The Element of Life, Coal, and Diamonds : Carbon

Carbon—element number 6—is the most versatile element in the periodic table. The singular ability of carbon atoms to form an incredibly diverse number of compounds is due to the ability of carbon atoms to form strong covalent bonds with other carbon atoms and to do so such that long chains and rings of atoms can form. Because the molecules that comprise living organisms—sugars, starches, fats, oils, proteins, and nucleic acids—are based on carbon, carbon is the “element of life,” and substances that contain at least carbon and hydrogen are called organic compounds. In fact, scientists do not know of any living organisms that do not contain carbon. In addition to the molecules of life, carbon has an important role in inorganic chemistry. Coal and diamond both represent elemental forms (allotropes) of carbon, and car-bon dioxide is one of the most important gases in Earth’s atmosphere.

THE ASTROPHYSICS OF CARBON

A young star fuses hydrogen into helium—a process called “hydrogen burning”—in its core until the only hydrogen left is in the outer shell of the star. At that time, the helium core begins to collapse under its own weight while hydrogen burning continues in the outer shell. Enormous amounts of heat and light are generated by the energy of the collapse, generating radiation pressure that makes the outer shell expand explosively while the core continues to contract. This is the red-giant phase of a star, so called because the expanding gas in the outer shell filters out all but the star’s red wavelengths.

The collapsing core meanwhile becomes denser and hotter until helium is able to fuse into carbon via the triple-alpha process (helium burning). Initially two helium nuclei, or alpha particles, fuse to form an unstable and very short-lived state of beryllium. Despite the brevity of its existence (about a tenth of a femtosecond), there is a chance for some of these ⁸Be nuclei to capture alpha particles to form 12C, the basis of life as it is known. The star now begins to form a central core of carbon. These reactions can be written as the following equations:

₂⁴He + ₂⁴He → ₄⁸Be

₄⁸Be + ₂⁴He → ₄¹²C + γ

where “γ” is a high-energy photon or “gamma ray.”

The helium burning in turn produces radiation pressure around the core, so there is no further collapse until all the helium is used up. The conditions are also suitable for some carbon atoms to capture alpha particles to produce ¹⁶O, so the red-giant stellar phase comprises helium, carbon, and oxygen nuclei. The carbon/oxygen ratio is of compelling interest in astrophysics because the subsequent heavy-ion burning stages rely heavily on this relative abundance. A clearer understanding of the alpha-capture process at this stage of stellar development would advance knowledge of what percentage of stars should progress to white dwarfs, red supergiants, or supernovae.

After all the helium is consumed at the core, without radiation pressure for support, a star will once again begin to collapse under its own weight. At sufficient pressure and temperature, carbon burning can take place, but only if the star’s weight is enough to effect extreme density (2 × 10⁸ kg/m³). For stars at least four times as massive as the Sun, car-bon fusions produce—with varying probabilities—neon, sodium, and magnesium.

EARTHBOUND: FROM COAL TO DIAMONDS

One of the most common elements on Earth, carbon comes in diverse forms, including graphite, amorphous carbon, diamond, and fullerene. The different forms, or allotropes, are distinguished by different bonding modes. All carbon deposits developed over millions of years from compaction of formerly living plant and animal cells. For that reason, coal and some of its by-products have earned the name fossil fuels.

Graphite, the purest form of coal, has been mined in this form in only one location—a large deposit in Cumbria, England. It is usually found with an admixture of other minerals such as quartz or mica, with the largest producers being China and India. Madagascar has become an important producer of large-flake graphite, some with an intriguing rhombohedral phase. The mining there, however, is increasingly becoming a threat to a unique biodiversity.

High-grade common coal, often extracted from anthracite, began its formation in swamp and wetland systems where oxidation rates were low, allowing large sedimentary and metamorphic deposits to form worldwide. Coal is easily extractable and easily transported, making it a popular choice for use as a fuel in electric power plants. The risks associated with mining procedures, however, have earned it the reputation as the deadliest source of power in history, and coal power plant emissions are a serious global warming hazard.

