Organic chemistry 4th by Paula Bruice

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  1. 1. 1 The first two chapters of the text cover a variety of topics that you need to get started with your study of organic chemistry. Chapter 1 reviews the topics from general chemistry that will be important to your study of organic chemistry. The chapter starts with a description of the structure of atoms and then proceeds to a description of the structure of molecules. Molecular orbital theory is introduced. Acidbase chemistry, which is central to understanding many organic reactions, is reviewed.You will see how the structure of a molecule affects its acidity and how the acidity of a solution affects molecular structure. To discuss organic compounds, you must be able to name them and visualize their structures when you read or hear their names. In Chapter 2, you will learn how to name five different classes of organic compounds. This will give you a good understanding of the basic rules followed in naming compounds. Because the compounds exam- ined in the chapter are either the reactants or the products of many of the reactions presented in the next 10 chap- ters, you will have the opportunity to review the nomen- clature of these compounds as you proceed through those chapters. The structures and physical properties of these compounds will be compared and contrasted, which makes learning about them a little easier than if each compound were presented separately. Because organic chemistry is a study of compounds that contain carbon, the last part of Chapter 2 discusses the spatial arrange- ment of the atoms in both chains and rings of carbon atoms. An Introduction to the Study of Organic Chemistry Chapter 1 Electronic Structure and Bonding Acids and Bases Chapter 2 An Introduction to Organic Compounds: Nomenclature, Physical Properties, and Representation of Structure PARTONE BRUI01-001_059r4 20-03-2003 2:58 PM Page 1
  2. 2. T o stay alive, early humans must have been able to tell the difference between two kinds of materials in their world. You can live on roots and berries, they might have said, but you cant live on dirt. You can stay warm by burning tree branches, but you cant burn rocks. By the eighteenth century, scientists thought they had grasped the nature of that difference, and in 1807, Jns Jakob Berzelius gave names to the two kinds of materials. Compounds derived from living organisms were believed to contain an unmeasurable vital forcethe essence of life. These he called organic. Compounds derived from mineralsthose lacking that vital forcewere inorganic. Because chemists could not create life in the laboratory, they assumed they could not create compounds with a vital force. With this mind-set, you can imagine how surprised chemists were in 1828 when Friedrich Whler produced ureaa compound known to be excreted by mammalsby heating ammonium cyanate, an inorganic mineral. For the first time, an organic compound had been obtained from something other than a living organism and certainly without the aid of any kind of vital force. Clearly, chemists needed a new definition for organic compounds. Organic compounds are now defined as compounds that contain carbon. Why is an entire branch of chemistry devoted to the study of carbon-containing compounds? We study organic chemistry because just about all of the molecules that C NH2 O heat NH4 OCN ammonium cyanate H2N urea + 2 1 Electronic Structure and Bonding Acids and Bases German chemist Friedrich Whler (18001882) began his professional life as a physician and later became a professor of chemistry at the Uni- versity of Gttingen. Whler codis- covered the fact that two different chemicals could have the same mo- lecular formula. He also developed methods of purifying aluminumat the time, the most expensive metal on Earthand beryllium. Jns Jakob Berzelius (17791848) not only coined the terms organic and inorganic, but also invented the system of chemical symbols still used today. He published the first list of accurate atomic weights and proposed the idea that atoms carry an electric charge. He purified or discovered the elements cerium, selenium, silicon, thorium, titanium, and zirconium. Ethane Ethene Ethyne BRUI01-001_059r4 20-03-2003 2:58 PM Page 2
