There was once a time when chemists thought "organic" referred only to things that were living, and that life was the result of a spiritual "life force." While there is nothing wrong with viewing life as having a spiritual component, spiritual matters are simply outside the realm of science, and to mix up the two is as silly as using mathematics to explain love (or vice versa). In fact, the "life force" has a name: carbon, the common denominator in all living things. Not everything that has carbon is living, nor are all the areas studied in organic chemistry—the branch of chemistry devoted to the study of carbon and its compounds—always concerned with living things. Organic chemistry addresses an array of subjects as vast as the number of possible compounds that can be made by strings of carbon atoms. We can thank organic chemistry for much of what makes life easier in the modern age: fuel for cars, for instance, or the plastics found in many of the products used in an average day.
HOW IT WORKS
Introduction to Carbon
As the element essential to all of life, and hence the basis for a vast field of study, carbon is addressed in its own essay. The Carbon essay, in addition to examining the chemical properties of carbon (discussed below), approaches a number of subjects, such as the allotropes of carbon. These include three crystalline forms (graphite, diamond, and buckminsterfullerene), as well as amorphous carbon. In addition, two oxides of carbon—carbon dioxide and carbon monoxide—are important, in the case of the former, to the natural carbon cycle, and in the case of the latter, to industry. Both also pose environmental dangers.
The purpose of this summary of the carbon essay is to provide a hint of the complexities involved with this sixth element on the periodic table, the 14th most abundant element on Earth. In the human body, carbon is second only to oxygen in abundance, and accounts for 18% of the body's mass. Capable of combining in seemingly endless ways, carbon, along with hydrogen, is at the center of huge families of compounds. These are the hydrocarbons, present in deposits of fossil fuels: natural gas, petroleum, and coal.
A PROPENSITY FOR LIMITLESS BONDING.
Carbon has a valence electron configuration of 2s22p2. Likewise all the members of Group 4 on the periodic table (Group 14 in the IUPAC version of the table)—sometimes known as the "carbon family"—have configurations of ns2np2, where n is the number of the period or row the element occupies on the table. There are two elements noted for their ability to form long strings of atoms and seemingly endless varieties of molecules: one is carbon, and the other is silicon, directly below it on the periodic table.
Just as carbon is at the center of a vast network of organic compounds, silicon holds the same function in the inorganic realm. It is found in virtually all types of rocks, except the calcium carbonates—which, as their name implies, contain carbon. In terms of known elemental mass, silicon is second only to oxygen in abundance on Earth. Silicon atoms are about one and a half times as large as those of carbon; thus not even silicon can compete with carbon's ability to form an almost limitless array of molecules in various shapes and sizes, and with various chemical properties.
Carbon is further distinguished by its high value of electronegativity, the relative ability of an atom to attract valence electrons. To mention a few basic aspects of chemical bonding, developed at considerably greater length in the Chemical Bonding essay, if two atoms have an electric charge and thus are ions, they form strong ionic bonds. Ionic bonding occurs when a metal bonds with a nonmetal. The other principal type of bond is a covalent bond, in which two uncharged atoms share eight valence electrons. If the electronegativity values of the two elements involved are equal, then they share the electrons equally; but if one element has a higher electronegativity value, the electrons will be more drawn to that element.
The electronegativity of carbon is certainly not the highest on the periodic table. That distinction belongs to fluorine, with an electronegativity value of 4.0, which makes it the most reactive of all elements. Fluorine is at the head of Group 7, the halogens, all of which are highly reactive and most of which have high electronegativity values. If one ignores the noble gases, which are virtually unreactive and occupy the extreme right-hand side of the periodic table, electronegativity values are highest in the upper right-hand side of the table—the location of fluorine—and lowest in the lower left. In other words, the value increases with group or column number (again, leaving out the noble gases in Group 8), and decreases with period or row number.
With an electronegativity of 2.5, carbon ties with sulfur and iodine (a halogen) for sixth place, behind only fluorine; oxygen (3.5); nitrogen and chlorine (3.0); and bromine (2.8). Thus its electronegativity is high, without being too high. Fluorine is not likely to form the long chains for which is carbon is known, simply because its electronegativity is so high, it overpowers other elements with which it comes into contact. In addition, with four valence electrons, carbon is ideally suited to find other elements (or other carbon atoms) for forming covalent bonds according to the octet rule, whereby most elements bond so that they have eight valence electrons.
Carbon's Multiple Bonds
Carbon—with its four valence electrons—happens to be tetravalent, or capable of bonding to four other atoms at once. It is not necessarily the case that an element has the ability to bond with as many other elements as it has valence electrons; in fact, this is rarely the case. Additionally, carbon is capable of forming not only a single bond, with one pair of shared valence electrons, but a double bond (two pairs) or even a triple bond (three pairs.)
Another special property of carbon is its ability to bond in long chains that constitute strings of carbons and other atoms. Furthermore, though sometimes carbon forms a typical molecule (for example, carbon dioxide, or CO2, is just one carbon atom with two oxygens), it is also capable of forming "molecules" that are really not molecules in the way that the word is typically used in chemistry. Graphite, for instance, is just a series of "sheets" of carbon atoms bonded tightly in a hexagonal, or six-sided, pattern, while a diamond is simply a huge "molecule" composed of carbon atoms strung together by covalent bonds.
