كتاب الطالب 2019 – 2020 كيمياء منهج إنجليزي صف ثاني عشر متقدم
Introduction to Hydrocarbons
MAINIDEA Hydrocarbons are carbon-containing organic compounds that provide a source of energy and raw materials
CHEM 4 YOU
If you have ridden in a car or a bus, you have used hydrocarbons. The gasoline and diesel fuel that are used in cars, trucks, and buses are hydrocarbons
Organic Compounds Chemists in the early nineteenth century knew that living things, such as the plants and panda shown in Figure 1, produce an immense variety of carbon compounds. Chemists referred to these compounds as organic compounds because they were produced by living organisms
Once Dalton's atomic theory was accepted in the early nineteenth century, chemists began to understand that compounds, including those made by living organisms, consisted of arrangements of atoms bonded together in certain combinations. They were able to synthesize many new and useful substances. However, scientists were not able to synthe. size organic compounds. Many scientists incorrectly concluded that they were unable to synthesize organic compounds because of vitalism. According to vitalism, organisms possessed a mysterious "vital force," enabling them to assemble carbon compounds. Disproving vitalism Friedrich Wöhler (1800 -1882), a German chemist, was the first scientist to realize that he had produced an organic compound, called urea, by synthesis in a laboratory. Wöhler's experiment did not immediately disprove vitalism, but it prompted a chain of similar experiments by other European chemists. Eventually, the idea that the synthesis of organic compounds required a vital force was discredited and scientists realized they could synthesize organic compounds in the laboratory
Models and hydrocarbons Chemists represent organic molecules in a variety of ways. Figure 4 shows four different ways to represent a methane molecule. Covalent bonds are represented by a single straight line, which denotes two shared electrons. Most often, chem ists use the type of model that best shows the information they want to highlight. As shown in Figure 4, molecular formulas give no infor mation about the geometry of the molecule. A structural formula shows the general arrangement of atoms in the molecule but not the exact, three-dimensional geometry. The ball-and-stick model demonstrates the geometry of the molecule clearly, but the space-filling model gives a more realistic picture of what a molecule would look like if you could see it. Keep in mind as you look at the models that the atoms are held closely together by electron-sharing bonds
Multiple carbon-carbon bonds Carbon atoms can bond to each other not only by single covalent bonds but also by double and triple covalent bonds, as shown in Figure 5. Recall that in a double bond, atoms share two pairs of electrons, in a triple bond, they share three pairs of electrons
In the nineteenth century, before chemists understood bonding and the structure of organic substances, they experimented with hydrocarbons obtained from heating animal fats and plant oils. They classified these hydrocarbons according to a chemical test in which they mixed each hydrocarbon with bromine and then measured how much reacted with the hydrocarbon. Some hydrocarbons would react with a small amount of bromine, some would react with more, and some would not react with any amount of bromine. Chemists called the hydrocarbons that reacted with bromine unsaturated hydrocarbons in the same sense that an unsaturated aqueous solution can dissolve more solute. Hydrocar bons that did not react with bromine were said to be saturated
Present-day chemists can now explain the experimental results obtained 170 years ago. Hydrocarbons that reacted with bromine had double or triple covalent bonds. Those compounds that did not react with bromine had only single covalent bonds. Today, a hydrocarbon having only single bonds is defined as a saturated hydrocarbon. A hydrocarbon that has at least one double or triple bond between carbon atoms is an unsaturated hydrocarbon. You will learn more about these different types of hydrocarbons later in this chapter
READING CHECK Explain the origin of the terms saturated and unsaturated hydrocarbons
Figure 6 also gives the names of the typical fractions separated from petroleum, along with their boiling points, hydrocarbon size ranges, and common uses. You might recognize some of the fractions because you use them every day. Unfortunately, fractional distillation towers, shown in Figure 7, do not yield fractions in the same propor tions that they are needed. For example, distillation seldom yields the amount of gasoline desired. However, it yields more of the heavier oils than the market demands
Many years ago, petroleum chemists and engineers developed a process to help match the supply with the demand. This process in which heavier fractions are converted to gasoline by breaking their large molecules into smaller molecules is called cracking. Cracking is done in the absence of oxygen and in the presence of a catalyst. In addition to breaking heavier hydrocarbons into molecules of the size range needed for gasoline, cracking also produces starting materials for the synthesis of many different products, including plastic products, films, and synthetic fabrics
READING CHECK Describe the process in which large-chain hydrocarbons are broken into more-desirable smaller-chain hydrocarbons
Rating gasoline None of the petroleum fractions is a pure substance. As shown in Figure 6, gasoline is not a pure substance, but rather a mixture of hydrocarbons. Most molecules with single covalent bonds in gasoline have 5 to 12 carbon atoms. However, the gasoline pumped into cars today is different from the gasoline used in automobiles in the early 1900s. The gasoline fraction that is distilled from petroleum is modified by adjusting its composition and adding substances to improve its performance in today's automobile engines and to reduce pollution from car exhaust
It is critical that the gasoline air mixture in the cylinder of an automo bile engine ignite at exactly the right instant and burn evenly. If it ignites too early or too late, much energy will be wasted, fuel efficiency will drop, and the engine will wear out prematurely. Most straight-chain hydrocarbons burn unevenly and tend to ignite from heat and pressure before the piston is in the proper position and the spark plug fires. This early ignition causes a rattling or pinging noise called knocking