Hydrocarbons

The hydrocarbons are compounds of carbon and hydrogen. There are two types, I saturated and unsaturated. Saturated hydrocarbons are those in which adjacent car- ; bon atoms are joined by a single covalent bond and all other bonds are satisfied by T hydrogen.

A saturated compound

Unsaturated hydrocarbons have at least two carbon atoms that are joined by more :: than one covalent bond and all remaining bonds are satisfied by hydrogen.

Unsaturated compounds

Saturated Hydrocarbons

The saturated hydrocarbons form a whole series of compounds starting with one : carbon atom and increasing one carbon atom, stepwise. These compounds are called ■ alkanes, or the methane series. The principal source is petroleum. Gasoline is a mixture containing several of them; diesel fuel is another such mixture.

The hydrocarbons serve as feedstocks for the preparation of a wide variety of organic chemicals. This knowledge serves as the basis of the great petrochemical industry within the petroleum industry. Saturated hydrocarbons are quite inert toward most chemical reagents. For this reason they were termed "paraffins" by early chemists (from the Latin parum affinis, meaning "little affinity").

Methane (CH4) is the simplest hydrocarbon. It is a gas of considerable importance to environmental engineers and scientists since it is a major end product of the anaerobic treatment process as applied to sewage sludge and other organic waste materials. It is a component of marsh gas and of natural gas and, in a mixture with air containing from 5 to 15 percent methane, it is highly explosive. This property allows its use as fuel for gas engines. Methane is commonly called "firedamp" by miners and makes their work particularly hazardous. Methane is also considered to be an important greenhouse gas; its concentration in the stratosphere affects the earth's heat balance, and thus temperature. On a per molecule basis, it is 21 times more effective at trapping heat in the atmosphere than carbon dioxide, the primary greenhouse gas.

Ethane (CH3—CH3) is the second member of the series. propane (CH3—CH2—CH3) is the third member of the series. Butane (C4HI0) is the fourth member of the series and is of interest because it occurs in two isomeric forms:

n-Butane Isobutane

Pentane (C5H12) is the fifth member of the series and exists in three isomeric forms:

H HCH

HHHHH HHHH H I H HC—C—C—C—CH HC—C—C—CH HC—C—CH HHHHH HlHH H|H

nprr UpU

riUrl tv^n

n-Pentane Isopentane Neopentane bp, 36.2°C bp, 28°C bp, 9.5°C

The third isomer of pentane might also be called tetramethylmethane or dimethylpropane, as the reader will shortly recognize.

As the number of carbon atoms increases in the molecule, the number of possible isomers increases accordingly. There are five possible isomers of hexane (QHI4) and 75 possible isomers of decane (C10H22).

Physical Properties Table 5.1 lists the names and physical constants of the normal saturated hydrocarbons of 1 to 10 carbon atoms per molecule. The term "normal" applies to the isomer that has all its carbon atoms finked in a straight chain. The others are referred to as branched-chain compounds. The branched form of butane and the simplest branched form of pentane are commonly given the prefix iso-.

The saturated hydrocarbons are colorless, practically odorless, and quite insoluble in water, particularly those with five or more carbon atoms. They dissolve read-

Table 5.11 Physical constants of some normal alkanes

Slllllllli

Table 5.11 Physical constants of some normal alkanes

Methane

CH4

-182.4

-161.5

0.423"162"

1

Ethane

-182.8

-88.6

0.545"®°

1

Propane

C3H8

-187.6

-42.1

0.49325'

1

Butane

C4H10

-138.2

-0.5

0.57325'

2

Pentane

csH12

-129.7

36.0

0.626

3

Hexane

QH14

-95.3

68.7

0.655

5

Heptane

-90.6

98.5

0.684

9

Octane

CsHla

-56.8

125.6

0.69925'

18

Nonane

CqS&X

-53.5

150.8

0.718

35

Decane

OioH22

-29.7

174.1

0.730

75

*Density of compound at 20°C (or at the temperature noted by superscript) over the maximum density of water, which is at 4°C.

Source: D. R. Linde <ed.): "Handbook of Chemistry and Physics," 82nd ed., CRC Press LLC, Boca Raton, 2001.

*Density of compound at 20°C (or at the temperature noted by superscript) over the maximum density of water, which is at 4°C.

Source: D. R. Linde <ed.): "Handbook of Chemistry and Physics," 82nd ed., CRC Press LLC, Boca Raton, 2001.

ily in many organic solvents. At room temperature all members through C5 are gases, those from C6 to C17 are liquids, and those above C!7 are solids. Data in Table 5,1 show that as molecular size increases, the melting and boiling points of alkanes increase. Solubility in water in general decreases with increasing size. Such relationships are important in understanding the behavior of organic compounds in the environment and in engineered reactors (Sec. 5.34).

Homologous Series It will be noted from Table 5.1 that each successive member of the series differs from the previous member by CH2. When the formulas of a series of compounds differ by a common increment, such as CH2, the series is referred to as being a homologous series. Such compounds can be expressed by a general formula. That for the alkane series is C„H2„+2.

Radicals The inert character of the alkanes has been mentioned; however, they may be made to react under the proper conditions, and a wide variety of compounds results. It becomes necessary, therefore, to establish some form of nomenclature to identify the products formed. When one hydrogen is replaced from a molecule of an alkane, the -ane ending is dropped and a -yl is added. The names of some radicals are shown in Table 5.2. The system serves quite well for the normal compounds but is of little value in naming derivatives of the isomers.

