forms the basis for millions of different chemicals, both natural and synthetic. The study of all carbon chemistry and its derivatives is called organic chemistry. The subgroup of hydrocarbons is the backbone of organic chemistry. Furthermore, the ability of carbon to form four strong bonds is the reason that hydrocarbons hold this position of importance.

In theory, an atom can form as many covalent bonds as it has orbitals that are not filled.

Carbon has four half-filled orbitals in its outer shell if it gains some energy, and each orbital has one electron. By contributing each of these four outer electrons to bonds in which the other atoms likewise share, carbon can attain a chemically stable configuration (such as an inert gas, which has its outer orbitals filled). This natural ability to form four strong bonds makes carbon a very valuable building block. Carbon atoms can bond with other carbon atoms to form chains, branches, and even rings. The result is an endless array of possible molecules.

The simplest hydrocarbon is the gas methane. As usual, carbon's outer electrons cause it to form four bonds. The formula for methane is CH₄ (one carbon with four hydrogens), but this fails to indicate its structure. The carbon is at the center with four hydrogens separately attached to it. The hydrogens are as far apart from one another as possible and so, in three dimensions, the angle formed by any two hydrogens and the carbon is 109.5 degrees. Methane is odorless, nontoxic, and flammable, and bacteria can produce it by digesting organic molecules. Methane is called swamp gas or marsh gas because it may be formed from decaying vegetation. The gas is the main molecule found in natural gas deposits trapped by certain underground rock formations.

The next hydrocarbon involves two carbons. If two carbons are bonded together with a single bond (and all other bonds attach to hydrogen), then ethane is formed. The formula for ethane is C₂H₆. Each carbon has three hydrogens attached to it and then uses the remaining bond to join with the other carbon.

If there are three single-bonded carbons with appropriate hydrogens attached, then the molecule is called propane. A chain of four single-bonded carbons, again with hydrogens attached to all the remaining bonds, is called butane. Propane (C₃H₈) and butane (C₄H₁₀) are normally gases but easily become liquids with an increase in pressure. Pressurized bottles and tanks of these liquefied hydrocarbons are sold for camping, outdoor cooking, and also to rural homes that are not near city gas lines. As the hydrocarbon molecules are released from their containers, they return to gases and mix with the air for burning in lamps or stoves. The larger a molecule gets, the more it is attracted to others around it. This causes the smaller propane to be more useful as a bottled gas in colder areas of the world. Butane stays liquid at 0 degrees Celsius.

Propane, however, is a gas until the temperature drops to -43 degrees Celsius, which allows its use in colder situations.

One could continue by adding carbon (methyl) units one by one. The next hydrocarbon would be pentane; the formula is C₅H₁₂. Hexane would follow with a formula of C₆H₁₄, and so on. It is well to note, however, that as the number of carbons increases beyond three, the number of possible arrangements of the carbons also increases. There are two ways to put four carbons together. They can be bonded "normally," one after the other in a chain: C-C-C-C, or they can be attached so that three of the carbons are attached to one central carbon. The formula for butane is still C₄H₁₀, but the properties of the two forms are different. Molecular form affects chemistry.

Therefore, "normal" or n-butane and isobutane are really different compounds. This is called structural isomerism. Each different C₄ arrangement is an isomer.

As the number of carbons increases, the number of possible isomers accelerates.

Pentane (C₅H₁₂) has three different isomers. C₉H₂₀ has thirty-five different arrangements on paper; and reassuringly, thirty-five different substances with the formula C₉H₂₀ have actually been identified in the laboratory.

Another variation can occur. Carbons do not always bond once with one another. They may bond twice or even three times. This reduces the number of hydrogens that can be attached.

The properties of the molecules are also different. An example is C₂H₄, called ethylene. The "-ene" ending indicates a double bond. The carbons may also be triple bonded, which allows only two hydrogens in the molecule. Therefore, the formula for acetylene is C₂H₂.

Molecules that have fewer hydrogens because of their multiple bonds are called "unsaturated."

Food labels use this terminology when indicating amounts of saturated and unsaturated fats.

