Английский. пособие Химики АЯ. Introduction
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Isomerism Many elements can form two or more covalent bonds, but only a few are able to form extended chains of covalent bonds. The outstanding example is carbon, which can form as many as four covalent bonds and can bond to itself indefinitely. Carbon has six electrons in total, two of which are paired in an atomic orbital closest to the nucleus. The remaining four are farther from the nucleus and are available for covalent bonding. When there is sufficient hydrogen present, carbon will react to form methane, CH4. When all four electron pairs occupy the four molecular orbitals of lowest energy, the molecule assumes the shape of a tetrahedron, with carbon at the centre and the four hydrogen atoms at the apexes. The C–H bond length is 110 picometres (1 picometre = 10–12 metre), and the angle between adjacent C–H bonds is close to 110°. Such tetrahedral symmetry is common to many carbon compounds and results in interesting structural possibilities. If two carbon atoms are joined together, with three hydrogen atoms bonded to each carbon atom, the molecule ethane is obtained. When four carbon atoms are joined together, two different structures are possible: a linear structure designated n-butane and a branched structure called iso-butane. These two structures have the same molecular formula, C4H10, but a different order of attachment of their constituent atoms. The two molecules are termed structural isomers. Each of them has unique chemical and physical properties, and they are different compounds. The number of possible isomers increases rapidly as the number of carbon atoms increases. There are five isomers for C6H14, 75 for C10H22, and 6.2 × 1013 for C40H82. When carbon forms bonds to atoms other than hydrogen, such as oxygen, nitrogen, and sulfur, the structural possibilities become even greater. It is this great potential for structural diversity that makes carbon compounds essential to living organisms. Even when the bonding sequence of carbon compounds is fixed, further structural variation is still possible. When two carbon atoms are joined together by two bonding pairs of electrons, a double bond is formed. A double bond forces the two carbon atoms and attached groups into a rigid, planar structure. As a result, a molecule such as CHCl = CHCl can exist in two nonidentical forms called geometric isomers. Structural rigidity also occurs in ring structures, and attached groups can be on the same side of a ring or on different sides. Yet another opportunity for isomerism arises when a carbon atom is bonded to four different groups. These can be attached in two different ways, one of which is the mirror image of the other. This type of isomerism is called optical isomerism, because the two isomers affect plane-polarized light differently. Two optical isomers are possible for every carbon atom that is bonded to four different groups. For a molecule bearing 10 such carbon atoms, the total number of possible isomers will be 210 = 1,024. Large biomolecules often have 10 or more carbon atoms for which such optical isomers are possible. Only one of all the possible isomers will be identical to the natural molecule. For this reason, the laboratory synthesis of large organic molecules is exceedingly difficult. Only in the last few decades of the 20th century have chemists succeeded in developing reagents and processes that yield specific optical isomers. They expect that new synthetic methods will make possible the synthesis of ever more complex natural products. Unit 3 How to Make Limewater Limewater is a solution of calcium hydroxide is water. The saturated solution of calcium hydroxide is referred to as limewater. It should not be confused with the acidic fruit lime or with lemon water. In other words, limewater is a clear, colorless, aqueous solution of calcium hydroxide. It is made by mixing calcium hydroxide in water (4 to 8 times the quantity of lime). The term can be used to refer to water that contains dissolved lime or calcium salts. The solubility of hydroxides in water increases as we go down the group. Calcium hydroxide is less soluble in comparison to barium hydroxide. 1 liter of pure water can dissolve about 1 gram of calcium hydroxide. Properties Limewater has an earthy smell. It tastes of calcium hydroxide (bitter). Whitewash: A paint made from calcium hydroxide and chalk is known as whitewash. It is lime water used as a paint. Milk of Lime: Calcium hydroxide is sparsely soluble in water. When excess of it is mixed with water, some of its particles remain suspended, imparting the solution a milky appearance. This mixture is known as milk of lime. It has a pH of 12.3 Making Lime Water The method involves mixing distilled water with calcium hydroxide and shaking the mixture thoroughly, making a saturated solution of calcium hydroxide. Materials 1 teaspoon of calcium hydroxide (slaked lime) ½ Jar of Water Steps to Prepare Pour in 1 teaspoon of slaked lime into a jar filled with water and place a cover on the jar. Shake it thoroughly. At first, shake for a minute or two and then allow the mixture to stand for 24 hours. After the given period, pour the solution into