Nomenclature The alkanes are characterized by names ending in -ane. The straight-chain compounds are termed normal compounds. The branched-chain

Table 5.21 Names of alkane-series radicals (alkyl groups)

ilpllslllpii

Methane

Methyl

ch3—

Ethane

Ethyl

C2Hs

Propane

«-Propyl

C3Hr-

Propane

Isopropyl

(CH3)2CH—

n-Butane

«-Butyl

C4H9

compounds and the derivatives of both straight- and branched-chain compounds are difficult to name with any degree of specificity. The IUPAC system, as proposed by the International Union of Pure and Applied Chemistry, is commonly used. In this system the compounds are named in terms of the longest continuous chain of carbon atoms in the molecule. A few examples will illustrate the method.

n-Pentane

n-Hexane

H H H H H H HC—C—C—C—C—CH H H J H H H HCH H

3-Methylhexane (an isomer of heptane)

HHHHHHH H H HC— C—C—C—C—C —C—C — CH HHH HHHHH H

4-Ethylnonane

(an isomer of undecane)

H H H H H H HC—C—C—C—C—CH H | H T H H HCH HCH H H

2,4-Diracthylhexane (an isomer of octane)

It will be noted that a chain is numbered from the end nearest the attached radical. The rule is to make the numbers as small as possible. The IUPAC system is applied to other compounds as well as to hydrocarbons.

Chemical Reactions Strong bases, acids, or aqueous solutions of oxidizing agents do not react with saturated hydrocarbons at room temperature. At elevated temperatures, strong oxidizing agents, such as concentrated sulfuric acid, oxidize the compounds to carbon dioxide and water. Other reactions of importance are as follows:

1. Oxidation with oxygen or air:

2. Substitution of hydrogen by halogens:

This reaction does not ordinarily occur in aqueous solutions and therefore is of little significance in environmental engineering and science.

3. Pyrolysis or cracking: High-molecular-weight hydrocarbons may be broken into smaller molecules by heat treatment. The process is used in the petroleum industry to increase the yield of light boiling fractions, suitable for sale as gasoline or for chemical synthesis, Heat treatment results in disruption of the large molecules as follows:

alkanes of lower mol. wt High-mol.-wt hca[ 4- alkenes alkanes ^ + hydrogen + naphthenes + carbon

4. Biological oxidation: Hydrocarbons are oxidized by certain bacteria under aerobic conditions. The oxidation proceeds through several steps. The first step is very slow biologically and involves conversion to alcohols with attack occurring on terminal carbon atoms, i.e., omega oxidation.

Hydrocarbon AicoKo!

Through additional oxidative steps, which will be discussed in Chap. 6, microorganisms convert the hydrocarbon to carbon dioxide and water and derive energy in the process.

Such reactions, particularly the intermediate steps, are of great interest to environmental engineering and science.

Unsaturated Hydrocarbons

The unsaturated hydrocarbons are usually separated into four classes.

Alkenes Each member of the alkane group except methane can lose hydrogen to form an unsaturated compound or alkene. The alkenes all contain one double bond between two adjacent carbon atoms,

Ethylene Butylene or or etlieoe 2-butene and their names all end in -ylene (older nomenclature) or -ene. The alkenes are also called olefins. Alkenes, particularly ethene, propene, and butene are formed in great quantities during the cracking or pyrolysis of petroleum.

The names, formulas, and physical constants of a number of important alkenes are given in Table 5.3. In naming specific alkenes, the IUPAC system chapter 5 Basic Concepts from Organic Chemistry 221

Table 5.3 i Physical constants of selected alkenes

Ethene CH2==CH2

Propene CH2=CHCH3

J-Butene CH2==CHCH2CH3

l-pentene CH2=CH(CH2)2CH3

1-Hexene CH2=CH(CH2)3CH3

1-Heptene CH2=CH(CH2),CH3

1-Octene CH2==CH(CH2)5CH3

1-Nonene CH2=CH(CH2)SCH3

1-Decene CH2==CH(CH2)7CH3

-■169

-103.7

0.568

1

™ 185.2

-47.6

0.505!S°

1

-185.3

-6.2

0.58825'

3

-165.2

29.9

0.640

5

-139.7

63.4

0.673

13

-119.7

93.6

0.697

27

-101.7

121.2

0.715

66

-81.3

149.9

0,72525'

153

-66.3

170.5

0.741

377

»Density of compound at 20°C (or at She temperature noted by superscript) over the maximum density of water, which is at 4°C.

Source: D. R. Linde (e&): "Handbook of Chemistry and Physics," 82nd ed., CRC Press LLC, Boca Raton, 2001.

must be employed on all compounds with over three carbon atoms. The nomenclature becomes quite complicated with branched-chain isomers. Fortunately, there is little reason to differentiate between normal and branched-chain compounds in this series.

Diolefins When aliphatic compounds contain two double bonds in the molecule, they are called alkadienes, some times dienes for short. The compound 1,3-butadi-ene is an important example:

ch2=chch==€h2 It has been used to make polymers.

Alkadienes Some organic compounds contain more than two double bonds per molecule. The red coloring matter of tomatoes, lycopene, and the yellow coloring matter of carrots are examples.

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  • eric
    Which hydrocarbons are the most valuable?
    1 year ago

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