(Note that, according to the rules where "ethyl" equals two carbons and "-yne" indicates that there is a triple bond, acetylene should be called "ethyne," but sometimes a common name is well established before the rules take over.) Acetylene torches are well known for their hot flames, which can be used to weld or cut steel. This high energy comes from the relatively easy breaking of the three bonds between the two carbons.

Double and triple bonds cause the molecule to become rigid. If two hands touch each other by only one finger, then it is possible to rotate the hands while still maintaining contact.

For example, while using one of the longer fingers for bonds, thumbs can be moved from positions of both up or one up and one down. If, however, two or three fingers are in contact, then no rotation is possible. If the thumbs are both up, then they must stay there; if one is up and one is down, then this condition cannot change. With hydrocarbons, this situation becomes important for the parts that surround a double bond. Allow the thumbs to represent the rest of the carbon chain instead of single hydrogens. If both carbon chains are on the same side, then the molecule is called cis. If they face in opposite directions, then the molecule is called trans. These types of differences are called geometrical isomers and also result in different properties.

Another variation is ring formation. Propane is able to connect its three carbons into a ring. With single bonds, this leaves two sites to attach hydrogens on each carbon. The resulting formula is C₃H₆ and the chemical is called cyclopropane. There are other cyclic compounds, and some have multiple bonds connecting carbons. The most important of these cyclic compounds is benzene. Benzene contains alternating double and single bonds. Its six carbons form a ring.

Other atoms can attach to the ring and the number of possible compounds is endless.

The basic idea of polymer chemistry is that units may repeat almost endlessly. Also, the molecules can be cross-linked with one another and affect the overall flexibility. Rubber is such a hydrocarbon with the formula (C₅H₈)n, where n is a very large number. The repeating and cross-linked C₅H₈ monomers form what is called a macromolecule. Polymers can be recognized by the prefix on their names. Polyethylene, polystyrene, and polypropylene are among the hydrocarbons. Many of the other common polymers involve additional atoms such as nitrogen, oxygen, chlorine, and fluorine, but carbon and hydrogen are the backbones; examples include polyurethane, polyester, polyvinyl chloride (PVC), and acrylics.

With so many different molecules to study, scientists had to develop rules for naming them. The longest chain of carbons determines the basic name of the molecule. Therefore, the "octane" (or isooctane) in the mixture called gasoline is, by the new rules, actually a pentane; while it has eight carbons, they are attached so that the longest chain is only five carbons long.

Consider one of the possible molecules made out of nine carbons, where the longest chain is seven. To name it according to the rules of the International Union of Pure and Applied Chemists (IUPAC), the carbons of the longest chain are numbered so that the positions of any attachments to that chain can be identified. The chain is numbered in the direction that produces the lowest sum. If the two carbons branch off at the second and fourth carbons, the molecule will be named, 2,4-dimethylheptane. The "2,4" gives the location of the methyl groups (carbons with three hydrogens). "Di" indicates that there are two methyls. "Hept" indicates that the longest chain is seven carbons long, and "-ane" indicates that all the bonds are single.

Applications

The properties of hydrocarbons vary greatly. Physically, the various compounds can be solid, liquid, or gas at standard temperature (0 degrees Celsius) and standard pressure (760 millimeters on a mercury barometer). The smallest hydrocarbons are gases. At five carbons long (pentane) through fifteen carbons long, the hydrocarbons are liquids. After sixteen carbons, they are solids. Viscosity (resistance to flow or thickness) of liquid hydrocarbons depends on how likely the molecules are to get tangled up as they move.

Chemically, hydrocarbons have as many properties and uses as there are compounds.

One generalization about hydrocarbons is found in their burning. Energy is released as heat and light. Complete combustion causes the hydrogen to be oxidized to water and the carbon to become carbon dioxide.

Oil wells contain a mixture of hydrocarbons that range from gaseous methane to solid paraffin. The mixture is called crude oil or petroleum, which literally means "rock oil."

Petroleum's origin is uncertain, but it is apparently the liquid remains of marine organisms decomposed by bacteria over long periods of time. Petroleum can be refined by heating the mixture to the gaseous state and then passing the vapors into a column still. This still, which is steam-heated, maintains progressively cooler temperatures as the crude oil vapors rise through it.