The hardest of carbon’s allotropes is the coveted and rare diamond, found famously in Africa—particularly South Africa, Botswana, and Angola—but also in Russia, India, Australia, and Canada. The United States boasts only one source of this gemstone: Crater of Diamonds State Park in Arkansas. Diamond mines are generally situated near a volcanic pipe, as the densely packed carbon, formed under extreme pressure in the depths of the Earth (more than 93 miles [ca. 150 km] beneath the surface), must be brought up by volcanic activity. Though fairly brittle, diamond is ideal for use in jewelry because of its high index of refraction, which allows for bending of light at angles unreachable by other stones. (Glass has an index of refraction about two-thirds that of diamond.)

Fullerene, a hollow-core allotrope of carbon, was first seen in 1992 in a sample of metamorphic rock from northwestern Russia, though it had been produced a few years earlier in university laboratories. Con-figurations range from soccer-ball shapes and icosahedrons to nanotubes and nanofibers.

Other recently discovered allotropes include a nanofoam—a web of light magnetic carbon clusters, lonsdaleite—a sort of disfigured hex-agonal diamond lattice, and an aggregated diamond nanorod.

DISCOVERY AND NAMING OF CARBON

Coal, charcoal, graphite, and diamonds were all known in prehistoric times, but they probably were not recognized as all being forms of the same element—carbon. Ancient civilizations were most likely to use charcoal as a source of fuel, and less likely to use carbon in its other forms. Even today, charcoal is a common fuel in various parts of the world. The name itself—carbon—is derived from carbo, the Latin word for charcoal.

The structures of (A) diamond, (B) graphite, and (C) buckminsterfullerene.

It was not until the beginning of modern chemistry that chemists began to recognize carbon’s varied forms. The biggest dilemma occurred with diamond, since its appearance is significantly different from the appearances of coal, charcoal, or graphite, and diamond is exceptionally harder than the others are. In 1694, chemists discovered that sunlight focused in by a large magnifying glass causes a diamond to disappear. In 1771, a diamond was heated and shown to burn completely without leaving any ash. It was not until 1796, however, that Smithson Tennant, an English chemist, demonstrated that when a diamond was burned, the only product was carbon dioxide, indicating that the diamond itself was a form of carbon.

Carbon mainly is found in the form of hydrocarbons—natural gas, oil, and coal—and carbonate-containing minerals such as limestone (CaCO3). Carbon also exists in the form of carbon dioxide, which makes up 0.0335 percent of Earth’s atmosphere.

There are three naturally occurring isotopes of carbon: carbon 12, which makes up 98.9 percent of all the carbon on Earth, carbon 13, which makes up 1.1 percent of the carbon, and carbon 14, which is radioactive (half-life = 5,730 years) and exists in only trace amounts. Carbon 11 can be produced artificially. Although it is radioactive with a half-life of only 20 minutes, it is an effective agent of medical diagnosis.

Another class of carbon allotropes was discovered in 1985 by Harold W. Kroto, James R. Heath, Sean O’Brien, Robert Curl, and Richard Smalley. Soccer-ball-shaped spheres of 60 carbon atoms with formulas like C₆₀ and C₇₀ were found in carbon soot and later recognized to be ubiquitous in interstellar clouds. C₆₀ is recognized as the most perfectly spherical known molecule. Because the arrangements of the car-bon atoms resemble the architecture of geodesic domes, which were invented by Richard Buckminster Fuller, this class of carbon allotropes came to be called fullerenes. Kroto, Curl, and Smalley shared the 1996 Nobel Prize in chemistry for this discovery.

A newly discovered form of carbon called graphene exhibits high thermal conductivity and an unprecedented electron mobility: Electrons in graphene move practically uninhibited by the atomic lattice. First symbolized in 2004 by researchers at the University of Manchester in England, this material—whose configuration resembles chicken wire, as would a single layer of graphite—is also extremely strong. Obvious applications in electronic circuits, which rely on electron mobility for signal speed, have made the study of graphene priority research in universities around the world.

Coal is usually found with an admixture of other minerals, such as quartz or mica, though it seldom resembles diamond. The largest producers in the world are China and India.