Organic chemistry is the study of carbon, its compounds, and their properties. The only major carbon compounds considered inorganic are carbonates (for instance, calcium carbonate, alluded to above, which is one of the major forms of mineral on Earth) and oxides, such as carbon dioxide and carbon monoxide. This leaves a huge array of compounds to be studied, as we shall see.
The term "organic" in everyday language connotes "living," but organic chemistry is involved with plenty of compounds not part of living organisms: petroleum, for instance, is an organic compound that ultimately comes from the decayed bodies of organisms that once were alive. It should be stressed that organic compounds do not have to be produced by living things, or even by things that once were alive; they can be produced artificially in a laboratory.
The breakthrough event in organic chemistry came in 1828, when German chemist Friedrich Wöhler (1800-1882) heated a sample of ammonium cyanate (NH4OCN) and converted it to urea (H2N-CO-NH2). Ammonium cyanite is an inorganic material, whereas urea, a waste product in the urine of mammals, is an organic one. "Without benefit of a kidney, a bladder, or a dog," as Wöhler later said, he had managed to transform an inorganic substance into an organic one.
Ammonium cyanate and urea are isomers: substances having the same formula, but possessing different chemical properties. Thus they have exactly the same numbers and proportions of atoms, yet these atoms are arranged in different ways. In urea, the carbon forms an organic chain, and in ammonium cyanate, it does not. Thus, to reduce the specifics of organic chemistry even further, this discipline can be said to constitute the study of carbon chains, and ways to rearrange them to create new substances.
Organic Chemistry and Modern Life
At first glance, the term "organic chemistry" might sound like something removed from everyday life, but this could not be further from the truth. The reality of the role played by organic chemistry in modern existence is summed up in a famous advertising slogan used by E. I. du Pont de Nemours and Company (usually referred to as "du Pont"): "Better Things for Better Living Through Chemistry."
Often rendered simply as "Better Living Through Chemistry," the advertising campaign made its debut in 1938, just as du Pont introduced a revolutionary product of organic chemistry: nylon, the creation of a brilliant young chemist named Wallace Carothers (1896-1937). Nylon, an example of a polymer (discussed below), started a revolution in plastics that was still unfolding three decades later, in 1967. That was the year of the film The Graduate, which included a famous interchange between the character of Benjamin Braddock (Dustin Hoffman) and an adult named Mr. McGuire (Walter Brooke):
- Mr. McGuire: I just want to say one word to you… just one word.
- Benjamin Braddock: Yes, sir.
- Mr. McGuire: Are you listening?
- Benjamin Braddock: Yes, sir, I am.
- Mr. McGuire: Plastics.
The meaning of this interchange was that plastics were the wave of the future, and that an intelligent young man such as Ben should invest his energies in this promising new field. Instead, Ben puts his attention into other things, quite removed from "plastics," and much of the plot revolves around his revolt against what he perceives as the "plastic" (that is, artificial) character of modern life.
In this way, The Graduate spoke for a whole generation that had become ambivalent concerning "better living through chemistry," a phrase that eventually was perceived as ironic in view of concerns about the environment and the many artificial products that make up modern life. Responding to this ambivalence, du Pont dropped the slogan in the late 1970s; yet the reality is that people truly do enjoy "better living through chemistry"—particularly organic chemistry.
APPLICATIONS OF ORGANIC CHEMISTRY.
What would the world be like without the fruits of organic chemistry? First, it would be necessary to take away all the various forms of rubber, vitamins, cloth, and paper made from organically based compounds. Aspirins and all types of other drugs; preservatives that keep food from spoiling; perfumes and toiletries; dyes and flavorings—all these things would have to go as well.
Synthetic fibers such as nylon—used in everything from toothbrushes to parachutes—would be out of the picture if it were not for the enormous progress made by organic chemistry. The same is true of plastics or polymers in general, which have literally hundreds upon hundreds of applications. Indeed, it is virtually impossible for a person in twenty-first century America to spend an entire day without coming into contact with at least one, and more likely dozens, of plastic products. Car parts, toys, computer housings, Velcro fasteners, PVC (polyvinyl chloride) plumbing pipes, and many more fixtures of modern life are all made possible by plastics and polymers.
Then there is the vast array of petrochemicals that power modern civilization. Best-known among these is gasoline, but there is also coal, still one of the most significant fuels used in electrical power plants, as well as natural gas and various other forms of oil used either directly or indirectly in providing heat, light, and electric power to homes. But the influence of petrochemicals extends far beyond their applications for fuel. For instance, the roofing materials and tar that (quite literally) keep a roof over people's heads, protecting them from sun and rain, are the product of petrochemicals—and ultimately, of organic chemistry.
Carbon, together with other elements, forms so many millions of organic compounds that even introductory textbooks on organic chemistry consist of many hundreds of pages. Fortunately, it is possible to classify broad groupings of organic compounds. The largest and most significant is that class of organic compounds known as hydrocarbons—chemical compounds whose molecules are made up of nothing but carbon and hydrogen atoms.
Every molecule in a hydrocarbon is built upon a "skeleton" composed of carbon atoms, either in closed rings or in long chains. The chains may be straight or branched, but in each case—rings or chains, straight chains or branched ones—the carbon bonds not used in tying the carbon atoms together are taken up by hydrogen atoms.