The heavier molecules with the higher boiling points condense before others and are removed.

Lubricating oils, greases, asphalt, and waxes condense at the bottom of the still, which is at about 350 degrees Celsius. Heating oil is then condensed at the next level up, which is at about 300 degrees Celsius. Next, kerosene and gasoline become liquids. The lightest molecules collect at the top.

Every fraction has its uses. Modern machinery requires various lubricating oils to reduce heat. About 4 percent of the crude is asphalt and road oil and is used to surface 90 percent of the roads in the United States. Many homes are heated with oil. Kerosene is still used for cooking and lighting.

Society has become dependent on hydrocarbons, especially for energy. Gasoline has become the chief product of petroleum. It is a mixture of about twenty-five hydrocarbons ranging from C₄H₁₀ to C₁₃H₂₈, including smooth-burning isooctane. Larger molecules in petroleum can be "cracked" or broken by using a combination of heat and pressure with a catalyst to produce more gasoline-sized molecules. The smooth-burning quality of gasoline is determined by comparing it to standard solutions that vary in amount of octane. Any gasoline mixture that runs in an engine (without premature explosions called "knocking") as well as a 90 percent octane, 10 percent heptane is given an octane rating of 90.

Methane, ethane, and other smaller hydrocarbons can also be found in wells. The gases are often under sufficient pressure to cause the liquids to gush out of the well when oil is "struck." This natural gas is used for cooking and to heat homes.

Another source of carbon molecules is coal. Coal is a mixture of carbon molecules, which formed in the past from compressed, decomposed vegetation. Soft coal can be heated in the absence of oxygen, and the gases produced can be condensed as coal tar, which contains many hydrocarbons.

Petrochemical plants use hydrocarbons from coal tar and petroleum as the starting points for many modern products. Polymers have changed many things with new products, including synthetic fibers and cloth, food wraps, emulsion paints, plastic boats and housewares, advertising signs, toys, radios, automobile parts, electrical plugs, flexible tubing, polyvinyl chloride plumbing pipes, and varnishes. Poly-cis-isopene, natural rubber, once harvested from trees, can now be manufactured. Also, many drugs and agricultural chemicals often start as hydrocarbons.

Context

Modern society relies on hydrocarbons. In 1854, when investors determined that "rock oil" coming out of the ground in Pennsylvania could be refined into a fuel for household lamps, people began to depend on hydrocarbons. Previously, artificial light had been provided by burning animal and vegetable fats. Distillates of coal (town gas) and turpentine were used.

High-quality lighting oil from the sperm whale was at an impressive price of $2.50 per gallon.

An inexpensive and reliable lighting fuel was needed. Also, the advances in machinery called for better lubrication to reduce friction and temperature.

By the summer of 1859, the first oil well was drilled. Kerosene was refined and sold; fortunes were made. Oil was to become the world's largest industry. Soon, naphtha, gasoline, fuel oil, and lubricants were refined. At first, the gasoline was used as a solvent and was sold as "stove naphtha" for cooking stoves. Petroleum jelly and paraffin followed. In 1882, Thomas Alva Edison's electric light bulb began cutting into the hydrocarbon market. Gasoline-driven automobiles appeared, and railroads and ships began to switch from coal to oil; demand grew. By 1913, the cracking process was used to produce more gasoline for the growing number of automobiles. World War I established the importance of petroleum in waging wars. In the 1920's, the technique of using an explosion to cause seismic waves in the earth was developed as an aid to locate oil. In 1950, the use of computers to analyze seismic data was explored. By the late 1970's, the United States petroleum industry was using more computer power than any other private industry in the world, with the major part of that power involved in seismic data processing.

The benefits of hydrocarbons have not been without problems. In the early 1970's, oil companies in the United States and other countries had difficulty supplying sufficient amounts of gasoline. Highway speeds were reduced to conserve gasoline. Oil companies improved methods of removing oil from wells. Methyl alcohol mixed with gasoline (gasohol) was made increasingly available. In 1991, a brief war was fought against Iraq to prevent its domination of the Persian Gulf oil supply.