THE CHEMISTRY OF CARBON

Chemical compounds are divided into two major groups: organic com-pounds and inorganic compounds, classifications assigned by the Swedish chemist Jöns Jakob Berzelius in 1807. Organic compounds were said to be those substances that are derived from living organisms—plants or animals. All other substances were said to be inorganic and would be derived from minerals. This distinction between organic and inorganic

The 1996 Nobel Prize in chemistry was awarded to Robert Curl (center), Richard Smalley (right), and Sir Harold Kroto (left) for their discovery of carbon fullerenes. (AP Photo/Soren Andersson)

substances seemed reasonable at the time. It was supported by the general observations that organic substances can be converted fairly easily into inorganic substances (such as carbon dioxide and water), but no one had observed the conversion of inorganic substances into organic substances.

The situation changed in 1827, when German chemist Friedrich Wöhler synthesized urea, one of the body’s waste products in urine, by heating ammonium cyanate, an inorganic compound. At first Wöhler could not believe the results. He repeated the experiment several times just to make sure he had not made a mistake. Finally, in 1828, he published his findings and surprised the chemical world.

Today chemists no longer classify organic versus inorganic com-pounds based on how they are derived. The current textbook definition of an organic compound is a pure substance that contains carbon and hydrogen and possibly other elements. In contrast, inorganic com-pounds are all the remaining pure substances (other than simple elements) that occur either naturally or artificially.

Carbon atoms readily form covalent bonds with other carbon atoms and with atoms of other nonmetals, especially hydrogen, nitro-gen, oxygen, phosphorus, sulfur, and the halogens. Carbon atoms form these bonds by sharing pairs of electrons with atoms of other elements. When two atoms share two electrons, the bond is called a single bond (symbolized in a structural formula by a single dash “–”). When four electrons are shared, the bond is called a double bond (symbolized by a double dash “=”). When six electrons are shared, the bond is called a triple bond (symbolized by a triple dash “<≡>”). A carbon atom will form enough bonds with other atoms so that a total of eight electrons is almost always shared.

To share a total of eight electrons with other atoms, a carbon atom may exhibit more than one type of bonding. 

The possibilities are the following: 

(1) form four single bonds, as in methane, CH4. 

(2) form a double bond and two single bonds, as in formaldehyde, H2CO. 

(3) form two double bonds, as in carbon dioxide, CO2.

(4) form a triple bond and a single bond, as in hydrogen cyanide, HCN.

When carbon atoms bond primarily to atoms like hydrogen or oxy-gen, small molecules like methane (CH4) or carbon dioxide (CO2) are possible. Alternatively, when carbon atoms bond to other carbon atoms, long chains of atoms are possible. Carbon chains also sometimes loop back on themselves and form rings. Different groups of carbon com-pounds are classified according to their structures as chains, as rings, or as compounds containing other elements besides carbon and hydro-gen—oxygen and nitrogen being the two most important other elements that may be part of organic compounds. What kinds of bonds form will determine the functional group to which that compound belongs and thus determine the compound’s chemical and physical properties. Examples of different kinds of bonds and the functional groups they represent are given in the following table.

Common Functional Groups of Carbon
Group	Elements	Bonds	Examples
Alkanes	C, H	All Single	C8H18 Octane, a component of gasoline
Alkenes	C, H	C=C Double Bond	H2C=CH2 Ethylene, a raw ingredient for common plastics
Alkynes	C, H	C≡C Triple Bond	HC = Acetylene, used in welding
Alcohols	C, H, O	All Single	H3C-CH2OH Ethanol, Alcoholic Beverages
Ethers	C, H, O	All Single	H3C-CH2-O-OH2-CH3 Diethyl ether, anesthetic
Aldehyde	C, H, O	C=C Double Bond	H2C=O Formaldehyde, preservative
Ketone	C, H, O	C=C Double Bond	O             || H3C – C – CH3 Acetone, paint thinner
Carboxylic Acids	C, H, O	C=C Double Bond & C-O Single Bond	H           O                  |         //           H – C – C                  |       |                 H     O – H Acetic Acid, component of vinegar
Amines	C, H, N	All Single	CH3-NH -CH3 Dimethyl amine, tanning agent

Hydrocarbons are classified according to whether the carbon atoms are linked by all single bonds (the alkanes), or whether a double bond (the alkenes) or triple bond (the alkynes) is present. Chains can also be classified as straight chains, in which no branching occurs, or as branched chains, in which there are side chains to the main chain of carbon atoms.

