Organic Compounds

Type of physical science: Chemistry

Field of study: Chemistry of molecules: types of molecules

Organic compounds are those that contain the element carbon. Such compounds are of central importance in all living systems--without carbon, there would be no life.

Overview

Organic compounds by definition are those compounds containing the element carbon.

This is appropriate, since the word organic means "derived from living organisms," and carbon is the element most essential to life. Compounds are pure substances that contain more than one type of element. Organic compounds always contain carbon, almost always contain hydrogen, often contain oxygen, and sometimes contain other elements such as nitrogen, sulfur, and phosphorus. Compounds that do not contain carbon are referred to as "inorganic."

Organic compounds are at least as widespread on Earth as life itself. They will be found anywhere there is, or was, life. For example, the fossil fuels (coal, natural gas, and petroleum) are mixtures of organic compounds. Sometimes these mixtures are quite complex; a sample of petroleum (crude oil) may contain more than five hundred different compounds. Each of these fuels was produced by the anaerobic ("without air") decomposition of matter that was once living.

A molecule is the smallest unit of a compound that retains its chemical identity.

Conversely, a compound is simply a large collection of identical molecules. In order to understand the chemical characteristics of organic compounds, it is perhaps more important to understand the specific structural aspects of organic molecules. A molecule is a tightly bound group of atoms acting as a unit. The force holding individual atoms together in an organic molecule is a "covalent bond": one or more pairs of electrons shared between the two bound atoms. Covalent bonding should be distinguished from "ionic" bonding, in which one or more electrons are transferred from one atom to another atom. Ionic bonds are frequently found in inorganic substances, but rarely in organic compounds.

The human body, excluding the water, is more than 50 percent carbon by mass. Yet, carbon constitutes only 0.09 percent by weight of the earth's surface, and a very large proportion of this is tied up in living things, their waste, and their decomposition products. It might seem odd that nature chose carbon as the element upon which life would be based, but there are several reasons for this. First, carbon is unique among the elements in its ability to engage in "catenation," or chain formation. For elements other than carbon, molecules containing chains of more than four or five identical atoms in a row are not commonly found, because this bonding arrangement would lead to instability in the molecule. For carbon, just the opposite is true. Very stable molecules can be formed in which there are long chains of carbon. Furthermore, these chains can be "branched" (as many as four carbon atoms can be attached to any given carbon atom), or the chains can loop back on themselves to form rings.

Second, carbon forms very strong covalent bonds with many other elements. This allows for the formation of a large number of organic compounds based solely on the "substitution pattern," that is, the specific distribution of noncarbon atoms around the carbon framework or "skeleton."

Third, carbon can take up to four covalent bonds, which means that as many as four atoms can be attached to a single carbon atom. This will happen any time each of the bonds to carbon results from the sharing of a single pair of electrons (a single bond). Carbon can be bound to another atom by sharing two pairs of electrons (a double bond), or by sharing three pairs of electrons (a triple bond). Generally, carbon atoms within molecules are associated with a total of four shared pairs of electrons. This allows for four combinations of bond types: four single bonds, one double and two single bonds, two double bonds, or one triple and one single bond. In the first combination, carbon will be bound to four other atoms; in the second case, only three atoms will be attached to carbon. In both the third and fourth cases, carbon will be bound to only two other atoms. No atom is ever observed to have more than six covalent bonds.

A chemical compound is represented by a formula that lists the number of each type of atom contained within the individual molecules of that compound. The formula C₄H₁₀ represents the compound butane, the fuel found in disposable cigarette lighters. Butane molecules are composed of four carbon atoms and ten hydrogen atoms, but the formula C₄H₁₀ actually refers to two different compounds, since there are two structurally distinct ways in which these atoms can be connected with covalent bonds. In one arrangement, the carbon atoms are lined up in a straight chain; in the other, they form a Y-shaped array--that is, a one-carbon branch attached to the center carbon of a three-carbon chain. In each of these configurations, hydrogen atoms are attached to the carbon atoms in such a way that each carbon has a total of four bonds. Hydrogen can never take more than one covalent bond; therefore, it can never be one of the center links of a chain.

