organic chemistry

n.

The chemistry of carbon compounds.

Concept

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.

Electronegativity

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 CO_2, 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

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.

Real-Life Applications

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.

Hydrocarbons

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.)

Alkanes

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 C_nH_2n+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: C_nH_2n. 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 C_nH_2n-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

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.

Petrochemicals

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.

Hydrocarbon Derivatives

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

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.

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The study of the structure, preparation, properties, and reactions of carbon compounds. The term organic was early applied to compounds derived from plant and animal sources. These substances from living systems were usually distillable liquids or low-melting solids and were flammable, in contrast to metals, salts, and oxides from mineral sources. Until about 1830 it was held by some that organic compounds contained some special quality, or vital force. This notion was dispelled, but the term organic remained and became broadened to include carbon compounds in general. See also Carbon.

Structure

The structures of organic compounds are described by a molecular framework of carbon atoms on which substituents may be located at various points.

Structures can be represented in several ways, as illustrated for the three-carbon alcohol 2-propanol (1) and the cyclic ketone 2-methyl-3-cyclohexenone (2) 1a

1b 1c 2 The expanded structure of 2-propanol (1a) shows all bonds and electron pairs, including unshared electrons on oxygen. More compact and convenient is the condensed structure (1b) in which the CC and CH bonds are implied. In the bond-line convention (1c), all CC bonds are indicated by a line, as shown for 2-propanol. Carbon atoms are not shown explicitly, but rather are implied at the ends of each line segment, together with enough hydrogen atoms to complete the tetravalency at each carbon. The bond-line convention is particularly convenient for cyclic structures such as 2-methyl-3-cyclohexenone; each vertex and the end of each line segment represents a carbon and appropriate number of hydrogens. See also Structural chemistry; Valence.

A functional group is an atom other than carbon or a multiple bond, such as the hydroxyl group (OH) of 2-propanol, the double bond (C&dbnd;C), or the carbonyl group (C&dbnd;O) of 2-methyl-3-cyclohexenone. The group defines a class of compounds and is the point at which characteristic reactions occur, for example, oxidation, reduction, or addition of an electrophilic or nucleophilic reagent. Some of the principal functional groups are shown in the table. See also Electrophilic and nucleophilic reagents.

The fact that there can be two or more compounds, known as isomers, with the same molecular composition was one of the key points in development of a structural theory. One type of isomerism, structural or constitutional, is illustrated by the two isomers that have the formula C₄H₁₀, butane (3a) and isobutane (2-methylpropane; 3b). The number of possible structural isomers becomes enormous in larger molecules. 3a

3b See also Molecular isomerism.

Several three-dimensional representations of butane, showing the tetrahedral geometry of the carbon atoms, are given in structures (4). As indicated 4a

4b 4c in these structures, butane can exist in several forms, called conformations, which differ in the relative positions of the carbon atoms, and thus the overall shape of the molecule. However, the barrier to rotation around the central CC bond is so low that these individual conformational isomers are not separable, and butane is thus a single compound. See also Conformational analysis.

In an alkene, rotation around the C&dbnd;C bond does not occur, and 2-butene, for example, exists as two isomeric compounds, cis (Z) and trans (E) (5a and 5b, respectively). 5a

5b

Stereoisomers are compounds that have the same bond sequence but differ in the spatial array of the bonds. When a carbon atom is bonded to four unlike atoms or groups, the tetrahedral geometry of carbon causes the atom to be dissymmetric or chiral. A compound with a chiral atom can exist in two isomeric forms, known as enantiomers. The relative positions of all atoms is identical in the two enantiomers, but they differ in handedness, a characteristic of an asymmetric object and its nonsuperposable mirror image, as in structures (6) of 1-chlorobutane. 6a

6b See also Stereochemistry.

When two chiral centers are present, two stereoisomers can arise from each enantiomer. Thus enantiomer (7a) of chlorobutane can lead to isomeric structures (7b) and (7c), 7

in which the relative positions of the atoms is not identical. In this case, the isomers are known as diastereoisomers. With n chiral centers, there can be 2ⁿ stereoisomers.

Acyclic compounds

The simplest organic compound is methane (CH₄). It is the first member of the homologous series of alkanes, in which successive compounds differ by an additional CH₂ group (CH₃CH₃, CH₃CH₂CH₃, and so forth). See also Alkane; Methane.

Higher alkanes, CH₃(CH₂)_nCH₃ (n = 3–20), and also branched isomers and cyclic hydrocarbons are the principal components of petroleum. These compounds have no reactive functional groups.

Both acyclic and cyclic carbon frameworks can contain multiple bonds; oxygen, nitrogen, and sulfur atoms; and other functional groups listed in the table.

Carbocyclic compounds

The two large groups of compounds with rings containing only carbon are alicyclic and aromatic. The parent hydrocarbons in the former series are cycloalkanes and in the latter, benzene. The structure of benzene is a planar six-numbered ring with six electrons in a delocalized array. See also Aromatic hydrocarbon; Benzene.

Heterocyclic compounds

A nitrogen, oxygen, or sulfur atom can take the place of carbon in either alicyclic or aromatic rings. The most numerous and important hetrocyclic compounds are those with nitrogen in a five- or six-membered aromatic system.

Synthesis reactions

The preparation of compounds occupies much of the effort of organic chemistry, and is the principal business of the chemical industry. The manufacture of drugs, pigments, and polymers entails the preparation of organic compounds on a scale of thousands to billions of kilograms per year, and there is constant research to develop new products and processes. Synthesis of new substances is carried out for many purposes beyond the goal of a commercial product. A compound of a specified structure may be needed to test a mechanistic proposal or to evaluate a biochemical response such as inhibition of an enzyme. Synthesis may provide a more dependable and less expensive source of a naturally occurring compound; moreover, a synthetic approach permits variations in the structure that may lead to enhanced biological activity.

