20 Hydrocarbon Compounds Turkmenistan has some of the world’s largest deposits of natural gas. This natural gas deposit in the city of Darvaza has been burning for over three decades! 708 S ON CU O F TEKS CHEMYSTERY 7B Nose for Hire Walking home from school one day, Anthony spotted a poster soliciting participants for a smell test. The poster offered participants “$50 for less than an hour of your time.” So he decided to go for it. When Anthony reported for the study, after filling out some paperwork, a researcher asked him to smell two chemical samples. The first one smelled like a freshly cut orange. The second had an odor that reminded him of pine trees. Anthony was curious. “What am I smelling?” he asked. “Limonene,” the researcher answered. “What about the second one?” Anthony asked. The researcher gave the same reply: “Limonene.” Anthony was perplexed. How could two substances, both with the name limonene, smell so different? As you read about hydrocarbons, think about how you could solve this mystery by writing structural formulas for these two covalent compounds. Take a video field trip with the Untamed Science crew to learn more about the relationship between structural formulas and physical properties. Texas Essential Knowledge and Skills READINESS TEKS: 7A Name ionic compounds containing main group or transition metals, covalent compounds, acids, and bases, using International Union of Pure and Applied Chemistry (IUPAC) nomenclature rules. 7B Write the chemical formulas of common polyatomic ions, ionic compounds containing main group or transition metals, covalent compounds, acids, and bases. 7C Construct electron dot formulas to illustrate ionic and covalent bonds. SUPPORTING TEKS: 7E Predict molecular structure for molecules with linear, trigonal planar, or tetrahedral electron pair geometries using Valence Shell Electron Pair Repulsion (VSEPR) theory. TEKS: 2H Organize, analyze, evaluate, make inferences, and predict trends from data. 2I Communicate valid conclusions supported by the data through methods such as lab reports, labeled drawings, graphs, journals, summaries, oral reports, and technology-based reports. 3D Evaluate the impact of research on scientific thought, society, and the environment. 3E Describe the connection between chemistry and future careers. Hydrocarbon Compounds 709 2 0.1 Hydrocarbons In this lesson, you will learn how to name hydrocarbon compounds using nomenclature rules established by the International Union of Pure and Applied Chemistry (TEKS 7A) and how to write their chemical and structural formulas (TEKS 7B). Also TEKS 7C, 7E, 2H. CHEMISTRY & YOU Q: Why are some fossil fuels gases, some liquids, and some solids? The gasoline used to fuel this motorcycle is a liquid at STP. So are the diesel fuel used in trucks and buses and the kerosene used in lanterns. Other fuels are gases or solids. For example, the fuel used in a furnace might be natural gas or a solid such as coal. All these fuels contain mixtures of compounds called hydrocar- bons. In this lesson, you will learn about the structure and properties of hydrocarbons. Key Questions Organic Chemistry Why does a carbon atom and Hydrocarbons TEKS 7B, 7A, 7C, 7E form four covalent bonds? Why does a carbon atom form four covalent bonds? What are two possible arrangements of carbon atoms Fewer than 200 years ago, it was thought that only living organisms in an alkane? could synthesize the carbon compounds found in their cells. So these compounds were classified as organic compounds, and the study of Vocabulary these compounds was known as organic chemistry. Many people • hydrocarbon thought that a mysterious vital force directed the formation of car- • alkane bon compounds. A German chemist, Friedrich Wöhler (1800–1882), • straight-chain alkane refuted this idea in 1828. He was able to use inorganic substances • homologous series to synthesize urea—an organic compound found in urine. Today, • condensed structural formula organic chemistry includes the chemistry of almost all carbon com- • substituent pounds, regardless of their origin. • alkyl group • branched-chain alkane Introduction to Hydrocarbons In a reference book that lists prop- erties of common compounds, the list of organic compounds is much ELPS 4.G.3 longer than the list of inorganic compounds. In fact, new organic Read with a partner the Key compounds are synthesized every day. The simplest organic com- Questions above. As you read the pounds are called hydrocarbons. By definition, a compound contains lesson together, note some possible at least two elements. A hydrocarbon is an organic compound that responses to each Key Question. contains only carbon and hydrogen. The two simplest hydrocarbons Then meet with another pair of students. Ask one another the Key are methane and ethane. Questions and respond to the Methane, which has the chemical formula CH4, is the major questions, using your notes. component of natural gas. It is sometimes called marsh gas because it is formed by the action of bacteria on decaying plants in swamps and other marshy areas. Livestock and termites also emit substantial quantities of methane as a product of digestion. 