Organic chemistry

Carbon, in its endless variety of compounds, is the essential ingredient of such varied everyday products as nylon and gasoline, perfume and plastic, DDT and TNT. Carbon atoms have a unique ability to form strong chemical bonds both with themselves and with a large variety of atoms of other elements. Equally unique is their ability to form giant, chainlike molecules called polymers. The extended structures of these molecules repeat a pattern of carbon-atom combinations many, many times. Without carbon bonding, life on earth could never have arisen. All the fundamental substances in living organisms, such as proteins and carbohydrates, are based on carbon-atom (“organic’1 chemistry.

Organic chemistry embraces not only substances produced by living organisms but also a vast array of synthetic chemicals, including many important in industry. Nearly all plastics and synthetic fibers are organic chemicals, as are such diverse materials as dry-cleaning fluids, artificial sweeteners, pesticides, and many pharmaceuticals.

Many of today’s important organic chemicals are produced from fossil fuels—mainly oil and natural gas, the remains of once-living matter. But when supplies of these raw materials run short, the same chemicals can still be produced from fossil-fuel by-products, such as carbon monoxide gas.

Carbon’s combining power

The outer shell of a carbon atom has four electrons and so is exactly half full. A single carbon atom can thus form four two-electron bonds, attaching itself to as many as four other atoms. By obtaining a share of one electron from each of the four atoms bonded to it, the carbon atom fills the four vacancies in its outer shell. It can also obtain an octet of electrons by bonding with just one other atom or from any number of atoms up to four.

Not only can a carbon atom form bonds with other carbon atoms, it can also bond readily with hydrogen atoms and with other atoms of the periodic table. Many organic substances are made up of the elements carbon, hydrogen, oxygen, and/or nitrogen. Other elements common in organic compounds include sulfur, phosphorus, and the halogens (fluorine, chlorine, bromine, and iodine).

Chemists have combined carbon atoms with atoms of most other elements. A whole new branch of chemistry—organometallic chemistry—has developed from this, devoted to the study of substances based on carbon-metal bonds (many of which are used in industry as catalysts).

The ability of carbon atoms to link together by more than a single bond is of fundamental importance. Two carbon atoms can be joined by two or even three pairs of electrons, forming double or triple carbon-carbon bonds. Carbon atoms joined by multiple bonds readily enter into combination with other atoms.

In simple compounds such as ethene (ethylene), the reactivity of the carbon-carbon double bond is put to great use in making a wide variety of industrially important chemicals.

In many of the more complex organic substances, double bonds are responsible for the color of the substance. Many organic compounds are brightly colored. Some, such as the green plant pigment chlorophyll, occur naturally. Others are synthesized for use as dyes and colorants. All colored organic compounds share one feature: They consist of sequences of atoms linked by double bonds (called chromophores), either in chains or rings. By altering the number and sequence of doubly bonded atoms, chemists can vary the colors of compounds.

Oil and natural gas are obtained by drilling. They are the chief starting materials for a wide range of important organic chemicals, such as plastics, pesticides, dyes, synthetic textiles, and some pharmaceuticals.

Natural pigments

Many naturally occurring organic pigments display a basic similarity: All are aromatic compounds in which oxygen atoms are essential to the molecular structure. Natural pigments account for most of the yellow, red, and blue colors in flowers and fruits. A group called catechins (yellow, powdery acid compounds) are used as dyestuffs and in special inks. Another group are the leucoanthocyanidins, of which indigo, the dye used in blue jeans, is probably the best known. It is obtained from the indigo plant and has been used as a dye since ancient times.

Indigo and Tyrian purple have both been used as dyes since ancient times, indigo was originally obtained from various species of the indigo plant [ln-digofera spj, principally from Indigofera tinctoria This plant is native to parts of eastern Asia and Central America. In 1897, however, the dye was synthesized. Today, almost all indigo is obtained synthetically. The structure of indigo is very similar to that of Tyrian purple, with one exception— Tyrian purple contains two bromine (Br) atoms. This can be seen by comparing the structures in the diagram. The presence of these bromines has the effect of displacing the color of the compound toward the red. This results in the purple color of Tyrian purple as opposed to the dark blue of indigo. Like indigo, Tyrian purple was originally obtained from a natural source—the aquatic mol-lusk Murex brandaris (the shell of which is shown here). Today, this mollusk has been superseded by synthetic dyes.

