Carbon and carbon group compounds

• Type of physical science: Chemistry
• Field of study: Chemical compounds

Carbon is a unique elemental material present in all the molecules making up living matter. Carbon atoms are capable of forming strong chemical bonds with one another, producing a vast array of structures in combination with very few elements.

Overview

Carbon is one of the most familiar of all the substances making up the world. To the chemist, the name “carbon” denotes one of the naturally occurring chemical elements, one of the limited number of pure basic materials from which everything in the material world is constructed. Charcoal, pencil lead, graphite, and diamonds are all nearly pure carbon. Additional carbon allotropes such as graphene, a single layer of carbon atoms arranged in a two-dimensional lattice, exhibit exceptional mechanical strength and electrical conductivity and are being further developed for applications in energy storage and electronics. While it is known that elements are complex structures and can, under appropriate conditions, be broken down into simpler components, it is usually correct to treat them as unchanging in chemical reactions.

The smallest subdivision of an element is called an atom and represents the most basic unit of all chemical discussion and understanding. The atom has one structural unit of overriding importance chemically: its electrons. Each of these nearly weightless bundles of energy has a unit of negative electrical charge. Although they are charged, electrons have the surprising property of being able to associate with one another and provide the force required to hold atoms together in more complex arrangements called molecules. It is the making and breaking of these electron pairs or bonds that allow for the change of one form of matter into another, the process called chemistry.

All the basic definitions presented thus far are true for all chemicals and chemical changes with the important exception of radioactive material. Yet, there are significant differences between carbon and all the other elements. The most far-reaching of these differences is the nearly unique ability of carbon to form a large network of strong chemical bonds with other carbon atoms. This ability is unique because only a few elements share electron pairs among themselves, and where they do, only a strictly limited number of atoms are present in the molecules.

Consider the common elements of the atmosphere: oxygen and nitrogen. The first of these is one of the most abundant elements on Earth, though hydrogen is the most abundant element in the universe, and yet it is rarely present except as two linked atoms in its natural, elemental state or as single atoms in other compounds. A given molecule often contains a larger number of oxygen atoms, but they are not bonded to one another. There is the important case of the ozone molecule, in which three oxygen atoms are linked together consecutively, and there are the very reactive peroxide molecules, in which two consecutive oxygen atoms are found. Both of these atomic arrangements are exceptions to the general rule. The same situation is found in the case of nitrogen, which occurs in nature as molecules containing two atoms. In compounds one finds a wider variety of two, three, or even four consecutive nitrogen atom combinations. While this is clearly a more flexible arrangement than is characteristic of oxygen, the single nitrogen atom in a sequence of bonds is the combination most frequently observed.

Carbon atoms form an extensive series of sequential bonds. Therefore, a variety of sequences is not only possible but also found routinely. Thus, carbon chains can be either continuous or branched. Their arrangement is analogous to an uninterrupted stretch of river or one with tributaries. Furthermore, the sequence of carbon atoms may link with itself and form a ring. Combinations of these three structural units are well known. Rings are attached to continuous or branched chains (sometimes both), chains can connect rings, or rings may be attached to one another.

Several concerns become immediately apparent. Perhaps the first and most worrisome relates to the number of possible structures that this property of carbon allows. The vastness of these possibilities may make the study of carbon structures very difficult. There is a way that not only leads to confident understanding but also is suitable for the most detailed exploration of this multifaceted terrain. In the middle of the nineteenth century, it became apparent that there were several unifying principles that helped to organize one’s thinking and ultimately led to the understanding of the patterns found in carbon compounds. The most useful of these generalizations is that, in all of its stable, electrically neutral compounds, carbon shares four pairs of electrons with other atoms. It makes no difference what the atoms bonded to carbon are; many are possible, but hydrogen, oxygen, nitrogen, sulfur, and phosphorus are the most common. In each of these cases and carbon as well, exactly four bonds are formed.

Taking the simplest example—that of compounds made only of carbon and hydrogen—one can illustrate this tetravalent property of carbon atoms. The simplest such compound ever found is the common marsh gas that one burns in stoves and that accounts for many explosions in coal mines. This compound is called methane, and it is known to consist of molecules containing four hydrogen atoms for every carbon atom. Its formula is CH₄.

If one examines the comparable compound containing two carbon atoms, six hydrogen atoms are found. Thus, ethane has the molecular formula C₂H₆ or the structural formula CH₃CH₃.

Each carbon atom has one bond to another carbon atom and three hydrogen atoms; a total of four bonds each. To illustrate the symbolic representation chemists use to think about the matter they study, use the letter “C” for a carbon atom, the letter “H” for a hydrogen atom, and a dash for a chemical bond or electron pair.

