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Chapter 7: Alkanes and Halogenated Hydrocarbons

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Opening Essay

7.1 Recognition of Organic Structures

7.2 Introduction to Alkanes

Straight Chain AlkanesBranched Chain AlkanesCycloalkanesClassification of Carbon Bonds

7.3 Properties of Alkanes

Melting Points and Boiling PointsSolubilityAlkane Properties and Environmental Hazards: A Closer Look

7.4 Chemical Reactivity of Alkanes

Combustion ReactionsHalogenation Reactions (Substitution Type)Cracking Alkanes

7.5 Chapter Summary

7.6 End-of-Chapter Exercises

7.7 References


Opening Essay

Hydrocarbons are the simplest organic compounds, but they have interesting physiological effects. These effects depend on the size of the hydrocarbon molecules and where on or in the body they are applied. Alkanes of low molar mass—those with from 1 to approximately 10 or so carbon atoms—are gases or light liquids that act as anesthetics. Inhaling (“sniffing”) these hydrocarbons in gasoline or aerosol propellants for their intoxicating effect is a major health problem that can lead to liver, kidney, or brain damage or to immediate death by asphyxiation by excluding oxygen. Pressurized canisters of propane and butane gas, both of which are intended for use as fuels, are abused as inhalants.

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Figure 7.1. A range of petroleum-based products that can be abused as inhalants. Photo By: Lance Cpl. Matthew K. Hacker

Swallowed, liquid alkanes do little harm while in the stomach. In the lungs, however, they cause “chemical” pneumonia by dissolving fatlike molecules from cell membranes in the tiny air sacs (alveoli). The lungs become unable to expel fluids, just as in pneumonia caused by bacteria or viruses. People who swallow gasoline or other liquid alkane mixtures should not be made to vomit, as this starrkingschool.netld increase the chance of getting alkanes into the lungs. (There is no home-treatment antidote for gasoline poisoning; call a poison control center.)

Liquid alkanes with approximately 5–16 carbon atoms per molecule wash away natural skin oils and cause drying and chapping of the skin, while heavier liquid alkanes (those with approximately 17 or more carbon atoms per molecule) act as emollients (skin softeners). Such alkane mixtures as mineral oil and petroleum jelly can be applied as a protective film. Water and aqueous solutions such as urine will not dissolve such a film, which explains why petroleum jelly protects a baby’s tender skin from diaper rash.


In this chapter we will investigate the alkanes, compounds containing only two elements, carbon and hydrogen, and having only single bonds. We will also investigate alkanes that have halogens incorporated into their structure. Recall that halogens are the elements in Family 7A on the periodic table and contain representative elements such as chlorine, fluorine, iodine, and bromine. There are several other kinds of hydrocarbons, distinguished by the types of bonding between carbon atoms and by the properties that result from that bonding. In Chapter 8 we will examine hydrocarbons with double bonds, with triple bonds, and with a special kind of bonding called aromaticity. Then in Chapter 9, we will study some compounds considered to be derived from hydrocarbons by replacing one or more hydrogen atoms with an oxygen-containing group. Chapter 10 focuses on organic acids and bases.

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7.1 Recognition of Organic Structures


Learning ObjectiveTo be able to recognize the composition and properties typical of organic and inorganic compounds.

Scientists of the 18th and early 19th centuries studied compounds obtained from plants and animals and labeled them organic because they were isolated from “organized” (living) systems. Compounds isolated from nonliving systems, such as rocks and ores, the atmosphere, and the oceans, were labeled inorganic. For many years, scientists thought organic compounds could be made by only living organisms because they possessed a vital force found only in living systems. The vital force theory began to decline in 1828, when the German chemist Friedrich Wöhler synthesized urea from inorganic starting materials. He reacted silver cyanate (AgOCN) and ammonium chloride (NH4Cl), expecting to get ammonium cyanate (NH4OCN). What he expected is described by the following equation.

AgOCN + NH4Cl → AgCl + NH4OCN

Instead, he found the product to be urea (NH2CONH2), a well-known organic material readily isolated from urine. This result led to a series of experiments in which a wide variety of organic compounds were made from inorganic starting materials. The vital force theory gradually went away as chemists learned that they could make many organic compounds in the laboratory.

Today organic chemistry has been reclassified as the study of compounds that contain carbon, and inorganic chemistry is the study of the chemistry of all other elements. It may seem strange that we divide chemistry into two branches—one that considers compounds of only one element and one that covers the 100-plus remaining elements. However, this division seems more reasonable when we consider that of tens of millions of compounds that have been characterized, the overwhelming majority are carbon compounds.


