Amylopectin forms the remaining 70 to 80 percent of starch. It also consists of long chains of α-glucose units joined between carbons number 1 and number 4, but amylopectin is a branched molecule with cross-linkages, between the carbon number 1 of one glucose unit and carbon number 6 of another glucose unit, occurring every twenty to twenty-five glucose units. The presence of up to a million glucose units in interconnecting chains makes amylopectin one of the largest molecules found in nature.
Part of the structure of amylopectin. The arrow shows the C#1 to C#6 α-cross-linkage responsible for the branching of amylopectin.
In starches, the α-linkage is responsible for other important properties besides our ability to digest them. Chains of amylose and amylopectin form into the shape of a helix rather than the tightly packed linear structure of cellulose. When water molecules have enough energy, they are able to penetrate into the more open helical coils; thus starch is water soluble whereas cellulose is not. As all cooks know, the water solubility of starch is strongly temperature dependent. If a suspension of starch and water is heated, granules of starch absorb more and more water until, at a certain temperature, the starch molecules are forced apart, resulting in a mesh of long molecules interspersed in the liquid. This is known as a gel. The cloudy suspension then becomes clear, and the mixture starts to thicken. Thus cooks use starch sources such as flour, tapioca, and cornstarch to thicken sauces.
The storage polysaccharide in animals is glycogen, formed mainly in the cells of the liver and skeletal muscle. Glycogen is a very similar molecule to amylopectin, but where amylopectin has carbon number 1 to carbon number 6 α-cross-linkages only every twenty or twenty-five glucose units, glycogen has these α-cross-links every ten glucose units. The resulting molecule is highly branched. This has a very important consequence for animals. An unbranched chain has only two ends, but a highly branched chain, with the same overall number of glucose units, has a large number of ends. When energy is needed quickly, many glucose units can be removed simultaneously from these many ends. Plants, unlike animals, do not need sudden bursts of energy to escape from predators or chase a prey, so fuel storage as the lesser branched amylopectin and unbranched amylose is sufficient for a plant’s lower metabolic rate. This small chemical difference, relating only to the number and not to the type of cross-link, is the basis for one of the fundamental differences between plants and animals.
The different branching in starch (amylose and amylopectin) compared with glycogen. The greater the branching, the greater the number of chain ends for enzymes to break down the linkages and the faster glucose can be metabolized.
CELLULOSE MAKES A BIG BANG
Although there is a very large amount of storage polysaccharide in the world, there is a lot more of the structural polysaccharide, cellulose. By some accounts half of all organic carbon is tied up in cellulose. An estimated 1014 kilograms (about a 100 billion tons) of cellulose is biosynthesized and degraded annually. As it is not only an abundant but also a replenishable resource, the possibility of using cellulose as a cheap and readily available starting material for new products long interested chemists and entrepreneurs.
By the 1830s it was found that cellulose would dissolve in concentrated nitric acid and that this solution, when poured into water, formed a highly flammable and explosive white powder. Commercialization of this compound had to wait until 1845 and a discovery by Friedrich Schönbein of Basel, Switzerland. Schönbein was experimenting with mixtures of nitric and sulfuric acids in the kitchen of his home, against the wishes of his wife, who perhaps understandably had strictly forbidden the use of her residence for such activities. On this particular day his wife was out, and Schönbein spilled some of the acid mixture. Anxious to clean up the mess quickly, he grabbed the first thing that came to hand—his wife’s cotton apron. He mopped up the spill and then hung the apron over the stove to dry. Before long, with an extremely loud bang and a great flash, the apron exploded. How Schönbein’s wife reacted when she came home to find her husband continuing his kitchen experiments on cotton and the nitric acid mix is not known. What is recorded is what Schönbein called his material—schiessbaumwolle, or guncotton. Cotton is 90 percent cellulose, and we now know that Schönbein’s guncotton was nitrocellulose, the compound formed when the nitro group (NO2) replaces the H of OH at a number of positions on the cellulose molecule. Not all these positions are necessarily nitrated, but the more nitration on cellulose, the more explosive is the guncotton produced.