Theoretically, there is no limit to the number of possible hydrocarbons. Not only does carbon form itself into apparently limitless molecular shapes, but hydrogen is a particularly good partner. It has the smallest atom of any element on the periodic table, and therefore it can bond to one of carbon's valence electrons without getting in the way of the other three.
There are two basic varieties of hydrocarbon, distinguished by shape: aliphatic and aromatic. The first of these forms straight or branched chains, as well as rings, while the second forms only benzene rings, discussed below. Within the aliphatic hydrocarbons are three varieties: those that form single bonds (alkanes), double bonds (alkenes), and triple bonds (alkynes.)
The alkanes are also known as saturated hydrocarbons, because all the bonds not used to make the skeleton itself are filled to their capacity (that is, saturated) with hydrogen atoms. The formula for any alkane is CnH2n+2, where n is the number of carbon atoms. In the case of a linear, unbranched alkane, every carbon atom has two hydrogen atoms attached, but the two end carbon atoms each have an extra hydrogen.
What follows are the names and formulas for the first eight normal, or unbranched, alkanes. Note that the first four of these received common names before their structures were known; from C5 onward, however, they were given names with Greek roots indicating the number of carbon atoms (e.g., octane, a reference to "eight.")
- Methane (CH4)
- Ethane (C2H6)
- Propane (C3H8)
- Butane (C4H10)
- Pentane (C5H12)
- Hexane (C6H14)
- Heptane (C7H16)
- Octane (C8H18)
The reader will undoubtedly notice a number of familiar names on this list. The first four, being the lowest in molecular mass, are gases at room temperature, while the heavier ones are oily liquids. Alkanes even heavier than those on this list tend to be waxy solids, an example being paraffin wax, for making candles. It should be noted that from butane on up, the alkanes have numerous structural isomers, depending on whether they are straight or branched, and these isomers have differing chemical properties.
Branched alkanes are named by indicating the branch attached to the principal chain. Branches, known as substituents, are named by taking the name of an alkane and replacing the suffix with yl—for example, methyl, ethyl, and so on. The general term for an alkane which functions as a substituent is alkyl.
Cycloalkanes are alkanes joined in a closed loop to form a ring-shaped molecule. They are named by using the names above, with cyclo-as a prefix. These start with propane, or rather cyclopropane, which has the minimum number of carbon atoms to form a closed shape: three atoms, forming a triangle.
ALKENES AND ALKYNES.
The names of the alkenes, hydrocarbons that contain one or more double bonds per molecule, are parallel to those of the alkanes, but the family ending is-ene. Likewise they have a common formula: CnH2n. Both alkenes and alkynes, discussed below, are unsaturated—in other words, some of the carbon atoms in them are free to form other bonds. Alkenes with more than one double bond are referred to as being polyunsaturated.
As with the alkenes, the names of alkynes (hydrocarbons containing one or more triple bonds per molecule) are parallel to those of the alkanes, only with the replacement of the suffix -yne in place of-ane. The formula for alkenes is CnH2n-2. Among the members of this group are acetylene, or C2H2, used for welding steel. Plastic polystyrene is another important product from this division of the hydrocarbon family.
Aromatic hydrocarbons, despite their name, do not necessarily have distinctive smells. In fact the name is a traditional one, and today these compounds are defined by the fact that they have benzene rings in the middle. Benzene has a formula C6H6, and a benzene ring is usually represented as a hexagon (the six carbon atoms and their attached hydrogen atoms) surrounding a circle, which represents all the bonding electrons as though they were everywhere in the molecule at once.
In this group are products such as naphthalene, toluene, and dimethyl benzene. These last two are used as solvents, as well as in the synthesis of drugs, dyes, and plastics. One of the more famous (or infamous) products in this part of the vast hydrocarbon network is trinitrotoluene, or TNT. Naphthalene is derived from coal tar, and used in the synthesis of other compounds. A crystalline solid with a powerful odor, it is found in mothballs and various deodorant-disinfectants.
As for petro-chemicals, these are simply derivatives of petroleum, itself a mixture of alkanes with some alkenes, as well as aromatic hydrocarbons. Through a process known as fractional distillation, the petrochemicals of the lowest molecular mass boil off first, and those having higher mass separate at higher temperatures.
Among the products derived from the fractional distillation of petroleum are the following, listed from the lowest temperature range (that is, the first material to be separated) to the highest: natural gas; petroleum ether, a solvent; naphtha, a solvent (used for example in paint thinner); gasoline; kerosene; fuel for heating and diesel fuel; lubricating oils; petroleum jelly; paraffin wax; and pitch, or tar. A host of other organic chemicals, including various drugs, plastics, paints, adhesives, fibers, detergents, synthetic rubber, and agricultural chemicals, owe their existence to petrochemicals.
Obviously, petroleum is not just for making gasoline, though of course this is the first product people think of when they hear the word "petroleum." Not all hydrocarbons in gasoline are desirable. Straight-chain or normal heptane, for instance, does not fire smoothly in an internal-combustion engine, and therefore disrupts the engine's rhythm. For this reason, it is given a rating of zero on a scale of desirability, while octane has a rating of 100. This is why gas stations list octane ratings at the pump: the higher the presence of octane, the better the gas is for one's automobile.