The term synthesis usually implies a planned sequence of steps leading from simple starting compounds to a desired end product. Each of these steps involves a reaction that may lead to formation of a CC bond or to the introduction, alteration, or removal of a functional group. Progress in synthesis depends on the availability of a wide range of reactions that bring about these changes in good yield, with a minimum of interfering by-products. An integral part of synthesis is the development of new methods and reagents that are selective for a desired transformation, and, very importantly, proceed with control of the stereochemistry. See also Asymmetric synthesis; Organic synthesis.

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The molecular science that deals primarily with materials constructed of carbon and hydrogen atoms. See organic compound.

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The branch of chemistry dealing with organic molecules.

Chemists often heat substances to see what will happen. Some solids melt; other solids and some liquids vaporize. If the liquid or vapor is trapped and the heat lowered, the same material comes back. But some substances burn, char, or coagulate. After that happens, cooling will not bring back the original substance. For the most part, this second class of materials can be recognized as products of living creatures, while the class that only melts includes metals and other items produced from the earth. In 1807 Jöns Jakob Berzelius named the first class--those that melt--inorganic and the second--those that burn--organic.

Although it was known that new chemicals could be produced from organic sources--Pierre-Eugene-Marcelin Berthelot made the first of these synthetics as early as 1778--it was widely believed at the beginning of the 19th century that organic chemicals could not be synthesized from inorganic ones. Consequently, Friedrich Wöhler was astonished when in 1828 he heated some ammonium cyanate, classed as inorganic, and got urea, an organic chemical. We now know that ammonium cyanate is not, strictly speaking, inorganic, but Wöhler generally gets the credit for the first synthesis from inorganic material. People were astounded in his time. The popular idea that some mysterious vital principle was in organic chemicals had been dealt a severe blow.

Even if the vitalists were wrong, it was clear that organic chemicals were very different in some ways from inorganic chemicals. For one thing, chemicals that seemed to be exactly the same behaved differently. Jean-Baptiste Biot observed in 1815 that tartaric acid produced by grapes polarized light, while seemingly the same acid produced in the laboratory did not polarize light--but both acids had the same chemicals in the same proportions, or the same chemical formula. Justus von Liebig and Wöhler in the 1820s analyzed various different organic compounds, finding several apparently different substances that had exactly the same chemical formulas. In 1830 Berzelius named such pairs of compounds isomers.

Louis Pasteur's first major project as a young chemist was to try to unravel the mystery of why the two varieties of tartaric acid behaved differently. He observed a tiny difference among the crystals of the type of tartaric acid that did not affect light. After painstakingly separating the crystals into two groups, he discovered that tartaric acid from one group of crystals polarized light just the way tartaric acid from grapes did. Tartaric acid from the other group also polarized light, but in the opposite direction. Pasteur correctly realized in 1844 that the two types of polarization cancelled each other out in the laboratory-made substance. He also understood that two different organic chemicals might have different properties and the same formula because the shape of the molecule might be different between the isomers.

In 1845 Adolph Wilhelm Hermann Kolbe became the first chemist to synthesize an organic compound (acetic acid) directly from chemical elements. Shortly after, the concepts of valence and bonding were introduced into chemistry. Friedrich Kekulé began to use diagrams based on bonding in organic chemistry in 1861. Kekulé's diagrams showed that Pasteur was correct. The shape of an organic molecule determines its properties.

Even before a basic understanding of organic molecules developed, chemists were beginning to synthesize new organic compounds with important properties not available in inorganics. The first such synthetic, nitrocellulose, was found by accident by Christian Schönbein in 1846. Also known as guncotton, it was very explosive. Nitrocellulose was discovered when Schönbein's wife's apron, which he had used to wipe up a spilled mixture of acids, exploded and vanished in a puff of smoke. When others tried to manufacture guncotton in quantity, many were killed by premature explosions. Another synthetic discovered soon after guncotton, nitroglycerine, was only marginally safer. Eventually, however, both substances were tamed into cordite and dynamite. The modern age of high explosives was at hand.

Ten years after the discovery of nitrocellulose, a young Englishman accidentally started another industry. William Perkin was trying to synthesize quinine when he produced the first synthetic dye, which we know as mauve. Perkin got rich on mauve and then went on to synthesize other chemicals. In 1875 he started his second industry by creating the first synthetic perfume ingredient, coumarin.

Although Perkin was English, he was something of an anomaly, for most of the organic chemists of the second half of the 19th century were German. In fact, Perkin's chemistry teacher, August Wilhelm von Hofmann, was a German chemist teaching in England. Hofmann synthesized his first dye, magenta, in 1858. After he returned to Germany, Hofmann continued to work on dyes and developed a number of violets. Other chemists in Germany worked on synthesizing exact copies of natural dyes from easily available chemicals, obtaining a red called alizarin in 1869 and indigo in 1880. All of these dyes became the basis of an immense German chemical industry. They also had an impact on biology, for biologists discovered that coloring bacteria or other cells with dyes made previously invisible structures apparent.

Another group of organic chemicals got their start in England and the United States. In 1865 English chemist Alexander Parkes found a way to convert nitrocellulose to a nonexplosive (but still quite flammable) substance that we know as celluloid, the first plastic. This was improved on by American inventor John Wesley Hyatt, who was looking for a replacement for ivory billiard balls. In the 20th century, English and American chemists continued to dominate the plastics industry, creating rayon, Bakelite, nylon, Teflon, Lucite, and polyester, among other synthetics.

In the late 19th century and the early 20th, the raw materials for most of these synthetics were coal, water, and air. Later in the 20th century, petroleum replaced coal, for there are generally fewer steps in a chemical process that starts with petroleum.

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