Hydrocarbons and related compounds can also exist in rings, in which case they are called cyclic compounds. An important example of a cyclic compound is benzene. The benzene ring is relatively stable, making benzene fairly nonreactive. Benzene is an important solvent in the chemical industry. Compounds derived from benzene are used as gasoline additives to boost performance.

Synthetic plastics, fibers, and elastomers (rubber) are major products of the chemical industry. All of these substances consist of very large carbon chains called macromolecules or polymers, which may have hundreds, or even thousands, of carbon atoms in them. Most synthetic polymers are manufactured from the raw ingredients found in natural gas or petroleum.

Molecules of alcohols and ethers contain an oxygen atom; both kinds of compounds have the general molecular formula CnH2n+2O. (For example, if n = 4, then the formula would be C4H10+2O.) Therefore, alcohols and ethers containing the same number of carbon atoms are isomers of each other, even though they have significantly different properties. Ethanol and dimethyl ether both have the molecular formula C2H6O but an important difference in their structural formulas.

The most common ether is diethyl ether. Because of diethyl ether’s volatility, it is commonly used as a starting fluid in motor vehicles. In the 19th century, diethyl ether was used as an anesthetic. Its low flash point, however, made it hazardous to use in the presence of oxygen and any flames or other ignition sources.

Ethanol is typically made from plant sugars, such as the glucose or maltose found in grains, through the process of fermentation in which the following reaction occurs in the presence of yeast:

C6H12O6 (s) → 2 C2H5OH (l) + 2 CO2 (g).

Ethanol is the alcohol present in all alcoholic beverages. (Any other alcohol would be far too toxic for human consumption. Even ethanol is toxic, hence the term intoxicated.)

The structure of benzene

Molecules of aldehydes and ketones also contain an oxygen atom, but in this case, the oxygen atom and a carbon atom are connected together with a double bond (see table above). A carbon-oxygen double bond is depicted in a structural formula as “C = O.” The general formula for an aldehyde or ketone is CnH2nO. An aldehyde has its oxygen atom attached to a carbon atom at the end of the hydrocarbon chain, and a ketone has its oxygen atom attached to a carbon atom that is not at the end of the chain. Both compounds have good solvent properties.

The structures of ethanol and dimethyl ether

The structure of diethyl ether

Esters and carboxylic acids contain two oxygen atoms—one with a single bond to a carbon atom, and the other with a double bond to the same carbon atom. Many esters occur naturally, are good nonpolar solvents, and tend to be fragrant, for which purpose they are often used in household products. Ethyl acetate, for example, is the solvent often used in fingernail polish remover.

The structure of ethyl acetate

Carboxylic acids occur naturally, as the following list shows:

Name of Compound               Where Found

formic acid                              the sting of ants 

acetic acid                               vinegar

butanoic acid                           rancid butter

caproic acid                             goats milk

oxalic acid                               rhubarb

Because carboxylic acid molecules contain an –OH group, the molecules can hydrogen-bond to each other, resulting in relatively high boiling points. They can also hydrogen-bond to water molecules resulting in miscibility with water.

Amines contain nitrogen atoms and are among the most abundant organic molecules found in nature. In contrast to carboxylic acids, which are nature’s weak acids, amines are nature’s weak bases and are similar to ammonia (NH3) in that sense. Many synthetic organic compounds are amines or contain nitrogen groups. Naturally occurring compounds include amino acids, peptides, proteins, alkaloids, and neurotransmitters. Synthetic compounds include decongestants, anesthetics, sedatives, and stimulants.

Because amines contain an –NH group, they exhibit hydrogen bonding, although to a lesser extent than alcohols do. Because of hydro-gen bonding, they have relatively high boiling points, and small amines