Molecules that have the same chemical formula but a different "spatial" arrangement of the atoms are said to be "isomers" of one another. Isomers that differ because of the order in which the atoms are connected to one another, as in the case of the butanes, are called "constitutional" isomers. Isomers are different compounds--the two isomers of butane have different chemical and physical properties. Straight-chain butane (usually called "n-butane" or "normal butane") boils at 0 degrees Celsius and freezes at -138 degrees Celsius; branched-chain butane (called "isobutane" or "2-methylpropane" boils at -12 degrees Celsius and freezes at -159 degrees Celsius. The difference in the physical properties of the two compounds is caused entirely by the difference in the structure of the two molecules.

Butane is a member of a class of organic compounds known as the "alkanes." Alkanes are compounds having the formula C_nH_2n+2, where "n" is any positive whole number. The number of possible alkane isomers grows rapidly as "n" increases. While there are only two constitutional isomers of butane, there are three isomers of pentane (C₅H₁₂), five of hexane (C₆H₁₄), nine of heptane (C₇H₁₆), and eighteen of octane (C₈H₁₈). The octane rating system for gasoline is based on the relative effectiveness of each of the octane isomers as fuel in an internal combustion engine--the smoothest-burning isomer ,,-trimethylpentane or "isooctane") is assigned a rating of 100. Straight-chain heptane (n-heptane), poor fuel for cars because of its tendency to produce "knocking" in the engine, is given a rating of "zero." Gasoline, which is always a very complex mixture of organic compounds, is rated in comparison with mixtures of isooctane and n-heptane: An octane rating of 90 means the gasoline behaves as though it were a mixture of 90 percent isooctane and 10 percent n-heptane.

When "heteroatoms" (atoms other than carbon or hydrogen) are introduced into a formula, the number of possible isomers increases even further. Chlorobutane (C₄H₉Cl, also called butyl chloride) is what one obtains by replacing one of the hydrogen atoms in butane with a chlorine atom. There are four constitutional isomers of chlorobutane because each of the "skeletal" isomers (different carbon skeletons) of butane has two distinguishable types of carbon atoms: those at the end of the chain, and those in the middle. These two distinct positions for carbon in each of the butane skeletons give rise to two isomers of chlorobutane, depending on the type of carbon to which chlorine is attached.

There is a more subtle form of isomerism displayed by many organic compounds called "stereoisomerism." Stereoisomers are molecules in which the atoms are connected to one another in the same order, but that still differ in the way the atoms are arranged in space. One particularly important type of stereoisomerism is seen in molecules that are related to one another in the same way a left hand is related to a right hand: They are nonsuperimposable mirror images. A molecule, or any object, that is not superimposable on its mirror image is said to be "chiral," and the individual mirror images are called "enantiomers." An enantiomeric pair will be possible any time a molecule contains one chiral center, that is, one carbon atom that bears four different substituents. (A substituent can be a single atom or a group of atoms.) One of the chlorobutane isomers described earlier is such a case. 2-Chlorobutane (or sec-butyl chloride) has a chlorine atom attached to one of the interior carbons of a four-carbon straight chain. As such, this carbon atom has four different substituents: the chlorine atom, a hydrogen atom, a "methyl" group (-CH₃), and an "ethyl" group (-CH₂-CH₃).

Unlike constitutional isomers, enantiomers have identical physical properties.

Individually, each of the 2-chlorobutane isomers boils at 68 degrees Celsius and freezes at -140 degrees Celsius. The most important way in which enantiomers differ from one another is in their interactions with other chiral molecules. A person knows immediately if one has tried to put a left shoe on a right foot; the fit is not right. A left shoe-right foot interaction is not the same as a right shoe-right foot interaction. This might seem like a trivial point, but it is of enormous significance in all manner of biochemical phenomena. The submicroscopic environments in which molecules of biochemical importance exert their effects are almost always chiral.