With carbon and hydrogen as the backbone, the hydrocarbons are capable of forming a vast array of hydrocarbon derivatives by combining with other elements. These other elements are arranged in functional groups—an atom or group of atoms whose presence identifies a specific family of compounds. Below we will briefly discuss some of the principal hydrocarbon derivatives, which are basically hydrocarbons with the addition of other molecules or single atoms.
Alcohols are oxygen-hydrogen molecules wedded to hydrocarbons. The two most important commercial types of alcohol are methanol, or wood alcohol; and ethanol, which is found in alcoholic beverages, such as beer, wine, and liquor. Though methanol is still known as "wood alcohol," it is no longer obtained by heating wood, but rather by the industrial hydrogenation of carbon monoxide. Used in adhesives, fibers, and plastics, it can also be applied as a fuel. Ethanol, too, can be burned in an internal-combustion engine, when combined with gasoline to make gasohol. Another significant alcohol is cholesterol, found in most living organisms. Though biochemically important, cholesterol can pose a risk to human health.
Aldehydes and ketones both involve a double-bonded carbon-oxygen molecule, known as a carbonyl group. In a ketone, the carbonyl group bonds to two hydrocarbons, while in an aldehyde, the carbonyl group is always at the end of a hydrocarbon chain. Therefore, instead of two hydrocarbons, there is always a hydrocarbon and at least one other hydrogen bonded to the carbon atom in the carbonyl. One prominent example of a ketone is acetone, used in nail polish remover. Aldehydes often appear in nature—for instance, as vanillin, which gives vanilla beans their pleasing aroma. The ketones carvone and camphor impart the characteristic flavors of spearmint leaves and caraway seeds.
CARBOXYLIC ACIDS AND ESTERS.
Carboxylic acids all have in common what is known as a carboxyl group, designated by the symbol -COOH. This consists of a carbon atom with a double bond to an oxygen atom, and a single bond to another oxygen atom that is, in turn, wedded to a hydrogen. All carboxylic acids can be generally symbolized by RCOOH, with R as the standard designation of any hydrocarbon. Lactic acid, generated by the human body, is a carboxylic acid: when a person overexerts, the muscles generate lactic acid, resulting in a feeling of fatigue until the body converts the acid to water and carbon dioxide. Another example of a carboxylic acid is butyric acid, responsible in part for the smells of rancid butter and human sweat.
When a carboxylic acid reacts with an alcohol, it forms an ester. An ester has a structure similar to that described for a carboxylic acid, with a few key differences. In addition to its bonds (one double, one single) with the oxygen atoms, the carbon atom is also attached to a hydrocarbon, which comes from the carboxylic acid. Furthermore, the single-bonded oxygen atom is attached not to a hydrogen, but to a second hydrocarbon, this one from the alcohol. One well-known ester is acetylsalicylic acid—better known as aspirin. Esters, which are a key factor in the aroma of various types of fruit, are often noted for their pleasant smell.
Polymers are long, stringy molecules made of smaller molecules called monomers. They appear in nature, but thanks to Carothers—a tragic figure, who committed suicide a year before Nylon made its public debut—as well as other scientists and inventors, synthetic polymers are a fundamental part of daily life.
The structure of even the simplest polymer, polyethylene, is far too complicated to discuss in ordinary language, but must be represented by chemical symbolism. Indeed, polymers are a subject unto themselves, but it is worth noting here just how many products used today involve polymers in some form or another.
Polyethylene, for instance, is the plastic used in garbage bags, electrical insulation, bottles, and a host of other applications. A variation on polyethylene is Teflon, used not only in nonstick cookware, but also in a number of other devices, such as bearings for low-temperature use. Polymers of various kinds are found in siding for houses, tire tread, toys, carpets and fabrics, and a variety of other products far too lengthy to enumerate.
WHERE TO LEARN MORE
Blashfield, Jean F. Carbon. Austin, TX: Raintree Steck-Vaughn, 1999.
"Carbon." Xrefer (Web site). <http://www.xrefer.com/entry/639742> (May 30, 2001).
Chemistry Help Online for Students (Web site). <http://members.tripod.com/chemistryhelp/> May 30, 2001).
Knapp, Brian J. Carbon Chemistry. Illustrated by David Woodroffe. Danbury, CT: Grolier Educational, 1998.
Loudon, G. Marc. Organic Chemistry. Menlo Park, CA: Benjamin/Cummings, 1988.
"Organic Chemistry" (Web site). <http://edie.cprost.sfu.ca/~rhlogan/organic.html> (May 30, 2001).
"Organic Chemistry." Frostburg State University Chemistry Helper (Web site). <http://www.chemhelper.com/> (May 30, 2001).
Sparrow, Giles. Carbon. New York: Benchmark Books, 1999.
Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.
Hydrocarbons that form single bonds. Alkanes are also called saturated hydrocarbons.
Hydrocarbons that form double bonds.
A general term for an alkane that functions as a substituent.
Hydrocarbons that form triple bonds.
Different versions of the same element, distinguished by molecular structure.
Having no definite structure.
A type of chemical bonding in which two atoms share valence electrons.
