We are only beginning to understand the unique properties of some naturally occurring members of the phenol family. Tetrahydrocannabinol (THC), the active ingredient in marijuana, is a phenol found in Cannabis sativa, the Indian hemp plant. Marijuana plants have been grown for centuries for the strong fibers found in the stem, which make excellent rope and a coarse cloth, and for the mildly intoxicating, sedative, and hallucinogenic properties of the THC molecule found—in some cannabis varieties—in all parts of the plant but most often concentrated in the female flower buds.
Tetrahydrocannabinol, the active ingredient of marijuana
Medicinal use of the tetrahydrocannabinol in marijuana to treat nausea, pain, and loss of appetite in patients suffering from cancer, AIDS, and other illnesses is now permitted in some states and countries.
Naturally occurring phenols often have two or more OH groups attached to the benzene ring. Gossypol is a toxic compound, classified as a polyphenol because it has six OH groups on four different benzene rings.
The gossypol molecule. The six phenol (OH) groups are indicated by arrows.
Extracted from seeds of the cotton plant, gossypol has been shown to be effective in suppressing sperm production in men, making it a possible candidate for a male chemical birth control method. The social implications of such a contraceptive could be significant.
The molecule with the complicated name of epigallocatechin-3-gallate, found in green tea, has even more phenolic OH groups.
The epigallocatechin-3-gallate molecule in green tea has eight phenolic groups.
It has recently been credited with providing protection against various types of cancer. Other studies have shown that polyphenolic compounds in red wine inhibit the production of a substance that is a factor in hardening of the arteries, possibly explaining why in countries where a lot of red wine is consumed there is a lower incidence of heart disease, despite a diet rich in butter, cheese, and other foods high in animal fat.
PHENOL IN PLASTICS
As valuable as the many different derivatives of phenol are, however, it is the parent compound, phenol itself, that has brought about the greatest changes in our world. As useful and influential as the phenol molecule was in the development of antiseptic surgery, it had a vastly different and possibly even more important role in the growth of an entirely new industry. Around the same time Lister was experimenting with carbolic acid, the use of ivory from animals for items as diverse as combs and cutlery, buttons and boxes, chessmen and piano keys was rapidly increasing. As more and more elephants were killed for their tusks, ivory was becoming scarce and expensive. Concern for the reduction of the elephant population was most noticeable in the United States, not for the conservationist reasons that we espouse today but because of the exploding popularity of the game of billiards. Billiard balls require extremely high-quality ivory for the ball to roll true. They must be cut from the very center of a flaw-free animal tusk, and only one out of every fifty tusks provides the consistent density necessary.
In the last decades of the nineteenth century, as supplies of ivory dwindled, the idea of making an artificial material to replace it appeared sensible. The first artificial billiard balls were made from pressed mixtures of substances such as wood pulp, bone dust, and soluble cotton paste impregnated by or coated with a hard resin. The main component of these resins was cellulose, often a nitrated form of cellulose. A later and more sophisticated version used the cellulose-based polymer celluloid. The hardness and density of celluloid could be controlled during the manufacturing process. Celluloid was the first thermoplastic material—that is, one that could be melted and remolded many times in a process that was the forerunner of the modern injection-molding machine, a method of repeatedly reproducing objects inexpensively with unskilled labor.
A major problem with cellulose-based polymers is their flammability and, especially where nitrocellulose is involved, their tendency to explode. There is no record of exploding celluloid billiard balls, but celluloid was a potential safety hazard. In the movie business film was originally composed of a celluloid polymer made from nitrocellulose, using camphor as a plasticizer for improving flexibility. After a disastrous 1897 fire in a movie theater in Paris killed 120 people, projection booths were lined with tin to prevent the spread of fire if the film happened to ignite. This precaution, however, did nothing for the safety of the projectionist.
In the early 1900s, Leo Baekeland, a young Belgian immigrant to the United States, developed the first truly synthetic version of the material we now call plastic. This was revolutionary, since varieties of polymers that had been made up to this time had been composed, at least partly, of the naturally occurring material cellulose. With his invention Baekeland initiated the Age of Plastics. A bright and inventive chemist who had obtained his doctorate from the University of Ghent at the age of twenty-one, he could have settled for the security of an academic life. He chose instead to emigrate to the New World, where he believed the opportunities to develop and manufacture his own chemical inventions would be greater.
A growing scarcity of good quality ivory from elephant tusks was relieved by the development of phenolic resins like Bakelite. (Photo courtesy Michael Beugger)
At first this choice seemed a mistake, as despite working diligently for a few years on a number of possible commercial products, he was on the verge of bankruptcy in 1893. Then, desperate for capital, Baekeland approached George Eastman, the founder of the photographic company Eastman Kodak, with an offer to sell him a new type of photographic paper he had devised. This paper was prepared with a silver chloride emulsion that eliminated the washing and heating steps of image development and boosted light sensitivity to such a level that it could be exposed by artificial light (gaslight in the 1890s). Amateur photographers could thus develop their photographs quickly and easily at home or send them to one of the new processing laboratories opening across the country.