Carbon can form a very large number of compounds if one considers only the additional element hydrogen and allows the use of single pairs of electrons for bonding. The common substance polyethylene is actually a complex mixture of very large molecules similar to those discussed. When dealing with molecules that have thousands of carbon atoms in each molecule, it becomes difficult to isolate and study pure, uniform compounds.

The restriction of involving only single electron pairs in bonding is unnecessarily limiting, since carbon forms excellent bonds with other carbon atoms using two or three electron pairs. These double and triple bonds are very different in chemical reactivity from the unreactive single bonds. They also introduce a new series of compounds analogous to, but distinct from, the single-bond examples. The great difference in chemical reactivity between these single- and multiple-bond compounds is characterized in the technical names “saturated” and “unsaturated,” respectively. Examples of these unsaturations, or multiple bonds, can be found in continuous chain hydrocarbons, branched chain hydrocarbons, and cyclic hydrocarbons. Indeed, with only a few special cases such as a triple bond in a five-carbon atom ring, unsaturations have been made in great number and variety.

In certain cases, a particular combination of these structural features will create a molecule with extraordinary properties. One of the most celebrated examples of this situation occurs when a specific number of multiple bonds are included in rings of specific numbers of atoms. The example that has been most extensively studied and is found most often in nature involves three carbon-carbon double bonds in a six-carbon atom ring. The molecule, called benzene, is the prime representative of the subdivision of carbon compounds designated as “aromatic” compounds. The name “aromatic” actually includes many compounds with unpleasant odors, but in this case, it is used exclusively to refer to the special arrangement of electrons involved. Such compounds show a much greater stability than would be expected on the basis of the unsaturations that appear to be present.

Benzene has the chemical formula C₆H₆.

Place the six carbon atoms in a ring and connect them with alternating double and single bonds.

It is discovered that each carbon atom has three bonds. Thus, adding one hydrogen atom at each carbon provides the usual four bonds found in carbon chemistry.

These few simple rules allow one to imagine a number and variety of carbon or organic compounds that approaches the infinite. Nevertheless, this is really only the beginning. As one seeks practical applications of this new knowledge, one finds that every one of the imagined structures can be easily multiplied into countless new and fascinating molecules.

Applications

The compounds discussed in the general nature of the carbon group are collectively called the hydrocarbons. Oil and coal supply huge quantities of these valuable synthetic and fuel substances. Yet, it is the modification of these molecules through the introduction of oxygen and nitrogen atoms that gives them their widest application. Carbon materials such as graphite are essential components of lithium-ion batteries, where they function as electrodes that store and release energy through reversible chemical reactions.

Like carbon and hydrogen, these heteroatoms have very regular valence or bonding capacities. Oxygen forms two bonds, while nitrogen forms three bonds in stable, electrically neutral compounds. Also, like carbon, multiple bonds are frequently formed—double bonds with oxygen and double or triple bonds for nitrogen.

If one considers only the possible single bonds of oxygen, then several possibilities are found. For example, an oxygen atom attached to a carbon atom can hold on its other available bond a hydrogen atom, another carbon atom, a nitrogen atom, or simply a negatively charged, saltlike unit called an ion. These simple and common examples need not be restricted to a single such arrangement in a given molecule, but can be expanded at least as many times as there are available carbon atoms.

The two-carbon molecule ethane illustrates this oxygen system. If one oxygen atom is inserted between one of the carbon and hydrogen atom pairs, then the common substance ethyl, or grain alcohol, is produced. It should be noted that ethyl alcohol is not actually made in this manner; the structural relationships possible are simply being visualized. If a second oxygen atom is inserted between the other carbon and one of its hydrogens, then the result is the compound ethylene glycol commonly used in antifreeze solutions. If one begins with ethyl alcohol and replaces the hydrogen atom attached to oxygen with another two-carbon fragment, then this addition is a second molecule of ethane with one of its six hydrogens removed. The resulting product is ether, which is used in surgery as an anesthetic.

One could carry out completely analogous structure designs with the one-carbon analog methane or the three-carbon member of the series, propane. The latter material is familiar in camp stoves and rural heating. Methane and propane should provide new limitations and opportunities relative to ethane. Using methane as an example, the introduction of an oxygen atom produces a substance similar to ethyl alcohol. Methyl or wood alcohol boils and freezes at its own characteristic temperatures, however, and is a deadly poison. Since methyl alcohol has only one carbon, it is impossible to place the second oxygen on a different carbon atom as in ethylene glycol. A second oxygen at the single carbon of methyl alcohol produces an entirely new class of compounds. The gas formaldehyde, commonly used to preserve tissue, has lost a molecule of hydrogen and now contains a carbon-oxygen double bond. A similar observation is made in the case of ethyl alcohol if the second oxygen is introduced at the carbon atom that is partially oxidized.