Note

The word organic has different meanings. Organic fertilizer, such as cow manure, is organic in the original sense; it is derived from living organisms. Organic foods generally are foods grown without synthetic pesticides or fertilizers. Organic chemistry is the chemistry of compounds of carbon. Carbon is unique among the other elements in that its atoms can form stable covalent bonds with each other and with atoms of other elements in a multitude of variations. The resulting molecules can contain from one to millions of carbon atoms.


Organic compounds, like inorganic compounds, obey all the natural laws. Often there is no clear distinction in the chemical or physical properties among organic and inorganic molecules. Nevertheless, it is useful to compare typical members of each class, as in Table 7.1 (Keep in mind, however, that there are exceptions to every category in this table.) To further illustrate typical differences among organic and inorganic compounds, Table 7.1 also lists properties of the inorganic compound sodium chloride (common table salt, NaCl) and the organic compound hexane (C6H14), a solvent that is used to extract soybean oil from soybeans (among other uses). Many compounds can be classified as organic or inorganic by the presence or absence of certain typical properties, as illustrated in Table 7.1.


Table 7.1 General Contrasting Properties and Examples of Organic and Inorganic Compounds

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Concept Review Exercises: Click to Complete the Questions

Key Takeaway
Organic chemistry is the study of carbon compounds, nearly all of which also contain hydrogen atoms.
More Practice

Classify each compound as organic or inorganic.

C6H10CoCl2C12H22O11

Classify each compound as organic or inorganic.

CH3NH2NaNH2Cu(NH3)6Cl2
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7.2 Introduction to Alkanes

Alkanes are organic compounds that consist entirely of single-bonded carbon and hydrogen atoms and lack any other functional groups. Alkanes have the general formula CnH2n+2 and can be subdivided into the following three groups: the linear straight-chain alkanes, branched alkanes, and cycloalkanes (Fig. 7.2). Alkanes are also saturated hydrocarbons, that is all of the carbon atoms are ‘saturated’ with hydrogen atoms and do not contain any carbon-carbon double bonds or triple bonds. Alkanes are the simplest and least reactive hydrocarbon species containing only carbons and hydrogens. They are commercially very important, being the principal constituent of gasoline and lubricating oils and are extensively employed in organic chemistry; though the role of pure alkanes (such as hexanes) is delegated mostly to solvents. The distinguishing feature of an alkane, making it distinct from other compounds that also exclusively contain carbon and hydrogen, is its lack of unsaturation. That is to say, it contains no double or triple bonds, which are highly reactive in organic chemistry. Though not totally devoid of reactivity, their lack of reactivity under most laboratory conditions makes them a relatively uninteresting, though very important component of organic chemistry. As you will learn about later, the energy confined within the carbon-carbon bond and the carbon-hydrogen bond is quite high and their rapid oxidation produces a large amount of heat, typically in the form of fire.

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Figure 7.2. Examples of Alkanes


Straight Chain Alkanes

The straight chain alkanes, methane (CH4), ethane (C2H6), and propane (C3H8) represent the beginning of a series of compounds in which any two members in a sequence differ by one carbon atom and two hydrogen atoms—namely, a CH2 unit (Fig. 7.3) 

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Figure 7.3 The Three Simplest Alkanes


The first 10 members of this series are given in Table 7.2. Note that as you increase the length of the carbon chain, the number of possible different structural isomers also increases.


Table 7.2 The First 10 Straight-Chain Alkanes

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From propane (C3H8) onward, you will notice that the only difference between longer chain hydrocarbons involves the addition of CH2 units as you move up the series (Fig. 7.4). Any family of compounds in which adjacent members differ from each other by a definite factor (here a CH2 group) is called a homologous series, and can be defined mathematically. The members of such a series, called homologs. In organic chemistry, homologs have properties that vary in a regular and predictable manner. Thus, the principle of homology gives organization to organic chemistry in much the same way that the periodic table gives organization to inorganic chemistry. Instead of a bewildering array of individual carbon compounds, we can study a few members of a homologous series and from them deduce some of the properties of other compounds in the series.


 

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Figure 7.4 Members of a Homologous Series

In Figure 7.4, note that each succeeding formula incorporates one carbon atom and two hydrogen atoms more than the previous formula. The principle of homology allows us to write a general formula for alkanes: CnH2n+ 2. Using this formula, we can write a molecular formula for any alkane with a given number of carbon atoms. For example, an alkane with eight carbon atoms has the molecular formula C8H(2 × 8) + 2 = C8H18.

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Concept Review Exercises

In the homologous series of alkanes, what is the molecular formula for the member just above C8H18?


Use the general formula for alkanes to write the molecular formula of the alkane with 12 carbon atoms.