The structure of part of a cellulose molecule. The arrows show where nitration can take place at the OH on C#2, 3, and 6 of each of the glucose units
A portion of the structure of nitrocellulose or “guncotton” showing nitration;—NO2 is substituted for -H at every possible OH position on each glucose unit of the cellulose.
Schönbein, recognizing the potential profit from his discovery, established factories to manufacture nitrocellulose, hoping it would become an alternative to gunpowder. But nitrocellulose can be an extremely dangerous compound unless it is kept dry and handled with proper care. At the time the destabilizing effect of residual nitric acid on the material was not understood, and thus a number of factories were accidentally destroyed by violent explosions, putting Schönbein out of business. It was not until the late 1860s, when proper methods were found to clean guncotton of excess nitric acid, that it could be made stable enough for use in commercial explosives.
Later, control of this nitration process led to different nitrocelluloses, including a higher-nitrate-content guncotton and the lower-nitrate-content materials collodion and celluloid. Collodion is a nitrocellulose mixed with alcohol and water and was used extensively in early photography. Celluloid, a nitrocellulose mixed with camphor, was one of the first successful plastics and was originally used as film for moving pictures. Another cellulose derivative, cellulose acetate, was found to be less flammable than nitrocellulose and quickly replaced it for many uses. The photography business and the movie industry, today enormous commercial enterprises, owe their beginnings to the chemical structure of the versatile cellulose molecule.
Cellulose is insoluble in almost all solvents but does dissolve in an alkaline solution of one organic solvent, carbon disulfide, forming a derivative of cellulose called cellulose xanthate. Cellulose xanthate is in the form of a viscous colloidal dispersion and was given the trade name of viscose. When viscose is forced through tiny holes and the resulting filament is treated with acid, the cellulose is regenerated in the form of fine threads that can be woven into a fabric known commercially as rayon. A similar process, where the viscose is extruded through a narrow slit, produces sheets of cellophane. Rayon and cellophane are usually considered to be synthetic textiles, but they are not totally man-made in the sense that they are just somewhat different forms derived from naturally occurring cellulose.
Both the α polymer of glucose (starch) and the β polymer (cellulose) are essential components of our diet and as such have had, and always will have, an indispensable function in human society. But it is the non-dietary roles of cellulose and its various derivatives that have created milestones in history. Cellulose, in the form of cotton, was responsible for two of the most influential events of the nineteenth century: the Industrial Revolution and the American Civil War. Cotton was the star of the Industrial Revolution, transforming the face of England through rural depopulation, urbanization, rapid industrialization, innovation and invention, social change, and prosperity. Cotton evoked one of the greatest crises in the history of the United States; slavery was the most important issue in the Civil War between abolitionist North and the southern states, whose economic system was based on slave-grown cotton.
Nitrocellulose (guncotton) was one of the very first explosive organic molecules made by man, and its discovery marked the start of a number of modern industries originally based on nitrated forms of cellulose: explosives, photography, and the movie business. The synthetic textile industry, with its beginnings from rayon—a diff
erent form of cellulose—has played a significant role in shaping the economy over the last century. Without these applications of the cellulose molecule, our world would be a very different place.
5. NITRO COMPOUNDS
SCHÖNBEIN’S WIFE’S exploding apron was not the first example of a man-made explosive molecule, nor would it be the last. When chemical reactions are very rapid, they can have an awesome power. Cellulose is only one of the many molecules we have altered to take advantage of the capacity for explosive reaction. Some of these compounds have been of enormous benefit; others have caused widespread destruction. Through their very explosive properties, these molecules have had a marked effect on the world.
Although the structures of explosive molecules vary widely, most often they contain a nitro group. This small combination of atoms, one nitrogen and two oxygens, NO2, attached at the right position, has vastly increased our ability to wage war, changed the fate of nations, and literally allowed us to move mountains.