A term describing a type of solid in which the constituent parts have a simple and definite geometric arrangement repeated in all directions.
A form of bonding in which two atoms share two pairs of valence electrons. Carbon is also capable of single bonds and triple bonds.
The relative ability of an atom to attract valence electrons.
An atom or group of atoms whose presence identifies a specific family of compounds.
Any chemical compound whose molecules are made up of nothing but carbon and hydrogen atoms.
Families of compounds formed by the joining of hydrocarbons with various functionalgroups.
Substances having the same chemical formula, but that are different chemically due to disparities in the arrangement of atoms.
A term describing the distribution of valence electrons that takes place in chemical bonding for most elements, which end up with eight valence electrons.
The study of carbon, its compounds, and their properties. (Some carbon-containing compounds, most notably oxides and carbonates, are not considered organic.)
A term describing a hydrocarbon in which each carbon is already bound to four other atoms. Alkanes are saturated hydrocarbons.
A form of bonding in which two atoms share one pair of valence electrons. Carbon is also capable of double bonds and triple bonds.
Branches of alkanes, named by taking the name of an alkane and replacing the suffix with yl—for example, methyl, ethyl, and so on.
Capable of bonding to four other elements.
A form of bonding in which two atoms share three pairs of valence electrons. Carbon is also capable of single bonds and double bonds.
A term describing a hydrocarbon in which the carbons involved in a multiple bond (a double bond or triple bond) are free to bond with other atoms. Alkenes and alkynes are bothunsaturated.
Electrons that occupy the highest principal energy level in an atom. These are the electrons involved in chemical bonding.
Organic chemistry is the chemistry of carbon compounds. All organic compounds contain carbon; however, there are some compounds of carbon that are not classified as organic. For example, salts such as carbonates (e.g., Na2CO3, CaCO3) and cyanides (e.g., NaCN, KCN) are usually designated as inorganic. Perhaps a more useful description might be: Organic compounds are compounds of carbon that usually contain hydrogen and that may also contain other elements such as oxygen, nitrogen, sulfur, phosphorus, or halogen (F, Cl, Br, or I). In any case, there are very few carbon compounds that are not organic, while there are millions that are.
Prehistoric civilizations obtained many useful chemicals from plants and animals. They were familiar with sugar, which they learned to ferment to make wine. Then they found that the wine could turn into vinegar. Ancient Egyptians used blue dye made from the indigo in madder root, and a royal purple dye extracted from a rare kind of mollusk. Soap was made by heating animal fat with base from wood ashes.
During the Middle Ages dry distillation of wood yielded mixtures of methyl alcohol, acetone, and acetic acid. Alchemists isolated cholesterol from gallstones, morphine from opium, and drugs such as quinine, strychnine, and brucine from various plants. Two hundred years ago chemists such as Antoine Lavoisier determined the elemental composition of many of these substances and noted that they all contained carbon and hydrogen, and that many also contained oxygen and nitrogen. It also appeared that there were two classes of materials: the mineral type (generally hard, high-melting, and noncombustible), and the organic type (often soft, liquid or low melting solids, and frequently easily combustible materials). Most organic chemicals could be burned to produce carbon dioxide; and any hydrogen present was converted to water (H2O). Because organic compounds had for centuries been isolated only from plants and animals, it was commonly believed that some "vital force" in living things was necessary to produce them. This belief persisted until 1828, when Friedrich Wöhler was able to make urea, a chemical found in the urine of animals, from the inorganic salt ammonium cyanate.
Since that time organic chemistry has grown into a vast and ever expanding field that encompasses millions of chemical compounds.
Scope of Organic Chemistry
The field of organic chemistry includes more than twenty million compounds for which properties have been determined and recorded in the literature. Many hundreds of new compounds are added every day. Much more than half of the world's chemists are organic chemists. Some new organic compounds are simply isolated from plants or animals; some are made by modifying naturally occurring chemicals; but most new organic compounds are actually synthesized in the laboratory from other (usually smaller) organic molecules. Over the years organic chemists have developed a broad array of reactions that allow them to make all kinds of complex products from simpler starting materials.
Singular Attributes of Carbon
When one considers the millions of chemical compounds that are known and notes that more than 95 percent of them are compounds of carbon, one realizes that carbon is unique. Why are there so many carbon compounds? It turns out that atoms of carbon are quite remarkable in a number of ways.
Carbon atoms form very strong bonds with other carbon atoms. The bonds are so strong that carbon can form long chains, some containing thousands of carbon atoms. (Carbon is the only element that can do this.)
A carbon atom forms four bonds, therefore carbon not only can form long chains, but it also forms chains that have branches. It is a major reason why carbon compounds exhibit so much isomerism. The simple compound decane (C10H22), for example, has 75 different isomers .
Carbon atoms can be bonded by double or triple bonds as well as single bonds. This multiple bonding is much more prevalent with carbon than with any other element.
Carbon atoms can form rings of various sizes. The rings may be saturated or unsaturated. The unsaturated 6-membered ring known as the benzene ring is the basis for an entire subfield of "aromatic" organic chemistry.
Carbon atoms form strong bonds not only with other carbon atoms but also with atoms of other elements. In addition to hydrogen, many carbon compounds also contain oxygen. Nitrogen, sulfur, phosphorus, and the halogens also frequently occur in carbon compounds.
