More than any other element, carbon has tremendous variability in the ways it forms bonds and also in the number of other elements to which it is able to bond. Thus there are many, many more compounds of carbon, both naturally occurring and man-made, than there are compounds of all the other elements combined. This may account for the fact that we will be dealing with many more organic than inorganic molecules in this book; or perhaps it is because both the authors are organic chemists.
CHEMICAL STRUCTURES: DO WE HAVE TO?
In writing this book, our biggest problem was determining how much chemistry to include in its pages. Some people advised us to minimize the chemistry, to leave it out and just tell the stories. Especially, we’ve been told, do not draw any chemical structures. But it is the connection between chemical structures and what they do, between how and why a compound has the chemical properties it has, and how and why that affected certain events in history, that we find the most fascinating. While you can certainly read this book without looking at the structures, we think understanding the chemical structures makes the interwoven relationship between chemistry and history come alive.
Organic compounds are mainly composed of only a few types of atoms: carbon (with chemical symbol C), hydrogen (H), oxygen (O), and nitrogen (N). Other elements may be present as well; for example, bromine (Br), chlorine (Cl), fluorine (F), iodine (I), phosphorus (P), and sulfur (S) are also found in organic compounds. The structures in this book are generally drawn to illustrate differences or similarities between compounds; mostly all that is required is to look at the drawing. The variation will often be arrowed, circled, or indicated in some other way. For example, the only difference between the two structures shown below is in the position where OH is attached to a C; it’s pointed out by an arrow in each case. For the first molecule the OH is on the second C from the left; for the second molecule the OH is attached to the first C from the left.
Molecule produced by honeybee queen
Molecule produced by honeybee worker
This is a very small difference, but is hugely important if you happen to be a honeybee. Queen honeybees produce the first molecule. Bees are able to recognize the difference between it and the second molecule, which is produced by honeybee workers. We can tell the difference between workers and queens by looking at the bees.
(Courtesy of Raymond and Sylvia Chamberlin)
Bees use chemical signaling to tell the difference. We could say they see through chemistry.
Chemists draw such structures to depict the way atoms are joined to each other through chemical bonds. Chemical symbols represent atoms, and bonds are drawn as straight lines. Sometimes there is more than one bond between the same two atoms; if there are two it is a double bond and shown as =. When three chemical links exist between the same two atoms, it is a triple bond and drawn as ≡.
In one of the simplest organic molecules, methane (or marsh gas), carbon is surrounded by four single bonds, one to each of four hydrogen atoms. The chemical formula is given as CH4, and the structure is drawn as:
Methane
The simplest organic compound that has a double bond is ethene (also called ethylene) with a formula of C2H4 and the structure:
Ethylene
Here carbon still has four bonds—the double bond counts as two. Despite being a simple compound, ethylene is very important. It is a plant hormone that is responsible for promoting the ripening of fruit. If apples, for example, aren’t stored with appropriate ventilation, the ethylene gas they produce will build up and cause them to overripen. This is why you can hasten the ripening of a hard avocado or kiwi fruit by putting it in a bag with an already ripe apple. Ethylene produced by the mature apple increases the rate of ripening of the other fruit.
The organic compound methanol, also known as methyl alcohol or wood alcohol, has the formula CH4O. This molecule contains an oxygen atom and the structure is drawn as:
Methanol
Here the oxygen atom, O, has two single bonds, one connected to the carbon atom and the other to a hydrogen atom. As always,carbon has a total of four bonds.
In compounds where there is a double bond between a carbon atom and an oxygen atom, as in acetic acid (the acid of vinegar), the formula, written as C2H4O2, does not directly indicate where the double bond is. This is the reason we draw chemical structures—to show exactly which atom is attached to which other atom and where the double or triple bonds are.
Acetic acid
We can draw these structures in an abbreviated or more condensed form. Acetic acid could also be drawn as:
where not all the bonds are shown. They are, of course, still there, but these shortened forms are faster to draw and show just as clearly the relationships among atoms.
This system of drawing structures works well for smaller examples, but when the molecules get bigger, it becomes time consuming and difficult to follow. For example, if we return to the queen honeybee recognition molecule:
and compare it to a fully drawn-out version showing all the bonds, the structure would look like:
Fully drawn-out structure of the queen honeybee molecule
This full structure is cumbersome to draw and looks very cluttered. For this reason, we often draw compounds using a number of shortcuts, the most common of which is to leave out many of the H atoms. This does not mean that they are not there; we just do not show them. A carbon atom always has four bonds, so if it does not look as if C has four bonds, be assured it does—the ones that are not shown bond to hydrogen atoms.
Queen honeybee recognition molecule
As well, the carbon atoms are often shown joined at an angle instead of in a straight line; this is more indicative of the true shape of the molecule. In this format the queen honeybee molecule looks like this:
An even more simplified version leaves out most of the carbon atoms:
Here the end of a line and any intersection represent a carbon atom. All other atoms (except most of the carbons and hydrogens) are still shown. By simplifying in this manner, it is easier to see the difference between the queen molecule and the worker molecule.