GUNPOWDER-THE FIRST EXPLOSIVE
Gunpowder (or black powder), the first explosive mixture ever invented, was used in ancient times in China, Arabia, and India. Early Chinese texts refer to “fire-chemical” or “fire-drug.” Its ingredients were not recorded until early in A.D. 1000, and even then the actual proportions required of the component nitrate salt, sulfur, and carbon were not given. Nitrate salt (called saltpeter or “Chinese snow”) is potassium nitrate, chemical formula KNO3. The carbon in gunpowder was in the form of wood charcoal and gives the powder its black color.
Gunpowder was initially used for firecrackers and fireworks, but by the middle of the eleventh century flaming objects—used as weapons and known as fire arrows—were launched by gunpowder. In 1067 the Chinese placed the production of sulfur and saltpeter under government control.
We have no certainty as to when gunpowder arrived in Europe. The Franciscan monk Roger Bacon, born in England and educated at Oxford University and the University of Paris, wrote of gunpowder around 1260, a number of years before Marco Polo’s return to Venice with stories of gunpowder in China. Bacon was also a physician and an experimentalist, knowledgeable in the sciences that we would now call astronomy, chemistry, and physics. He was also fluent in Arabic, and it is likely that he learned about gunpowder from a nomadic tribe, the Saracens, who acted as middlemen between the Orient and the West. Bacon must have been aware of the destructive potential of gunpowder, as his description of its composition was in the form of an anagram that had to be deciphered to reveal the ratio: seven parts saltpeter, five parts charcoal, and five parts sulfur. His puzzle remained unsolved for 650 years before finally being decoded by a British army colonel. By then gunpowder had, of course, been in use for centuries.
Present-day gunpowder varies somewhat in composition but contains a larger proportion of saltpeter than Bacon’s formulation. The chemical reaction for the explosion of gunpowder can be written as
This chemical equation tells us the ratios of substances reacting and the ratios of the products obtained. The subscript (s) means the substance is a solid, and (g) means it is a gas. You can see from the equation that all the reactants are solids, but eight molecules of gases are formed: three carbon dioxide, three carbon monoxide, and two nitrogens. It is the hot, expanding gases produced from the rapid burning of gunpowder that propel a cannonball or bullet. The solid potassium carbonate and sulfide formed are dispersed as tiny particles, the characteristic dense smoke of exploding gunpowder.
Thought to have been produced somewhere around 1300 to 1325, the first firearm, the firelock, was a tube of iron loaded with gunpowder, which was ignited by the insertion of a heated wire. As more sophisticated firearms developed (the musket, the flintlock, the wheellock), the need for different rates of burning of gunpowder became apparent. Sidearms needed faster-burning powder, rifles a slower-burning powder, and cannons and rockets an even slower burn. A mixture of alcohol and water was used to produce a powder that caked and could be crushed and screened to give fine, medium, and coarse fractions. The finer the powder, the faster the burn, so it was possible to manufacture gunpowder that was appropriate for the various applications. The water used for manufacture was frequently supplied as urine from workers in the gunpowder mill; the urine of a heavy wine drinker was believed to create particularly potent gunpowder. Urine from a clergyman, or better yet a bishop, was also considered to give a superior product.
EXPLOSIVE CHEMISTRY
The production of gases and their consequent fast expansion from the heat of the reaction is the driving force behind explosives. Gases have a much greater volume than do similar amounts of solids or liquids. The destructive power of an explosion is due to the shock wave caused by the very rapid increase in volume as gases form. The shock wave for gunpowder travels around a hundred meters per second, but for “high” explosives (TNT or nitroglycerin, for example) it can be up to six thousand meters per second.
All explosive reactions give off large amounts of heat. Such reactions are said to be highly exothermic. The large amounts of heat act dramatically to increase the volume of the gases—the higher the temperature the larger the volume of gas. Heat comes from the energy difference between the molecules on each side of the explosive reaction equation. The molecules produced (on the right of the equation) have less energy tied up in their chemical bonds than the starting molecules (on the left). The compounds that form are more stable. In explosive reactions of nitro compounds, the extremely stable nitrogen molecule, N2, is formed. The stability of the N2 molecule is due to the strength of the triple bond that holds the two nitrogen atoms together.