Various kinds of functional groups occur widely among carbon compounds, and many different kinds of isomers are possible.
Compounds of carbon and hydrogen only are called hydrocarbons. These are the simplest compounds of organic chemistry. The most basic group of hydrocarbons are the alkanes, which contain only single bonds. The simplest member of the alkane series is methane, CH4, the main component of natural gas. The names of some alkanes are listed in Table 1. Alkanes sometimes
have ring structures. Since a 4-carbon chain of the alkane series is called butane, a ring of 4 carbon atoms is called cyclobutane.
Simple hydrocarbons that contain one or more double bonds are called alkenes. They are named like alkanes, but their names end in " –ene." The simplest alkene has two carbon atoms and is called ethene. A 3-carbon chain that has a double bond is called propene.
A 5-carbon hydrocarbon chain with a double bond is called pentene, and if the double bond links the second and third carbons, it is 2-pentene. Like cycloalkanes, alkenes have the general formula CnH2n. Alkenes having ring structures are called cycloalkenes. A 5-carbon ring with a double bond is called cyclopentene.
Hydrocarbons that contain one or more triple bonds are called alkynes, and is the name ending is "–yne." A 2-carbon alkyne is therefore named ethyne. (However, the compound is often referred to by its common name, which is acetylene. )
Compounds that contain double or triple bonds are said to be "unsaturated"—because they are not "saturated" with hydrogen atoms. Unsaturated compounds are reactive materials that readily add hydrogen when heated over a catalyst such as nickel. The reverse reaction also occurs. Heating ethane with steam is an important commercial process for making ethene (or ethylene). This is an important commercial process called "steam cracking."
When a 6-carbon ring contains 2 double bonds, it is called cyclohexadiene, but when it has 3 double bonds, it is not called cyclohexatriene; this is because a 6-carbon ring with three double bonds takes on a special kind of stability. The double bonds become completely conjugated and no longer behave as double bonds. The ring, known as a "benzene ring," is said to be aromatic.
The removal of a hydrogen atom from a hydrocarbon molecule leaves an alkyl group that readily attaches to a functional group, or forms a branch on a hydrocarbon chain. The groups are named after the corresponding hydrocarbons. For example, CH3– is named methyl; CH3CH2–, ethyl; CH2= CH–, ethenyl; CH3CH2CH2–, propyl; and so on. A benzene ring from which a hydrogen atom has been removed is often referred to as a phenyl. The branched molecules shown here would be given names as follows
Theoretically there is no limit to the length of hydrocarbon chains. Very large hydrocarbon molecules (polymers) have been made containing as many as 100,000 carbon atoms. However, such molecules are hard to make and very difficult to melt and to shape into useful products.
Hydrocarbons are obtained primarily from fossil fuels—especially petroleum and natural gas. Natural gas is a mixture that is largely methane mixed with varying amounts of ethane and other light hydrocarbons, while petroleum is a complex mixture of many different hydrocarbons. Coal, the other fossil fuel, is a much more complicated material from which many kinds of organic compounds, some of them hydrocarbons, can be obtained.
Alkane molecules are rather unreactive (except for being very flammable), but alkenes react with many other substances. When a drop of bromine is added to an alkene, for example, the deep orange color of the bromine immediately disappears as the bromine adds across the double bond to form a dibromo derivative. The double bond is called a "functional group" because its presence in a molecule causes reactivity at that particular site. There are a dozen or so functional groups that appear frequently in organic compounds. Some of the most common ones are listed in Table 2. The same molecule may contain several functional groups. Aspirin, for example, is both a carboxylic acid and an ester , and cholesterol is an alkene as well as an alcohol.
Isomers are molecules with the same molecular formula but different structures. There is only one structure for methane, ethane, or propane; but butane, C4H10, can have either of two different structure:
The linear molecule (1) is called butane, or normal butane (n -butane), whereas the branched molecule (2) is methylpropane (rather than 2-methylpropane, as the methyl group has to be in a 2-position). If the methyl group of (2) were attached to a terminal carbon, the resultant molecule would be the same as (1). Methylpropane (2) is also called iso butane.
In a conjugated system, there are alternating double and single bonds, allowing electrons to flow back and forth. Molecules that contain such conjugated systems are said to be stabilized by "resonance." In the benzene ring every other bond is a double bond, all the way around the ring. This results in a special kind of stabilization called "aromaticity," in which the electrons are delocalized and free to travel all around the ring. Certain ring compounds, like benzene, that contain such a conjugated system of double and single bonds are described as "aromatic."
Pentane has 3 isomers: pentane (or n -pentane), methylbutane (or iso- pentane), and dimethyl propane (or neo pentane). Hexane has 5 isomers: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane. Heptane has 9 different isomers, octane has 18, nonane has 35, and decane has 75. An increase in the number of carbon atoms greatly increases the possibilities for isomerism. There are more than 4,000 isomers of C15H32 and more than 366,000 isomers of C20H42. The formula C30H62 has more than 4 billion. Of course, most of them have never been isolated as pure compounds (but could be, if there were any point in doing it).