710 Recall that a carbon atom has four valence electrons and a hydrogen atom READING SUPPORT has one valence electron. So one carbon atom can form a single covalent bond Build Study Skills: Cluster with four hydrogen atoms as shown in the electron dot formula below. Diagram As you read through the chapter, construct a cluster diagram C + 4H organizing the different types of hydrocarbons. Use Carbon Hydrogen Methane color-coding to distinguish atom atoms molecule among the groups. How Because carbon has four valence electrons, a carbon atom always will this diagram help you forms four covalent bonds. Remembering this principle will help you to learn about hydrocarbons? write correct structures for organic compounds. Methane is not typical of the vast majority of organic compounds because there isn’t a bond between carbon atoms in a methane molecule. But there is a carbon-carbon bond in ethane. In an ethane molecule, two carbon atoms share a pair of electrons. The remaining six valence electrons form bonding pairs with the electrons from six hydrogen atoms. H H 2 C + 6H C C + 6H H C C H Carbon Hydrogen H H atoms atoms Ethane molecule The ability of carbon to form stable carbon-carbon bonds is one reason that carbon can form so many different compounds. Representing Hydrocarbons Table 20.1 shows the structural formulas, ball-and-stick models, and space-filling models for methane and ethane. Structural formulas are a convenient way to show the arrangement of atoms in a molecule. But two-dimensional structural formulas do not provide accu- rate information about how the atoms in a molecule are arranged in space. Three-dimensional molecular models represent the shapes of molecules more accurately. Throughout this chapter and the next, ball-and-stick models and space-filling models will be used along with structural formulas to represent organic molecules. Hybrid orbital theory and VSEPR theory are used to pre- dict the molecular shapes. Table 20.1 Different Ways of Representing Hydrocarbons IUPAC name Structural formula Ball-and-stick model Space-filling model H Methane H C H H H H Ethane H C C H H H Hydrocarbon Compounds 711 Properties of Hydrocarbons The electron pair in a carbon-hydrogen bond or a carbon-carbon bond is shared almost equally by the nuclei of the atoms forming the bond. Thus, hydrocarbons are nonpolar molecules. The attractions between nonpolar molecules are weak van der Waals forces. So hydrocarbons with low molar masses tend to be gases or liquids that boil at a low temperature. Recall the general rule “like dissolves like.” Two nonpolar compounds will form a solution, as will two polar compounds. But a nonpolar compound and a polar compound will not form a solution. For example, because oil is a mixture of hydrocarbons, oil and water do not mix. So oil will float on top of water, as shown in Figure 20.1. Alkanes TEKS 7A, 7B What are two possible arrangements of carbon atoms in an alkane? Methane and ethane are examples of alkanes. An alkane is a hydrocarbon in which there are only single covalent bonds. In any alkane, all the carbon- Figure 20.1 Oil and Water carbon bonds are single covalent bonds, and all the other bonds are carbon- The nonpolar molecules in this cooking oil are not attracted to the hydrogen bonds. The carbon atoms in an alkane can be arranged in a polar water molecules, so they do straight chain or in a chain that has branches. not mix. Straight-Chain Alkanes Ethane is the simplest straight-chain alkane, Interpret Photos What evidence of insolubility do you see in the which is an alkane that contains any number of carbon atoms, one after the photo? other, in a chain. Propane (C3H8) has three carbon atoms bonded in a chain with eight electrons shared with eight hydrogen atoms. Butane (C4H10) has a chain of four carbons and ten hydrogens. Figure 20.2 shows how propane and butane can be used. a Figure 20.2 Hydrocarbon Fuels Hydrocarbons are commonly used as fuels. a. Pressurized tanks of propane are used to fuel the burners in hot-air balloons. b. Butane serves as the fuel for many kitchen torches. Propane b Butane 712 Chapter 20 • Lesson 1 Interpret G r a p h s TEKS 2H Melting and Boiling Points Figure 20.3 The graph illustrates of Straight-Chain Alkanes how the melting and boiling points 200 vary with the number of carbons in 150 straight-chain alkanes. a. Describe Determine whether 100 each alkane is a solid, liquid, or 50 gas at room temperature. Boiling points b. Evaluate Graphs How 0 do the added carbons affect the 50 boiling points of the straight-chain Melting points alkanes? Temperature (°C) Temperature 100 c.