There are literally millions of organic compounds that are known, that have been isolated, and whose physical properties and chemical behavior have been studied. In large measure, this large number results from the great diversity of structural form that carbon atoms can give to molecules, even in cases where these molecules have relatively simple formulas.

Perhaps nature picked carbon as the element upon which life would be based because it was "efficient" to do so. By using carbon, the maximum amount of chemical information can be packed into the minimum amount of space.

Applications

The impact of the practical applications of organic chemistry on modern society cannot be overstated. As of 1989, twenty-nine of the top fifty manufactured chemicals in the United States were organic compounds. All plastics and virtually all drugs are organic compounds, and most of these are "synthetic" as opposed to naturally occurring materials. Dyes, food additives, pesticides, detergents, and certain explosives are among the other important synthetic organic compounds produced by the chemical industry.

The synthesis of organic compounds from simpler substances would not be the highly sophisticated discipline that it is were it not for the simplifying concept of the "functional" group.

A functional group is an atom or group of atoms that imparts a characteristic chemical reactivity on the molecules that contain it. Whereas the physical properties of a compound are largely dependent on the size and shape of the molecule, the chemical reactivity usually is not. The reactivity of a compound is dependent mainly on the functional group or groups it contains--the size and shape of the carbon skeleton is of secondary importance.

For example, the "alkenes" are defined as compounds containing a carbon-carbon double bond (represented C=C). This is a useful definition because as far as chemical reactivity is concerned, one alkene is just like any other, at least in most cases. One reaction characteristic of alkenes is the addition of hydrogen to give an alkane as the product ("hydrogenation"). Thus, subjecting ethylene (C₂H₄, or CH₂=CH₂) to an atmosphere of hydrogen gas (H₂) in the presence of an appropriate catalyst, such as nickel, palladium, or platinum, gives ethane (C₂H₆, or CH₃-CH₃) as the product. The same reaction conditions will add one hydrogen atom to each of the carbon atoms of virtually any C=C double bond.

The reader may already be familiar with hydrogenation reactions without even knowing it. The list of ingredients on a tub of margarine will almost certainly show "partially hydrogenated vegetable oil" as the main constituent. Molecules of vegetable oils typically contain many C=C double bonds and are liquids at room temperature. Partial hydrogenation changes some of these double bonds to C-C single bonds, the physical consequence being that the semisynthetic oil is a solid at room temperature. As such, margarine mimics the physical state and appearance of butter, which it is intended to replace.

Another reaction characteristic of alkenes is "addition polymerization," the process by which most synthetic polymers (plastics) are made. In a polymerization reaction, many small molecules (the "monomers") link up, usually in a stepwise, linear fashion to produce one huge molecule, the polymer ("many units"). Thus, ethylene is polymerized to give polyethylene, which essentially is a gigantic alkane; a given polyethylene molecule will incorporate anywhere from several hundred to many thousands of ethylene subunits. Styrofoam, Plexiglas, PVC (polyvinyl chloride), and Teflon are a few of the other common plastics made by addition polymerization of alkenes.

The pharmaceutical industry is also wholly dependent on organic chemistry. Nearly all the drugs and medicines in common use are synthetic organic compounds that had to be built up from simpler substances using the appropriate "methodology." Methodology in organic synthesis refers to the recipes or procedures necessary for bringing about various functional group transformations, or for carbon-carbon bond formation. The synthesis of a given drug may require anywhere from only a few to many dozens of chemical reactions. In order to be successful, chemists working in the pharmaceutical industry need a thorough knowledge of synthetic methodology, as well as the pitfalls and limitations associated with each of these reactions. An analogy can be drawn between organic synthesis and the process a chef goes through in preparing a particular dish from simpler foodstuffs. The chef's ability to prepare various meals is limited by the knowledge of raw materials, spices, cooking techniques, and particular recipes.

Likewise, the organic chemist's ability to produce a desired "target" compound is limited by the available synthetic methodology and by the skill and creativity in employing it. An area of ongoing, active research in organic chemistry is the development of new methodology in synthesis.