another container. Do not stir the sediments vigorously. The clearer solution must be stored into a clean bottle or jar until its next use. If excess calcium hydroxide is added, the solution has a milky appearance due to suspended calcium hydroxide particles. Applications Calcium hydroxide in lime water reacts with carbon dioxide to give calcium carbonate which forms an insoluble suspension in the solution. This property makes limewater useful in detecting the presence of carbon dioxide. A simple experiment that demonstrates this reaction is to exhale into limewater and observe the change in its color. The carbon dioxide breathed out reacts with calcium hydroxide to form calcium carbonate. And the solution becomes cloudy. Waste containing sulfur dioxide is treated with limewater to remove the toxic sulfur dioxide from it. Calcium hydroxide in limewater reacts with sulfur dioxide to give calcium sulfite as a precipitate. Lime water finds applications in cooking. In making tortillas, it is used for soaking maize. In the process, vitamin B and amino acid trytophan are liberated. Soaking in lime water also causes the kernels' skin to peel off. Lime water is used as a color solvent in fresco painting. Painting is done on wet plaster with the use of pigments dissolved in limewater. Organisms in reef tanks consume calcium from water. Limewater is added to the tanks to restore the lost calcium. Unit 4 Analytical chemistry Most of the materials that occur on Earth, such as wood, coal, minerals, or air, are mixtures of many different and distinct chemical substances. Each pure chemical substance (e. g., oxygen, iron, or water) has a characteristic set of properties that gives it its chemical identity. Iron, for example, is a common silver-white metal that melts at 1,535 °C, is very malleable, and readily combines with oxygen to form the common substances hematite and magnetite. The detection of iron in a mixture of metals, or in a compound such as magnetite, is a branch of analytical chemistry called qualitative analysis. Measurement of the actual amount of a certain substance in a compound or mixture is termed quantitative analysis. Quantitative analytic measurement has determined, for instance, that iron makes up 72.3 percent, by mass, of magnetite, the mineral commonly seen as black sand along beaches and stream banks. Over the years, chemists have discovered chemical reactions that indicate the presence of such elemental substances by the production of easily visible and identifiable products. Iron can be detected by chemical means if it is present in a sample to an amount of 1 part per million or greater. Some very simple qualitative tests reveal the presence of specific chemical elements in even smaller amounts. The yellow colour imparted to a flame by sodium is visible if the sample being ignited has as little as one-billionth of a gram of sodium. Such analytic tests have allowed chemists to identify the types and amounts of impurities in various substances and to determine the properties of very pure materials. Substances used in common laboratory experiments generally have impurity levels of less than 0.1 percent. For special applications, one can purchase chemicals that have impurities totaling less than 0.001 percent. The identification of pure substances and the analysis of chemical mixtures enable all other chemical disciplines to flourish. Unit 5 Biochemistry As understanding of inanimate chemistry grew during the 19th century, attempts to interpret the physiological processes of living organisms in terms of molecular structure and reactivity gave rise to the discipline of biochemistry. Biochemists employ the techniques and theories of chemistry to probe the molecular basis of life. An organism is investigated on the premise that its physiological processes are the consequence of many thousands of chemical reactions occurring in a highly integrated manner. Biochemists have established, among other things, the principles that underlie energy transfer in cells, the chemical structure of cell membranes, the coding and transmission of hereditary information, muscular and nerve function, and biosynthetic pathways. In fact, related biomolecules have been found to fulfill similar roles in organisms as different as bacteria and human beings. The study of biomolecules, however, presents many difficulties. Such molecules are often very large and exhibit great structural complexity; moreover, the chemical reactions they undergo are usually exceedingly fast. The separation of the two strands of DNA, for instance, occurs in one-millionth of a second. Such rapid rates of reaction are possible only through the intermediary action of biomolecules called enzymes. Enzymes are proteins that owe their remarkable rate-accelerating abilities to their three-dimensional chemical structure. Not surprisingly, biochemical discoveries have had a great impact on the understanding and treatment of disease. Many ailments due to inborn errors of metabolism have been traced to specific genetic defects. Other diseases result from disruptions in normal biochemical pathways. Frequently, symptoms can be alleviated by drugs, and the discovery, mode of action, and degradation of therapeutic agents is