With propane, three oxygen atoms may be inserted, one on each carbon atom. The product is glycerine, which can be derived from propane-based structures through chemical modification and which finds uses ranging from candy making to high explosives. Glycerine is also a common constituent of fats and oils, thus having an important role in the complex chemistry of human living cells.

It is entirely possible to make these same extensions and many others with every hydrocarbon previously introduced. The alcohols are just as common in cyclic or ring compounds. The hydrocarbons may have unsaturations as well and oxygenated groups. In the aromatic series, the alcohol group (-OH) takes on a quite different character by virtue of its close association with the electron-rich ring. The common disinfectant phenol (historically called carbolic acid) is the simplest member of this series, and many photographic developers contain such an arrangement of atoms.

The elements nitrogen and sulfur are frequently found in chemical combinations analogous to those described for oxygen. Because nitrogen has three covalent bonding sites, or four in charged instances, there are more extensive possibilities available for nitrogen. The unpleasant odor most people find in fish is the result of simple nitrogen compounds that are similar in structure to the alcohols. Many of the most important natural and synthetic pharmaceutical substances contain nitrogen; many also contain sulfur. In proteins, nitrogen, oxygen, and sulfur all show their unique chemical properties in forming huge natural polymers and regulating their life-giving functions.

Context

Since the early years of the nineteenth century, when John Dalton proposed his atomic theory, chemists have continually searched for appropriate models of the varied forms of material found in the natural world. Historically, this search has been both demanding and highly successful in the world of living organisms. The study of biology has often looked to organic chemistry for answers to its fascinating problems. In the years between World War I and II, this scientific interdependence became so intense that the field of inquiry known as biochemistry was born. Both of these fields of study involve attention to the chemistry of the single element carbon, which has a degree of centrality that borders on the incomprehensible.

Why is it that, with approximately ninety different elemental substances available, one of them should form the essential structure of all molecules associated with living matter? While the scientific method does not lead to answers to such absolute questions, the current models of the carbon atom and its compounds have been examined profitably to see how they help to shed light on the world of natural molecules. The enormous variety and the fragile nature of the molecules of living matter presented formidable problems from the beginning of modern chemistry.

Friedrich Wohler, the nineteenth-century German master who is generally given credit for opening up organic chemistry to serious exploration, once felt despair at ever making sense of such a complex field. He compared the organic or carbon chemistry of the 1830s to “a primeval tropical forest full of the most remarkable things, a dreadful endless jungle into which one does not dare enter for there seems to be no way out.” Beginning in the nineteenth and continuing into the twentieth century, organic chemists were the creators of a variety of materials: the production of natural products such as ethyl alcohol and biologically derived compounds such as penicillin; the creation of materials never found in nature, such as sulfa drugs and polyethylene; and the modification of natural materials to improve their desirable properties and remove their undesirable properties, such as morphine and steroids.

As one looks at the misuse of tranquilizers, the relationship between fluorocarbons and the ozone layer, and the mounting problems of waste disposal, chemists are forced to ponder if they should have left the purple dye for royalty and not created beautifully colored fabrics and photographs at the cost of increased risk of cancer, if they have done their jobs too well, and if they were wise in the statement of their jobs. The scientist has a basic faith in the truth and in gaining an understanding of the truth. Carbon compounds have extended and improved human life.

Principal terms

ATOM: the smallest particle of an element that retains all the chemical and physical attributes of the substance

BOND: one or more pairs of electrons that are responsible for holding atoms together; in carbon compounds, these are shared pairs or covalent bonds

COMPOUND: a pure chemical composed of two or more elements and bonded to one another

COVALENCE: the number of chemical bonds or pairs of electrons shared by an atom with other atoms in making a molecule

ELEMENT: a pure chemical substance that can be broken up into simpler units only at the expense of losing its chemical characteristics

FORMULA: the chemical composition of a compound; used to designate the number and kinds of atoms and their structure

MOLECULE: a collection of atoms bonded together to form a structure that exists as a discrete entity and possesses at least enough stability to be detected

OXIDATION: the chemical process of removing electrons from an atom or molecule, often involving the introduction of oxygen

REDUCTION: the chemical process of adding electrons or hydrogen, which must always accompany oxidation

STRUCTURE: the exact arrangement of atoms in a molecule, especially their spatial distribution

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