For molecules other than hydrocarbons, still other kinds of isomers are possible. The simple formula C2H6O can represent ethyl alcohol or dimethyl ether; and C3H6O could stand for an alcohol, an ether, an aldehyde , or a ketone (among other things). The larger a molecule is, and the greater the variety of atoms and functional groups it contains, the more numerous its isomers.
There is still another kind of isomerism that stems from the existence of "right-" and "left-handed" molecules. It is sometimes referred to as optical isomerism because the molecules that make up a pair of these isomers usually differ only in the way they rotate plane polarized light.
There are so many millions of organic compounds that simply finding names for them all is a major challenge. It was not until the late nineteenth century that chemists developed a logical system for naming organic compounds. Compounds had often been named according to their sources. The 1-carbon carboxylic acid, for example, was first obtained from ants, and so it was called formic acid, from the Latin word for ants (formicae ). The 2-carbon acid was obtained from vinegar (acetum in Latin), and was called acetic acid.
To bring some order to the naming process an international meeting was held in 1892 at Geneva, Switzerland. The group later became known as the International Union of Pure and Applied Chemistry (IUPAC). Its objective was to establish a naming process that would provide each compound with a unique and systematic name. An initial set of rules was adopted at that first meeting in Geneva, and IUPAC has continued that work. Its systematic naming rules are used by organic chemists all over the world. The names of the alkanes form the basis for the system, with functional groups usually being indicated with appropriate suffixes. Some examples are given in Table 2.
Organic chemistry is concerned with the many compounds of carbon, their names, their isomers, and their properties, but it is mostly concerned with their reactions. Organic chemists have developed a huge array of chemical reactions that can convert one organic compound to another. Some reactions involve addition of one molecule to another; some involve decomposition of molecules; some involve substitution of one atom or group by another; and some even involve the rearrangement of molecules, with some atoms moving into new positions. Some reactions require energy in the form of heat or radiation; and some require a special kind of catalyst or some sort
|NAMING ORGANIC COMPOUNDS|
|Functional Group||Type Compound||Example||IUPAC Name||Common Name|
|carboxyl||carboxylic acid||HCOOH||methanoic acid||formic acid|
|carboxyl||ester||HCOOCH2CH3||ethyl methanoate||ethyl formate|
of solvent. Of course, not all organic reactions are highly successful. One reaction might be a very simple one giving essentially 100 percent of the desired product; but another might be a complex multistep process yielding less than 5 percent overall of the wanted product.
Organic reactions can often give remarkable control as to what products should be formed. Adding water to propene for example, produces 2-propanol in the presence of acid, but it yields 1-propanol if treated first with B2H6 and then H2O2 in the presence of base.
Future Sources of Organic Chemicals
Fossil fuels have been our primary natural source for many organic chemicals for more than a century, but our fossil fuel resources are finite, and they are being rapidly depleted (especially oil and gas). What will be our sources of organic materials in the future? Since fossil fuels are nonrenewable resources, it is believed that the twenty-first century will see a shift toward greater dependence on renewable raw materials. The largest U.S. chemical company has a goal of becoming 25 percent based on renewable resources by 2010. It is already producing 1,3-propanediol from cornstarch using a gene-tailored E. coli bacterium. This diol is used in Du Pont's fiber Sorona, which is said to combine the best features of both polyester and nylon fibers. Succinic acid and polyhydroxybutyrate are also obtainable from renewable crops, and the list of such renewable raw materials is destined to grow. For example, ethylene (or ethene), CH2=CH2, which is a highly important commercial chemical used in making many industrial chemicals and polymers, is presently made by steam cracking of ethane obtained from oil or natural gas; but ethylene can also be made by dehydration of ethyl alcohol made by fermentation of sugar. Efforts are even being made to use biowaste materials, such as corn husks, nutshells, and wood chips as industrial raw materials.
Organic chemists often need to examine products for identification, purity analysis, or structure determination. There are some marvelous tools available to help them do these things. Chromatography , spectroscopy , and crystallography are especially widely used in organic chemistry.
Column chromatography, gas chromatography, and liquid chromatography are all important methods for separating mixtures of organic compounds. Spectroscopic tools include ultraviolet (UV), infrared (IR), nuclear magnetic resonance (NMR), and mass spectroscopy (MS), each capable of providing a different kind of information about an organic compound. Although it is limited to substances that can be prepared as pure crystals, x-ray crystallography is probably the ultimate tool for determining molecular structure.
Careers in Organic Chemistry
Some organic chemists are involved in basic research at government or academic institutions, but most have careers in industry. The industries vary from oil and chemical companies to industries producing food, pharmaceuticals, cosmetics, detergents, paints, plastics, pesticides, textiles, or other kinds of products. Many organic chemists work in laboratories, where they do various kinds of analysis or research, but many others do not. Some are teachers, or writers, or science librarians. Some study law and become patent attorneys; some study medicine and become medical researchers; and some study business and become administrators of companies, colleges, or other institutions. Organic chemistry is an enormous field full of many kinds of career possibilities.
see also Fossil Fuels; Lavoisier, Antoine; Organic Halogen Compounds; WÖhler, Friedrich.
Kenneth E. Kolb
Atkins, Robert C., and Carey, Francis A. (2002). Organic Chemistry: A Brief Course, 3rd edition. Boston: McGraw-Hill.