another of the major areas of study in biochemistry. Bacterial infections can be treated with sulfonamides, penicillins, and tetracyclines, and research into viral infections has revealed the effectiveness of acyclovir against the herpes virus. There is much current interest in the details of carcinogenesis and cancer chemotherapy. It is known, for example, that cancer can result when cancer-causing molecules, or carcinogens as they are called, react with nucleic acids and proteins and interfere with their normal modes of action. Researchers have developed tests that can identify molecules likelyto be carcinogenic. The hope, of course, is that progress in the prevention and treatment of cancer will accelerate once the biochemical basis of the disease is more fully understood. The molecular basis of biologic processes is an essential feature of the fast-growing disciplines of molecular biology and biotechnology. Chemistry has developed methods for rapidly and accurately determining the structure of proteins and DNA. In addition, efficient laboratory methods for the synthesis of genes are being devised. Ultimately, the correction of genetic diseases by replacement of defective genes with normal ones may become possible. Polymer chemistry The simple substance ethylene is a gas composed of molecules with the formula CH2CH2. Under certain conditions, many ethylene molecules will join together to form a long chain called polyethylene, with the formula (CH2CH2)n, where n is a variable but large number. Polyethylene is a tough, durable solid material quite different from ethylene. It is an example of a polymer, which is a large molecule made up of many smaller molecules (monomers), usually joined together in a linear fashion. Many naturally occurring substances, including cellulose, starch, cotton, wool, rubber, leather, proteins, and DNA, are polymers. Polyethylene, nylon, and acrylics are examples of synthetic polymers. The study of such materials lies within the domain of polymer chemistry, a specialty that has flourished in the 20th century. The investigation of natural polymers overlaps considerably with biochemistry, but the synthesis of new polymers, the investigation of polymerization processes, and the characterization of the structure and properties of polymeric materials all pose unique problems for polymer chemists. Polymer chemists have designed and synthesized polymers that vary in hardness, flexibility, softening temperature, solubility in water, and biodegradability. They have produced polymeric materials that are as strong as steel yet lighter and more resistant to corrosion. Oil, natural gas, and water pipelines are now routinely constructed of plastic pipe. In recent years, automakers have increased their use of plastic components to build lighter vehicles that consume less fuel. Other industries such as those involved in the manufacture of textiles, rubber, paper, and packaging materials are built upon polymer chemistry. Besides producing new kinds of polymeric materials, researchers are concerned with developing special catalysts that are required by the large-scale industrial synthesis of commercial polymers. Without such catalysts, the polymerization process would be very slow in certain cases. Industrial chemistry The manufacture, sale, and distribution of chemical products is one of the cornerstones of a developed country. Chemists play an important role in the manufacture, inspection, and safe handling of chemical products, as well as in product development and general management. The manufacture of basic chemicals such as oxygen, chlorine, ammonia, and sulfuric acid provides the raw materials for industries producing textiles, agricultural products, metals, paints, and pulp and paper. Specialty chemicals are produced in smaller amounts for industries involved with such products as pharmaceuticals, foodstuffs, packaging, detergents, flavours, and fragrances. To a large extent, the chemical industry takes the products and reactions common to “bench-top” chemical processes and scales them up to industrial quantities. The monitoring and control of bulk chemical processes, especially with regard to heat transfer, pose problems usually tackled by chemists and chemical engineers. The disposal of by-products also is a major problem for bulk chemical producers. These and other challenges of industrial chemistry set it apart from the more purely intellectual disciplines of chemistry discussed above. Yet, within the chemical industry, there is a considerable amount of fundamental research undertaken within traditional specialties. Most large chemical companies have research-and-development capability. Pharmaceutical firms, for example, operate large research laboratories in which chemists test molecules for pharmacological activity. The new products and processes that are discovered in such laboratories are often patented and become a source of profit for the company funding the research. A great deal of the research conducted in the chemical industry can be termed applied research because its goals are closely tied to the products and processes of the company concerned. New technologies often require much chemical expertise. The fabrication of, say, electronic