It is now also easier to compare these compounds to those emitted by other insects. For example, bombykol, the pheromone or sex attractant molecule produced by the male silkworm moth, has sixteen carbon atoms (as opposed to the ten atoms in the honeybee queen molecule, also a pheromone), has two double bonds instead of one, and lacks the COOH arrangement.
It is particularly useful to leave out many of the carbon and hydrogen atoms when dealing with what are called cyclic compounds—a fairly common structure in which the carbon atoms form a ring. The following structure represents the molecule cyclohexane, C6H12:
Abbreviated or condensed version of the chemical structure of cyclohexane. Every intersection represents a carbon atom; hydrogen atoms are not shown.
If drawn out in full, cyclohexane would appear as:
The fully drawn-out chemical structure of cyclohexane showing all atoms and all bonds
As you can see, when we put in all the bonds and write in all the atoms, the resulting diagram can be confusing. When you get to more complicated structures such as the antidepressant drug Prozac, the fully drawn-out version (below) makes it really hard to see the structure.
Fully drawn-out structure of Prozac
But the simplified version is much clearer:
Prozac
Another term frequently used to describe aspects of a chemical structure is aromatic. The dictionary says that aromatic means “having a fragrant, spicy, pungent, or heady smell, implying a pleasant odor.”
Chemically speaking, an aromatic compound often does have a smell, although not necessarily a pleasant one. The word aromatic, when applied to a chemical, means that the compound contains the ring structure of benzene (shown below), which is most commonly drawn as a condensed structure.
Looking at the drawing of Prozac, you can see that it contains two of these aromatic rings. Prozac is therefore defined as an aromat
ic compound.
The two aromatic rings in Prozac
This is only a short introduction to organic chemical structures, but it is actually all that you need to understand what we describe in this book. We will compare structures to show how they differ and how they are the same, and we will show how extremely small changes to a molecule sometimes produce profound effects. Following the connections among the particular shapes and related properties of various molecules reveals the influence of chemical structures on the development of civilization.
1. PEPPERS, NUTMEG, AND CLOVES
CHRISTOS E ESPICIARIAS!—for Christ and spices—was the jubilant cry from Vasco da Gama’s sailors as, in May 1498, they approached India and the goal of gaining untold wealth from spices that for centuries had been the monopoly of the merchants of Venice. In medieval Europe one spice, pepper, was so valuable that a pound of this dried berry was enough to buy the freedom of a feudal laborer bound to the estate of a nobleman. Although pepper now appears on dinner tables all over the world, the demand for it and for the fragrant molecules of cinnamon, cloves, nutmeg, and ginger fueled a global search that ushered in the Age of Discovery.
A BRIEF HISTORY OF PEPPER
Pepper, from the tropical vine Piper nigrum, originating in India, is still the most commonly used of all spices. Today its major producers are the equatorial regions of India, Brazil, Indonesia, and Malaysia. The vine is a strong, woody climber that can grow up to twenty feet or more. The plants begin to bear a red globular fruit within two to five years and under the right conditions continue to produce for forty years. One vine can produce ten kilograms of the spice each season.
About three-quarters of all pepper is sold as black pepper, produced by a fungal fermentation of unripe pepper berries. White pepper, obtained from the dried ripe fruit after removal of the berry skin and pulp, makes up most of the remainder. A very small percentage of pepper is sold as green pepper; the green berries, harvested just as they are beginning to ripen, are pickled in brine. Other colors of peppercorn, such as are sometimes found in specialty stores, are artificially dyed or are really other types of berries.
It is assumed that Arab traders introduced pepper to Europe, initially by the ancient spice routes that led through Damascus and across the Red Sea. Pepper was known in Greece by the fifth century B.C. At that time its use was medicinal rather than culinary, frequently as an antidote to poison. The Romans, however, made extensive use of pepper and other spices in their food.
By the first century A.D., over half the imports to the Mediterranean from Asia and the east coast of Africa were spices, with pepper from India accounting for much of this. Spices were used in food for two reasons: as a preservative and as a flavor enhancer. The city of Rome was large, transportation was slow, refrigeration was not yet invented, and the problem of obtaining fresh food and keeping it fresh must have been enormous. Consumers had only their noses to help them detect food that was off; “best before” labels were centuries in the future. Pepper and other spices disguised the taste of rotten or rancid fare and probably helped slow further decay. The taste of dried, smoked, and salted food could also be made more palatable by a heavy use of these seasonings.
By medieval times much European trade with the East was conducted through Baghdad (in modern Iraq) and then to Constantinople (now Istanbul) via the southern shores of the Black Sea. From Constantinople spices were shipped to the port city of Venice, which had almost complete dominance of the trade for the last four centuries of the Middle Ages.