Bailey, Philip S., Jr., and Bailey, Christina A. (2000). Organic Chemistry: A Brief Survey of Concepts and Applications, 6th edition. Upper Saddle River, NJ: Prentice Hall.
Brown, William H., and Foote, Christopher S. (2002). Organic Chemistry, 3rd edition. San Diego: Saunders.
Fessenden, Ralph J.; Fessenden, Joan S.; and Logue, Marshall (1998). Organic Chemistry, 6th edition. Pacific Grove, CA: Brooks/Cole.
Solomons, T. W. Graham (1997). Fundamentals of Organic Chemistry, 5th edition. New York: Wiley.
Organic chemistry is the study of compounds of carbon. The name organic goes back to a much earlier time in history when chemists thought that chemical compounds in living organisms were fundamentally different from those that occur in nonliving things. The belief was that the chemicals that could be extracted from or that were produced by living organisms had a special "vitalism" or "breath of life" given to them by some supernatural being. As such, they presented fundamentally different kinds of problems than did the chemicals found in rocks, minerals, water, air, and other nonliving entities. The chemical compounds associated with living organisms were given the name organic to emphasize their connection with life.
In 1828, German chemist Friedrich Wöhler (1800–1882) proved that this theory of vitalism was untrue. He found a very simple way to convert chemical compounds from living organisms into comparable compounds from nonliving entities.
As a result of Wöhler's research, the definition of organic chemistry changed. The new definition was based on the observation that every compound discovered in living organisms had one property in common: they all contained the element carbon. As a result, the modern definition of organic chemistry—as the study of carbon compounds—was adopted.
Organic and inorganic chemistry
One important point that Wöhler's research showed was that the principles and techniques of chemistry apply equally well to compounds found in living organisms and in nonliving things. Nonetheless, some important differences between organic and inorganic (not organic) compounds exist. These include the following:
Major Organic Families and the Functional Groups They Contain
|Alkane||carbon-carbon singal bonds only: C-C|
|Alkene||at least one carbon-carbon double bond: C=C|
|Alkyne||at least one carbon-carbon triple bond: C≡C|
|Alcohol||hydroxyl group: C-OH|
|Ether||carbon-oxygen-carbon bonding: C-O-C|
|Aldehyde and ketone||carboxyl (C=O) group: C-C=O|
|Carboxylic acid||carboxylic (C=O) group:|
|Ester||ester (C=O) group:|
|Amine||amine (NH2) group: C-NH2|
1. The number of organic compounds vastly exceeds the number of inorganic compounds. The ratio of carbon-based compounds to non-carbon-based compounds is at least ten to one, with close to 10 million organic compounds known today. The reason for this dramatic difference is a special property of the carbon atom: its ability to join with other carbon atoms in very long chains, in rings, and in other kinds of geometric arrangements. It is not at all unusual for dozens, hundreds, or thousands of carbon atoms to bond to each other within a single compound—a property that no other element exhibits.
2. In general, organic compounds tend to have much lower melting and boiling points than do inorganic compounds.
3. In general, organic compounds are less likely to dissolve in water than are inorganic compounds.
4. Organic compounds are likely to be more flammable but poorer conductors of heat and electricity than are inorganic compounds.
5. Organic reactions tend to take place more slowly and to produce a much more complex set of products than do inorganic reactions.
Functional groups and organic families
The huge number of organic compounds requires that some system be developed for organizing them. The criterion on which those compounds are organized is the presence of various functional groups. A functional group is an arrangement of atoms that is responsible for certain characteristic physical and chemical properties in a compound. For example, one such functional group is the hydroxyl group, consisting of an oxygen atom and hydrogen atom joined to each other. It is represented by the formula —OH.
All organic compounds with the same functional group are said to belong to the same organic family. Any organic compound that contains a hydroxyl group, for instance, is called an alcohol. All alcohols are similar to each other in that: (1) they contain one or more hydroxyl groups, and (2) because of those groups, they have similar physical and chemical properties. For example, alcohols tend to be more soluble in water than other organic compounds because the hydroxyl groups in the alcohol form bonds with water molecules.
The simplest organic compounds are the hydrocarbons, compounds that contain only two elements: carbon and hydrogen. The class of hydrocarbons can be divided into subgroups depending on the way in which carbon and hydrogen atoms are joined to each other. In some hydrocarbons, for example, carbon and hydrogen atoms are joined to each other only by single bonds. A single bond is a chemical bond that consists of a pair of electrons. Such hydrocarbons are known as saturated hydrocarbons.
In other hydrocarbons, carbon and hydrogen atoms are joined to each other by double or triple bonds. A double bond consists of two pairs of electrons, and a triple bond consists of three pairs of electrons. The symbols used for single, double, and triple bonds, respectively, are —, =, and ≡. Hydrocarbons containing double and triple bonds are said to be unsaturated.
Hydrocarbons can also be open-chain or ring compounds. In an open-chain hydrocarbon, the carbon atoms are all arranged in a straight line, like a strand of spaghetti. In a ring hydrocarbon, the carbons are arranged in a continuous loop, such as a square, a pentagon, or a triangle.
or·gan·ic chem·is·try • n. the chemistry of carbon compounds (other than simple salts such as carbonates, oxides, and carbides).