microcircuits involves close to 100 separate chemical steps from start to finish. Thus, the chemical industry evolves with the technological advances of the modern world and at the same time often contributes to the rate of progress. Unit 6 Green Chemistry The term green chemistry is defined as: the invention, design and application of chemical products and processes to reduce or to eliminate the use and generation of hazardous substances. While this short definition appears straightforward, it marks a significant departure from the manner in which environmental issues have been considered or ignored in the upfront design of the molecules and molecular transformations that are at the heart of the chemical enterprise. Looking at the definition of green chemistry, the first thing one sees is the concept of invention and design. By requiring that the impacts of chemical products and chemical processes are included as design criteria, the definition of green chemistry inextricably links hazard considerations to performance criteria. Another aspect of the definition of green chemistry is found in the phrase “use and generation”. Rather than focusing only on those undesirable substances that might be inadvertently produced in a process, green chemistry also includes all substances that are part of the process. Therefore, green chemistry is a tool not only for minimizing the negative impact of those procedures aimed at optimizing efficiency, although clearly both impact minimization and process optimization are legitimate and complementary objectives of the subject. Green chemistry, however, also recognizes that there are significant consequences to the use of hazardous substances, ranging from regulatory, handling and transport, and liability issues, to name a few. To limit the definition to deal with waste only, would be to address only part of the problem. Green chemistry is applicable to all aspects of the product life cycle as well. Finally, the definition of green chemistry includes the term “hazardous”. It is important to note that green chemistry is a way of dealing with risk reduction and pollution prevention by addressing the intrinsic hazards of the substances rather than those circumstances and conditions of their use that might increase their risk. Why is it important for green chemistry to adopt a hazard-based approach? To understand this, we have to revisit the concept of risk. Risk, in its most fundamental terms, is the product of hazard and exposure: Risk = Hazard X Exposure A substance manifesting some quantifiable hazard, together with a quantifiable exposure to that hazard, will allow us to calculate the risk associated with that substance. Virtually all common approaches to risk reduction focus on reducing exposure to hazardous substances. Regulations often require increases in control technologies and treatment technology, and in personal protective equipment such as respirators, gloves, etc., in order to reduce risk by restricting exposure. By achieving risk reduction through hazard reduction, green chemistry addresses concerns about the cost and potential for failure of exposure controls. Regardless of the type of exposure control, ranging from engineering controls through personal protective gear, there is always going to be an upfront capital cost; to what degree this cost can be recouped will be situation-specific, but it will always be there. In contrast, there is no additional upfront capital cost necessarily associated with green chemistry. While some green chemistry options may require capital investment, others may actually lower total cost of operations from the outset. This result is frequently the case in some of the easiest ways of implementing green chemistry technologies. Exposure controls, because they rely on either equipment or human activity to accomplish their goals, are capable of failing. Respirators can rupture, air scrubbers can break down, and so forth. When failure occurs, risk is maximized because the resultant exposure is to a constant hazard. Green chemistry, in contrast, does not rely on equipment, human activity, or circumstances of use but, instead, changes the intrinsic hazard properties of the chemical products and transformations. Consequently, green chemistry is not as vulnerable to failure, as are the traditional approaches to hazard control. The definition of green chemistry also illustrates another important point about the use of the term “hazard”. This term is not restricted to physical hazards such as explosiveness, flammability, and corrosibility, but certainly also includes acute and chronic toxicity, carcinogenicity, and ecological toxicity. Furthermore, for the purposes of this definition, hazards must include global threats such as global warming, stratospheric ozone depletion, resource depletion and bioaccumulation, and persistent chemicals. To include this broad perspective is both philosophically and pragmatically consistent. It would certainly be unreasonable to address only some subset of hazards while ignoring or not addressing others. But more importantly, intrinsically hazardous properties constitute those issues that can be addressed through the proper design or redesign of chemistry and chemicals. |