  3. 3. Section 1.1 The Structure of an Atom 3 make life possibleproteins, enzymes, vitamins, lipids, carbohydrates, and nucleic acidscontain carbon, so the chemical reactions that take place in living systems, in- cluding our own bodies, are organic reactions. Most of the compounds found in naturethose we rely on for food, medicine, clothing (cotton, wool, silk), and energy (natural gas, petroleum)are organic as well. Important organic compounds are not, however, limited to the ones we find in nature. Chemists have learned to synthesize millions of organic compounds never found in nature, including synthetic fabrics, plastics, synthetic rubber, medicines, and even things like photographic film and Super glue. Many of these synthetic compounds prevent shortages of naturally occur- ring products. For example, it has been estimated that if synthetic materials were not available for clothing, all of the arable land in the United States would have to be used for the production of cotton and wool just to provide enough material to clothe us. Currently, there are about 16 million known organic compounds, and many more are possible. What makes carbon so special? Why are there so many carbon-containing com- pounds? The answer lies in carbons position in the periodic table. Carbon is in the center of the second row of elements. The atoms to the left of carbon have a tendency to give up electrons, whereas the atoms to the right have a tendency to accept electrons (Section 1.3). Because carbon is in the middle, it neither readily gives up nor readily accepts elec- trons. Instead, it shares electrons. Carbon can share electrons with several different kinds of atoms, and it can also share electrons with other carbon atoms. Consequently, carbon is able to form millions of stable compounds with a wide range of chemical properties simply by sharing electrons. When we study organic chemistry, we study how organic compounds react. When an organic compound reacts, some old bonds break and some new bonds form. Bonds form when two atoms share electrons, and bonds break when two atoms no longer share electrons. How readily a bond forms and how easily it breaks depend on the par- ticular electrons that are shared, which, in turn, depend on the atoms to which the elec- trons belong. So if we are going to start our study of organic chemistry at the beginning, we must start with an understanding of the structure of an atomwhat electrons an atom has and where they are located. 1.1 The Structure of an Atom An atom consists of a tiny dense nucleus surrounded by electrons that are spread throughout a relatively large volume of space around the nucleus. The nucleus con- tains positively charged protons and neutral neutrons, so it is positively charged. The electrons are negatively charged. Because the amount of positive charge on a proton equals the amount of negative charge on an electron, a neutral atom has an equal num- ber of protons and electrons. Atoms can gain electrons and thereby become negatively charged, or they can lose electrons and become positively charged. However, the num- ber of protons in an atom does not change. Protons and neutrons have approximately the same mass and are about 1800 times more massive than an electron. This means that most of the mass of an atom is in its nucleus. However, most of the volume of an atom is occupied by its electrons, and that is where our focus will be because it is the electrons that form chemical bonds. Li Be B C the second row of the periodic table N O F BRUI01-001_059r4 20-03-2003 2:58 PM Page 3
  4. 4. 4 C H A P T E R 1 Electronic Structure and Bonding Acids and Bases Louis Victor Pierre Raymond duc de Broglie (18921987) was born in France and studied history at the Sorbonne. During World War I, he was stationed in the Eiffel Tower as a radio engineer. Intrigued by his expo- sure to radio communications, he re- turned to school after the war, earned a Ph.D. in physics, and became a professor of theoretical physics at the Facult des Sciences at the Sorbonne. He received the Nobel Prize in physics in 1929, five years after ob- taining his degree, for his work that showed electrons to have properties of both particles and waves. In 1945, he became an adviser to the French Atomic Energy Commissariat. Erwin Schrdinger (18871961) was teaching physics at the Universi- ty of Berlin when Hitler rose to power. Although not Jewish, Schrdinger left Germany to return to his native Austria, only to see it taken over later by the Nazis. He moved to the School for Advanced Studies in Dublin and then to Oxford University. In 1933, he shared the Nobel Prize in physics with Paul Dirac, a professor of physics at Cam- bridge University, for mathematical work on quantum mechanics. The atomic number of an atom equals the number of protons in its nucleus. The atomic number is also the number of electrons that surround the nucleus of a neutral atom. For example, the atomic number of carbon is 6, which means that a neutral car- bon atom has six protons and six electrons. Because the number of protons in an atom does not change, the atomic number of a particular element is always the sameall carbon atoms have an atomic number of 6. The mass number of an atom is the sum of its protons and neutrons. Not all carbon atoms have the same mass number, because, even though they all have the same num- ber of protons, they do not all have the same number of neutrons. For example, 98.89% of naturally occurring carbon atoms have six neutronsgiving them a mass number of 12and 1.11% have seven neutronsgiving them a mass number of 13. These two different kinds of carbon atoms and are called isotopes. Isotopes have the same atomic number (i.e., the same number of protons), but diff