While he was taking the train to meet with Eastman, Baekeland decided he would ask for $50,000 for his new photographic paper, since it was a great improvement over the celluloid product with associated fire hazards that was then being used by Eastman’s company. If forced to compromise, Baekeland told himself he would accept no less than $25,000, still a reasonably large amount of money in those days. Eastman was so impressed with Baekeland’s photographic paper, however, that he immediately offered him the then-enormous sum of $750,000. A stunned Baekeland accepted and used the money to establish a modern laboratory next to his home.
With his financial problem solved, Baekeland turned his attention to creating a synthetic version of shellac, a material that had been used for many years as a lacquer and wood preservative and is still used today. Shellac is obtained from an excretion of the female lac or Laccifer lacca beetle, a native of Southeast Asia. The beetles attach themselves to trees, suck the sap, and eventually become encased in a covering of their own secretion. After reproducing, the beetles die, and their cases or shells—hence the shell part of the word shellac—are gathered and melted. The liquid thus obtained is filtered to remove the bodies of the dead beetles. It takes fifteen thousand lac beetles six months to produce a single pound of shellac. As long as the shellac was used only as a thin coating, the price was affordable, but with increased use of shellac by the electrical industry, which was rapidly expanding at the beginning of the twentieth century, the demand for shellac soared. The cost of making an electrical insulator, even just using shellac-impregnated paper, was high, and Baekeland realized that artificial shellac would become a necessity for insulators in this increasing market.
Baekeland’s first approach to the problem of making a shellac involved the reaction of phenol—the same molecule with which Lister had successfully transformed surgery—and formaldehyde, a compound derived from methanol (or wood alcohol) and in those days used extensively as an embalming agent by undertakers and for preserving animal specimens.
Previous attempts to combine these compounds had produced discour
aging results. Rapid, uncontrolled reactions led to insoluble and infusible materials that were too brittle and inflexible to be useful. But Baekeland recognized that such properties could be just what was needed in a synthetic shellac for electrical insulators, if only he could control the reaction so the material could be processed into a usable form.
By 1907, using a reaction in which he was able to control both heat and pressure, Baekeland had produced a liquid that rapidly hardened into a transparent, amber-colored solid conforming exactly to the shape of the mold or vessel into which it was poured. He named the material Bakelite and called the modified pressure cooker-like device used to produce it the Bakelizer. We can perhaps excuse the self-promotion inherent in these names when we consider that Baekeland had spent five years working with this one reaction in order to synthesize this substance.
While shellac would become distorted with heat, Bakelite retained its shape at high temperatures. Once set, it could not be melted and remolded. Bakelite was a thermoset material; that is, it was frozen in its shape forever, as opposed to a thermoplastic material like celluloid. This phenolic resin’s unique thermoset property was due to its chemical structure: formaldehyde in Bakelite can react at three different places on the benzene ring of phenol, causing cross-links among the polymer chains. Rigidity in Bakelite is attributed to these very short cross-links attached to already rigid and planar benzene rings.
Schematic formula for Bakelite showing -CH2- cross-links between phenol molecules. These are only some possible ways of linking; in the actual material the linkages are random.
When used for electrical insulators, Bakelite’s performance was superior to that of any other material. It was more heat resistant than shellac or any shellac-impregnated paper versions; it was less brittle than ceramic or glass insulators; and it had better electrical resistance than porcelain or mica. Bakelite did not react with sun, water, salt air, or ozone and was impervious to acid and solvents. It didn’t easily crack, chip, discolor, fade, burn, or melt.
Subsequently, though not the original intent of its inventor, Bakelite was found to be the ideal material for billiard balls. Bakelite’s elasticity was very similar to that of ivory, and Bakelite billiard balls, when they collided, made the same agreeable clicking sound as did ivory billiard balls, an important factor that was lacking in celluloid versions. By 1912 almost all nonivory billiard balls were made of Bakelite. Numerous other applications followed, and within the space of a few years Bakelite was everywhere. Telephones, bowls, washing machine agitators, pipe stems, furniture, car parts, fountain pens, plates, glasses, radios, cameras, kitchen equipment, handles for knives, brushes, drawers, bathroom fittings, and even artworks and decorative items were all being made of Bakelite. Bakelite became known as the “material for a thousand uses”—though nowadays other phenolic resins have superseded the original brown material. Later resins were colorless and could be easily tinted.
A PHENOL FOR FLAVOR
The creation of Bakelite is not the only example where a phenol molecule was the basis of the development of an artificial substance to take the place of a natural substance for which demand had exceeded supply. The market for vanillin has long surpassed the supply available from the vanilla orchid. So synthetic vanillin is manufactured and comes from a surprising source: the waste pulp liquor from the sulfite treatment of wood pulp in the making of paper. Waste liquor consists mainly of lignin, a substance found in and between the cell walls of land plants. Lignin contributes to the rigidity of plants and makes up about 25 percent of the dry weight of wood. It is not one compound but a variable cross-bonded polymer of different phenolic units.