Even when the substance in question is not entirely synthetic, knowledge of the reactivity of organic compounds is of vital importance. The production of modern gasoline from crude oil serves as an illustration. As it comes from the ground, crude oil is not a very useful substance; it must be refined. The refining process begins with "fractional distillation" (also called "straight-run distillation"), whereby the crude oil is heated and its components separated by taking advantage of the differences in their boiling points. The fraction with the lowest boiling point (below 30 degrees Celsius) is roughly equivalent to natural gas in its composition; the next most volatile material (straight-run gasoline) boils over a range from 30 degrees Celsius to approximately 180 degrees Celsius; less volatile materials are kerosene (jet fuel), heating oil, and lubricating oil. The residue from distillation is used for making paraffin wax, petroleum jelly, asphalt, and petroleum coke. Because straight-run gasoline is composed largely of unbranched alkanes, it is poor fuel for cars (octane rating of 70). In addition, the gasoline fraction constitutes only a small proportion of the total volume of crude oil, and so modern society's needs for large quantities of high-octane gasoline are met through chemical reactions.

First, a process known as "cracking" (heating in the absence of air) converts the higher-boiling and less useful fractions into lower-boiling materials such as gaso- line. The extremely high heat used in the cracking process breaks the large molecules (high boiling points) into smaller ones (lower boiling points). "Catalytic cracking" produces highly branched alkanes, and hence, high-octane gasoline. "Steam cracking" produces alkenes, which are the starting materials for the production of many plastics and other petrochemicals. Second, the octane rating of gasoline can be improved through any of several processes. Isomerization reactions convert straight-chain alkanes into branched-chain molecules that are better suited for combustion in automobile engines. Likewise, "catalytic reforming" reactions convert low-octane alkanes into high-octane "aromatic" compounds. Aromatic molecules are those containing a ring of six carbon atoms having alternating single and double bonds between adjacent members of the ring.

Benzene, the simplest aromatic compound (C₆H₆), has the remarkably high octane rating of 106.

Context

The evolution of organic chemistry as a distinct discipline began in the early 1800's.

Prior to this time, organic compounds were defined as those derived from living sources, and inorganic compounds were those derived from minerals. Although it had been noticed that organic compounds were invariably composed of carbon and hydrogen, and sometimes included other elements as well, no one had singled out carbon as being special in some way. Likewise, certain minerals contained compounds of carbon (the "carbonates," for example), and these were considered to be inorganic substances. By this time, significant progress had been made in isolating, characterizing, and even synthesizing many inorganic compounds. Similar progress had not been made with organic compounds, largely because of their greater sensitivity to decomposition and their generally greater complexity. A number of organic compounds had been isolated and characterized, but no one had been able to prepare an organic compound using an inorganic source of carbon. Most chemists did not think it was possible; they believed that it was a mysterious "vital force," present only in living things, that allowed for the production of organic compounds. The "vitalistic" theory suffered a severe blow in 1828 when Friedrich Wohler prepared urea, a well-recognized organic compound isolated from urine, from the reaction between a metal cyanate and an ammonium salt, both inorganic compounds. Further erosion to the vitalistic theory inevitably followed, until the distinction between inorganic and organic compounds was, in essence, entirely blurred.

The development of the structural theory of organic compounds continued throughout the nineteenth century, with the most notable contributions from August Friedrich Kekule and Archibald Scott Couper, who independently concluded that carbon could, and usually did, take four bonds; Jacobus Henricus van't Hoff and Joseph-Archille Le Bel realized that the disposition of four substituents around carbon had to be tetrahedral. These advances in structural theory allowed chemists not only to understand how isomers of a given compound could possibly exist but also to predict the number of possible isomers to which a given formula should give rise.

Advances in industrial chemistry did not lag far behind. The beginning of the organic chemical industry began in 1856 with the production of "Perkin's mauve," the first synthetic dye.