Flavonols, flavones, and anthocyanins make up a third group. Flavonols and flavones account for the ivory and yellow colors of many plants, while anthocyanins are responsible for most of the red and blue colors.

Naphthoquinones and anthraquinones make up a fourth group. A familiar example of the former is lawsone, the substance in the leaves of the henna shrub that gives the orange-red color to henna dye. Carminic acid, a typical anthraquinone, is the principal pigment in cochineal, the scarlet dye obtained from the dried, pulverized bodies of the insect Coccus cacti.

Other common natural pigments include chlorophyll, carotene, and rhodopsin. Green chlorophyll plays a key role in photosynthesis, the process by which plants use the energy of the sun to manufacture nutrients. Carotenoids are responsible for the orange, yellow, and red pigments in many flowers and animals. A well-known example is beta-carotene, which gives carrots their orange color. Rhodopsin, also known as visual purple, occurs in the retina of the eye and is essential to sight.

Computer-generated simulation of the molecular structure of an organic compound can be examined on a display terminal. Computer modeling programs are vital to the development and synthesis of new drugs.

Improving on nature

Many complex organic chemicals are produced by synthetic chemists who combine chemical elements and compounds to duplicate naturally occurring substances. They also produce compounds that do not occur naturally, including drugs, pesticides, and dyestuffs. Azo dyes, synthesized (produced artificially) in the laboratory, are distinguished by the presence of at least one nitrogen-nitrogen double bond in their structures. Well-known examples are Congo red, a brown-red compound, and butter yellow, a vibrant orange substance.

Many of the most widely used azo dyes contain sulfur (in the form of sulfonate groups) in their structures. Sulfonate groups make the dyes soluble in water and also bind the dyes tightly to the large complex molecules of textiles. Because some azo dyes change color when reacted with an acid or base, they are used by chemists to help identify the substances present in a solution.

The molecular structures of natural substances are studied and altered to create products better suited to our needs. Chemists create variations of naturally occurring penicillin that can be used for different kinds of therapeutic treatment. Similarly, the chemistry of a naturally occurring insecticide in pyrethrum flowers has been modified to give a range of synthetic insecticides that are more useful than the natural insecticide.

Chemists construct complex organic compounds from simpler ones using a variety of reactions. These reactions are the techniques of organic synthesis, the artificial production of organic compounds. To make just one compound, a chemist may have to perform a long series of reactions, using many different reagents. (Reagents are chemical substances used to start or change chemical reactions.)

Chemists have several reasons for synthesizing a compound. A specific organic product such as a drug may be in demand commercially. Drugs are often synthesized by carefully constructed, complex synthetic routes. Or a chemist may wish to know the structure and identity of a natural product isolated from a plant or an animal. Because some natural products possess valuable properties (perhaps medicinal or industrial), their structures must be determined. Chemists do this by synthesizing compounds that correspond exactly to the natural product, both in chemical reactions and physical properties. If the compound proves to be identical with the natural product, this confirms its structure. A third reason is that chemists often make completely new compounds to test theories of reactivity in chemistry. New compounds may also turn out to have important properties.

Advances in computer technology are making synthesis much easier. Huge volumes of information can be stored and processed by computers, including details of synthetic methods, alternative methods of changing chemical groupings, and data on how different chemical agents behave in different chemical reactions.

Chemists have made a number of semisynthetic forms of the drug penicillin. Most of them have one or more properties that penicillin G, the most effective of the early penicillin forms, lacks.

Computers are also valuable in scaling-up and processing operations. Traditionally, scale-up follows a familiar pattern. Having established the need to make a particular product, one or two synthetic routes are investigated. The most favorable route is then developed into a process that can be carried out in a production (scaled-up) environment.

Computers can also be used to regulate and monitor reaction conditions. And the remote operation of a computer permits safe handling of toxic raw materials, intermediates, and products. Hazardous reactions can also be supervised from a safe distance.

Azo dyes, synthetic dyes created in the laboratory, are used in a wide variety of products, including printers inks.