There is a difference in the lignin composition between softwoods and hardwoods as shown by the structures of the building blocks of their respective lignins. In lignin, as in Bakelite, the rigidity of wood depends on the degree of cross-linking between phenolic molecules. The triple-substituted phenols, found only in hardwoods, allow for more cross-linkages and thus explain why hardwoods are “harder” than softwoods.
A representative structure for lignin showing some of the cross-links between these building-block units is illustrated below. It has definite similarities to Baekeland’s Bakelite.
Part of the structure of lignin (left). The dotted lines indicate connection to the rest of the molecule. The structure of Bakelite (right) also has cross-links between phenol units.
The circled part in the drawing of lignin (below) highlights part of the structure that is very similar to the vanillin molecule. When a lignin molecule is broken up under controlled conditions, vanillin can be produced.
Lignin (left), with the circled part of its structure very similar to the vanillin molecule (right)
Synthetic vanillin is not just a chemical imitation of the real thing; rather, it is pure vanillin molecules made from a natural source and is chemically absolutely identical to vanillin from the vanilla bean. Vanilla flavoring obtained from using the whole vanilla bean does, however, contain trace amounts of other compounds that, together with the vanillin molecule, give the overall flavor of true vanilla. Artificial vanilla flavoring contains synthetic vanillin molecules in a solution containing caramel as a coloring agent.
Odd as it may sound, there is a chemical connection between vanilla and the phenol molecule, found as carbolic acid. Under great pressures and moderate temperatures over a long time, coal forms from decomposing plant materials, including, of course, lignin from woody tissues, as well as cellulose, another major component of vegetation. In the process of heating coal to obtain the important coal gas fuel for homes and industries, a black viscous liquid with an acrid smell is obtained. This is coal tar, Lister’s source of carbolic acid. His antiseptic phenol was ultimately derived from lignin.
It was phenol that first permitted antiseptic surgery, allowing operations to be performed without risk of life-threatening infection. Phenol changed the prospects of survival for thousands injured in accidents or wars. Without phenol and later antiseptics the amazing surgical feats of today—hip replacements, open-heart surgery, organ transplants, neurosurgery, and microsurgical repairs—would never have been possible.
By investing in Baekeland’s invention for photographic paper, George Eastman was able to offer a better film that, together with the introduction in 1900 of a very inexpensive camera—the Kodak Brownie, which sold for one dollar—changed photography from a pursuit of the wealthy to a hobby available to everyone. Eastman’s investment financed the development—with phenol as a starting material—of the first truly synthetic material of the Age of Plastics, Bakelite, used to make the insulators necessary for the widespread use of electrical energy, a major factor in the modern industrial world.
The phenols we have discussed have changed our lives in many large ways (antiseptic surgery, the development of plastics, explosive phenols) and in many small ways (potential health factors, spicy foods, natural dyes, affordable vanilla). With such a wide variety of structures it is likely that phenols will continue to shape history.
8. ISOPRENE
CAN YOU IMAGINE what the world would be like without tires for our cars and trucks and planes? Without gaskets and fan belts for our engines, elastic in our clothes, waterproof soles for our shoes? Where would we be without such mundane but useful items as rubber bands?
Rubber and rubber products are so common that we probably never think about what rubber is and how it has changed our lives. Although mankind has been aware of its existence for hundreds of years, only in the last century and a half has rubber become an essential component of civilization. Its chemical structure gives rubber its unique properties, and the chemical manipulation of this structure produced a molecule from which fortunes have been made, lives have been lost, and countries have been changed forever.
ORIGINS OF RUBBER
Some form of rubber was long known throughout most of Central and South America. The first use of rubber, for both decorative and practical purposes, is often attributed to Indian
tribes of the Amazon basin. Rubber balls from a Mesoamerican archaeological site near Veracruz, Mexico, date back to between 1600 and 1200 B.C. On his second voyage to the New World, in 1495, Christopher Columbus saw Indians on the island of Hispaniola playing with heavy balls made of a plant gum that bounced surprisingly high. “Better than those fill’d with Wind in Spain,” he reported, presumably referring to the inflated animal bladders used by the Spanish for ball games. Columbus brought some of this new material back to Europe, as did other travelers to the New World after him. The samples of rubber latex remained mainly a novelty item, however; they became sticky and smelly during hot weather and hard and brittle in European winters.
A Frenchman named Charles-Marie de La Condamine was the first to investigate whether there was a serious use for this strange substance. La Condamine—variously described as a mathematician, a geographer, and an astronomer, as well as a playboy and an adventurer—had been sent by the French Academy of Sciences to measure a meridian through Peru to help in determining whether the Earth was actually slightly flattened at the poles. After completing his work for the academy, La Condamine took the opportunity to explore the South American jungle, arriving back in Paris in 1735 with a number of balls of the coagulated gum from the caoutchouc tree (the “tree that weeps”). He had observed the Omegus Indians of Ecuador collecting the sticky white caoutchouc sap, then holding it over a smoky fire and molding it into a variety of shapes to make containers, balls, hats, and boots. Unfortunately, La Condamine’s samples of raw sap, which remained as latex when not preserved through smoking, fermented during shipping and arrived in Europe as a useless smelly mass.
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