Furthermore, the exhaust fumes from automobiles cause as much as 80 percent of all carbon monoxide, 66 percent of all hydrocarbons, and 50 percent of all nitrogen oxide emissions.

Oil spills have damaged the environment. Increasing amounts of carbon dioxide in the atmosphere from burning fossil fuels may be warming the planet. The consequences could be serious if the average temperature on Earth increases by only one or two degrees.

Finally, supplies of oil and coal are ultimately limited and should be used wisely.

Estimates vary, but at early 1990's rates, the earth could virtually run out of oil and coal within one hundred to two hundred years. Clearly, the hydrocarbon era is going to end, and alternate sources of energy will need to be developed.

Principal terms

ATOM: the smallest particle of an element that still has the properties of that element

BOND: an orbital overlap between atoms that allows the sharing of electrons, thus creating a stable "filled" condition

CARBON: an element that forms the backbone of hydrocarbon molecules; an atom of carbon can form four bonds with other atoms

ELECTRON: a negative particle; in an atom, the electrons are arranged in shells around the nucleus, with the outermost electrons forming bonds and thus determining the chemistry of the atom

HYDROGEN: an element whose atom forms one bond

ISOMER: a molecule with an alternate arrangement of the same atoms

MOLECULE: the smallest unit of a substance with the properties of that substance

MONOMER: the basic unit from which a polymer is built

ORBITAL: the space around the nucleus in which the electron is most likely to be found; an orbital with two electrons is filled

POLYMER: a molecule that is composed of repeating units

Bibliography

Asimov, Isaac. THE WORLD OF CARBON. London: Abelard-Schuman, 1958. Although dated, this book is still valuable. Asimov explains the basics of organic chemistry well and gives interesting examples. His writing style, with its short and crisp sentences, is effective.

Atkins, P. W. MOLECULES. New York: Scientific American Library, 1987. A tour of common molecules that assumes no prior knowledge of chemistry. Atkins effectively uses drawings of space-filling models of the molecules in combination with structural formulas. Outstanding discussion. Glossary.

National Science Board. ONLY ONE SCIENCE: TWELFTH ANNUAL REPORT OF THE NATIONAL SCIENCE BOARD. Washington, D.C.: Author, 1981. This report contains several very readable essays relating to hydrocarbons among other subjects. The hydrocarbon articles include: "The Seismic Exploration for Oil and Gas," "Pesticides and Pest Control," and "Synthetic Fibers." The seismic exploration essay is best. Well illustrated, with old photographs and line drawings. Seismic data are shown.

Pauling, Linus, and Roger Hayward. THE ARCHITECTURE OF MOLECULES. San Francisco: W. H. Freeman, 1964. An outstanding book with beautiful illustrations of the structures of various molecules. Of particular interest are the full-page drawings and discussions of many of the hydrocarbons mentioned in this article. Nobel laureate Linus Pauling, discoverer of the α helix structure of protein, provides the background on each molecule.

Salem, Lionel. MARVELS OF THE MOLECULE. New York: VCH, 1987. A nice introduction to molecules and bonding that demystifies modern chemistry. Salem avoids heavier terms in the text when possible but provides a special glossary to allow translation back into standard chemical language. A well-illustrated book about the interesting properties of many different molecules. Discusses what the color of an object tells one about its bonds.

Tarbell, Dean Stanley, and Tracy Tarbell, eds. ESSAYS ON THE HISTORY OF ORGANIC CHEMISTRY IN THE UNITED STATES, 1875-1955. Nashville, Tenn.: Folio, 1986. A collection of appraisals of the growth of organic chemistry in the United States. This is a rather unpretentious book for those with an interest in the history of science and its ideas. It will reward those who are fascinated by the story of discovery and the effects of the personalities involved in science.

Yergin, Daniel. THE PRIZE. New York: Simon & Schuster, 1991. This fascinating book tells the history of how people have come to depend on oil. The interactions of the international politics, armed conflicts, big business, effects on society and technology are discussed in an engaging style. A must for understanding the context of the use of oil.

Molecular structure of methane

Isomers of butane

A double bond

Chemical Bond Angles and Lengths

Group IV Elements

Liquefaction of Gases