Methods for the large-scale production of various drugs were developed, so that by 1900, the Bayer Company was selling aspirin as a pain reliever (along with heroin, which was then promoted as a cough suppressant). The early growth of the plastics industry occurred during the 1930's and especially the 1940's, when, during World War II, supplies of natural rubber were cut off, precipitating the need for a synthetic substitute.

Incredible progress has been made in the area of organic synthesis since the nineteenth century. Substances as complex as vitamin B12 have been produced by totally synthetic means.

Molecular devices that act as microscopic switches, wires, or other electrical circuit components have been prepared, as have artificial enzymes. Patents have been issued for new "life-forms," produced through genetic engineering. The day seems to be drawing ever nearer when scientists will be able to create life, or its synthetic equivalent, in the laboratory. With carbon, the possibilities are endless.

Principal terms

ATOM: the smallest unit of matter that retains its chemical identity; an "element" is a large collection of atoms of the same type

CHIRALITY: the property ascribed to objects that are not superimposable on their mirror images

COVALENT BOND: a bond between two atoms within a molecule that results from the sharing of one, two, or three pairs of electrons between the bound atoms; called single, double, and triple bonds, respectively

FUNCTIONAL GROUP: an atom or group of atoms that confers a characteristic chemical reactivity upon molecules that contain it

ISOMERS: the term given to compounds having the same chemical formula (the same number and type of atoms in their molecules), but that differ in the way the atoms are arranged within the molecules

MOLECULE: a group of tightly bound atoms that act as a unit; a "compound" is a large collection of identical molecules

ORGANIC SYNTHESIS: the process by which chemists build up complex organic compounds from simpler starting materials

Bibliography

Benfey, O. Theodor. FROM VITAL FORCE TO STRUCTURAL FORMULAS. Boston: Houghton Mifflin, 1964. This slim volume covers the early history of organic chemistry in some detail. The rise and fall of the vitalistic theory, and the steps taken by nineteenth century scientists in developing a self-consistent theory of organic structure are presented in a clear and illuminating fashion. Indexed, with references at the end of each chapter.

McMurry, John. FUNDAMENTALS OF ORGANIC CHEMISTRY. Belmont, Calif.: Wadsworth, 1986. This textbook is intended as a one-semester introduction to the basic aspects of organic chemistry. Successive chapters explain the nomenclature, structure, physical properties, preparation, and reactivity of representative members of each of the important functional group classes. Especially entertaining are the "Interludes," brief chapters concerning some interesting subtopic, such as "The Chemistry of Vision" or "Chemical Warfare in Nature." Includes index and appendices.

Nickon, Alex, and Ernest F. Silversmith. ORGANIC CHEMISTRY: THE NAME GAME. Elmsford, N.Y.: Pergamon Press, 1987. Deals with the origins of the nonsystematic nomenclature of specific organic compounds. Some of these molecules are interesting by virtue of their shapes, others because of their histories, but the anecdotes presented are always entertaining. Full appreciation does require some basic knowledge of organic chemistry.

Szmant, H. Harry. ORGANIC BUILDING BLOCKS OF THE CHEMICAL INDUSTRY. New York: Wiley-Interscience, 1989. Szmant begins by covering the history, structure, and economics of the chemical industry. A thorough treatment is given to the sources and production methods of the major industrial organic chemicals. Index.

Windholtz, Martha, ed. THE MERCK INDEX. 11th ed. Rahway, N.J.: Merck, 1989. Intended as a reference for chemists, pharmacists, and medical doctors, this book gives brief descriptions for more than ten thousand chemical substances, most of which are organic compounds, although the more important inorganic compounds are listed as well. Each entry includes the chemical and structural formulas, various physical properties, methods of preparation, toxicity, and medical or therapeutic uses of the substance in question. The entries are in alphabetical order and a thorough cross index is provided for compounds that have more than one name. A formula index is provided, along with many useful tables.

Carbon and Carbon Group Compounds

Group IV Elements

Isomeric Forms of Molecules