From the sixth century A.D., Venice had grown substantially by marketing the salt produced from its lagoons. It had prospered over the centuries as a result of canny political decisions that let the city maintain its independence while trading with all nations. Almost two hundred years of holy Crusades, starting in the late eleventh century, allowed the merchants of Venice to consolidate their position as the world’s spice kings. Supplying transport, warships, arms, and money to Crusaders from western Europe was a profitable investment that directly benefited the Republic of Venice. The Crusaders, returning from the warm countries of the Middle East to their cooler northern homelands, wanted to take with them the exotic spices they had come to appreciate on their journeys. Pepper may have initially been a novelty item, a luxury few could afford, but its ability to disguise rancidity, give character to tasteless dried food, and seemingly reduce the salty taste of salted food very soon made it indispensable. The merchants of Venice had gained a vast new market, and traders from all over Europe came to buy spices, especially pepper.
By the fifteenth century the Venetian monopoly of the spice trade was so complete and the profit margins so great that other nations started to look seriously at the possibility of finding competing routes to India—in particular a sea route around Africa. Prince Henry the Navigator, son of King John I of Portugal, commissioned a comprehensive shipbuilding program that produced a fleet of sturdy merchant ships able to withstand the extreme weather conditions found in the open ocean. The Age of Discovery was about to begin, driven in large part by a demand for peppercorns.
During the mid-fifteenth century Portuguese explorers ventured as far south as Cape Verde, on the northwestern coast of Africa. By 1483 the Portuguese navigator Diago Cão had explored farther south to the mouth of the Congo River. Only four years later, another Portuguese seaman, Bartholomeu Dias, rounded the Cape of Good Hope, establishing a feasible route for his fellow countryman, Vasco da Gama, to reach India in 1498.
The Indian rulers in Calicut, a principality on India’s southwest coast, wanted gold in return for their peppercorns, which was not what the Portuguese had in mind if they were to take over world dominance in pepper. So five years later, da Gama, returning with guns and soldiers, defeated Calicut and brought the pepper trade under Portuguese control. This was the start of a Portuguese empire that eventually extended eastward from Africa through India and Indonesia and westward to Brazil.
Spain had also set its sights on the spice trade, pepper in particular. In 1492 Christopher Columbus, a Genoese navigator convinced that an alternative, and possibly shorter, route to the eastern edge of India could be found by sailing westward, persuaded King Ferdinand V and Queen Isabella of Spain to finance a voyage of discovery. Columbus was right in some but not in all of his convictions. You can get to India by going westward from Europe, but it is not a shorter route. The then unknown continents of North and South America as well as the vast Pacific Ocean are considerable obstacles.
What is it in pepper that built up the great city of Venice, that ushered in the Age of Discovery, and that sent Columbus off to find the New World? The active ingredient of both black and white pepper is piperine, a compound with the chemical formula C17H19O3N and this structure:
Piperine
The hot sensation we experience when ingesting piperine is not really a taste but a response by our pain nerves to a chemical stimulus. How this works is not fully known, but it is thought to be due to the shape of the piperine molecule, which is able to fit onto a protein on the pain nerve endings in our mouths and other parts of the body. This causes the protein to change shape and sends a signal along the nerve to the brain, saying something like “Ow, that’s hot.”
The story of the hot molecule piperine and of Columbus does not end with his failure to find a western trade route to India. When he hit land in October 1492, Columbus assumed—or maybe hoped—that he had reached part of India. Despite the lack of grand cities and wealthy kingdoms that he had expected to find in the Indies, he called the land he discovered the West Indies and the people living there Indians. On his second voyage to the West Indies, Columbus found, in Haiti, another hot spice. Though it was totally different from the pepper he knew, he nevertheless took the chili pepper back to Spain.
The new spice traveled eastward with the Portuguese around Africa to India and beyond. Within fifty years the chili pepper had spread around the world and was quickly incorporated into local cuisines, especially those o
f Africa and of eastern and southern Asia. For the many millions of us who love its fiery heat, the chili pepper is, without a doubt, one of the most important and lasting benefits of Columbus’s voyages.
HOT CHEMISTRY
Unlike the single species of peppercorn, chili peppers grow on a number of species of the Capsicum genus. Native to tropical America and probably originating in Mexico, they have been used by humans for at least nine thousand years. Within any one species of chili pepper, there is tremendous variation. Capsicum annuum, for example, is an annual that includes bell peppers, sweet peppers, pimentos, banana peppers, paprika, cayenne peppers, and many others. Tabasco peppers grow on a woody perennial, Capsicum frutescens.
Chili peppers come in many colors, sizes, and shapes, but in all of them the chemical compound responsible for their pungent flavor and often intense heat is capsaicin, with the chemical formula C18H27O3N and a structure that has similarities to that of piperine:
Both structures have a nitrogen atom (N) next to a carbon atom (C) doubly bonded to oxygen (O), and both have a single aromatic ring with a chain of carbon atoms. That both molecules are “hot” is perhaps not surprising if the hot sensation results from the shape of the molecule.
Penny le Couteur & Jay Burreson Page 2