Structure of the N2 molecule
That this triple bond is very strong means that a lot of energy is needed to break it. Conversely, when the N2 triple bond is made, a lot of energy is released, which is exactly what is wanted in an explosive reaction.
Besides production of heat and of gases, a third important property of explosive reactions is that they must be extremely rapid. If the explosive reaction were to occur slowly, the resulting heat would dissipate and the gases would diffuse into the surroundings without the violent pressure surge, damaging shock wave, and high temperatures characteristic of an explosion. The oxygen required for such a reaction has to come from the molecule that is exploding. It cannot come from the air, because oxygen from the atmosphere is not available quickly enough. Thus nitro compounds, in which nitrogen and oxygen are bonded together, are often explosive, while other compounds containing both nitrogen and oxygen, but not bonded together, are not.
This can be seen using isomers as an example, isomers being compounds that have the same chemical formula but different structures. Para-nitrotoluene and para-aminobenzoic acid both have seven carbon atoms, seven hydrogen atoms, one nitrogen atom, and two oxygen atoms for identical chemical formulae of C7H7NO2, but these atoms are arranged differently in each molecule.
Para- or p-nitrotoluene (the para just tells you that the CH3 and NO3 groups are at opposite ends of the molecule) can be explosive, whereas p-aminobenzoic acid is not at all explosive. In fact you have probably rubbed it over your skin in the summer; it is PABA, the active ingredient in many sunscreen products. Compounds such as PABA absorb ultraviolet light at the very wavelengths that have been found to be most damaging to skin cells. Absorption of ultraviolet light at particular wavelengths depends on the presence in the compound of alternating double and single bonds, possibly also with oxygen and nitrogen atoms attached. Variation in the number of bonds or atoms of this alternating pattern changes the wavelength of absorption. Other compounds that absorb at the required wavelengths can be used as sunscreens provided they also do not wash off easily in water, have no toxic or allergic effects, no unpleasant smell or taste, and do not decompose in the sun.
The explosiveness of a nitrated molecule depends on the number of nitro groups attached. Nitrotoluene has only one nitro group. Further nitration can add two or three more nitro groups, resulting in di- or trinitrotoluenes respecti
vely. While nitrotoluene and dinitrotoluene can explode, they do not pack the same power as the high-explosive trinitrotoluene (TNT) molecule.
The nitro groups are indicated by arrows.
Advances in explosives came about in the nineteenth century when chemists began studying the effects of nitric acid on organic compounds. Only a few years after Friedrich Schönbein destroyed his wife’s apron with his experiments, an Italian chemist, Ascanio Sobrero, of Turin, prepared another highly explosive nitro molecule. Sobrero had been studying the effects of nitric acid on other organic compounds. He dripped glycerol, also known as glycerin and readily obtained from animal fat, into a cooled mixture of sulfuric and nitric acids and poured the resulting mixture into water. An oily layer of what is now known as nitroglycerin separated out. Using a procedure that was normal in Sobrero’s time but unthinkable today, he tasted the new compound and recorded his comments: “a trace placed on the tongue but not swallowed gives rise to a most pulsating, violent headache, accompanied by great weakness of the limbs.”
Later investigations into the severe headaches suffered by workers in the explosives industry showed that these headaches were due to the dilation of blood vessels caused by handling nitroglycerin. This discovery resulted in the prescription of nitroglyerin for treatment of the heart disease angina pectoris.
For angina sufferers, dilation of previously constricted blood vessels supplying the heart muscle allows an adequate flow of blood and relieves the pain of angina. We now know that in the body nitroglycerin releases the simple molecule nitric oxide (NO), which is responsible for the dilation effect. Research on this aspect of nitric oxide led to the development of the anti-impotence drug Viagra, which also depends on the blood-vessel-dilating effect of nitric oxide.
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