Kelter P., Mosher M., Scott A. Chemistry. The Practical Science

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508 Chapter 12 Carbon

Products of the Fractional Distillation of Crude Oil and Their Uses

Fraction Formulas Uses Boiling Point (°C)

Natural gas CH

to C

Fuel, cooking gas <0

Petroleum ether C

to C

Solvent for organic compounds 30–70

Gasoline C

to C

Fuel, solvent 70–200

Kerosene C

to C

Rocket and jet engine fuel, 200–300

domestic heating

Heating oil C

to C

Industrial heating, fuel for 300–370

producing electricity

Lubricating oil C

to C

Lubricants for automobiles >350

and machines

Residue C

and up Asphalt, parafﬁn Solid

TABLE 12.5

into further processing steps to convert them into useful products. Table 12.5 also

lists some of the major uses made of each fraction. We will explore some of these

uses in more detail, and in doing so we will investigate the chemical properties

and reactivities of the hydrocarbons in each fraction.

12.5 Processing Hydrocarbons

The hydrocarbons within the fractions obtained by distillation are normally sub-

jected to further processing before being used as fuels or as feedstocks for the

chemical industry. The two major types of processing are known as cracking and

reforming.

Cracking

Many of the most useful hydrocarbons are the smaller ones, with fewer than

10 carbon atoms. At the reﬁnery, long-chain hydrocarbons are isolated from the

crude oil by fractional distillation. Then they are broken into a mixture of

shorter-chain hydrocarbons, including some branched and unsaturated hydro-

carbons, by means of a process called

cracking. This process uses heat to break

some of the carbon–carbon bonds in the long-chain hydrocarbons. The products

of cracking often contain short saturated and unsaturated hydrocarbons. There

are various types of cracking:

■

Thermal cracking uses heat alone.

■

Catalytic cracking uses heat plus a catalyst.

■

Hydrocracking uses heat plus a catalyst in the presence of hydrogen gas, en-

couraging formation of saturated rather than unsaturated products.

■

Steam cracking uses heat plus steam and a catalyst.

Reforming

Another crucial process at the reﬁnery is called reforming. In this process hydro-

carbons distilled from petroleum are passed through a catalyst bed and converted

into more useful forms.When a mixture is reformed, the predominantly straight-

chain hydrocarbons are made into a mixture containing more aromatic and

branched-chain hydrocarbons.

Reforming is a key step in making the gasoline we use as automobile

fuel. Gasoline must contain a suitable proportion of branched and aromatic

12.5 Processing Hydrocarbons 509

hydrocarbons to prevent the problem known as knocking. This is the rapid explo-

sion of the gas–air mixture as it is compressed in the automobile cylinder before

the spark plug creates the spark that is intended to cause the mixture to explode.

In some cases knocking damages a car’s engine, but most often it results only in

poor engine performance and reduced gas mileage. Addition of reformed distil-

lates to gasoline reduces knocking.

Gasoline

The gasoline we use to power automobiles is a complex mixture of well over a

hundred different hydrocarbons, which can include molecules such as pentane,

benzene, ethylbenzene, toluene (methylbenzene), and xylene (dimethylbenzene).

The precise composition of the mixture varies with the brand of gasoline, the lo-

cation, and the time of year, but the hydrocarbon molecules in automobile gas

generally contain between 5 and 17 carbon atoms, many being in the C

-to-C

range. One factor inﬂuencing the optimum mixture of hydrocarbons is the need

to produce a fuel with a vapor pressure that allows it to evaporate to an appro-

priate degree under the conditions in which it will be used.

Toluene Xylene

The extent to which a particular type of gasoline will burn smoothly in an

engine (without knocking) is indicated by using the

octane rating. This is based

on the early twentieth-century discovery that pure isooctane produced minimal

knocking in an engine, whereas pure heptane produced a very high level of

knocking. On the octane scale, 2,2,4-trimethylpentane (also known as isooctane)

is given the octane rating of 100, and heptane is given the octane rating of 0. By

comparing mixtures of isooctane and heptane to different formulations of

gasoline, an octane rating for the gasoline can be assigned. For example, a high-

quality gasoline, which produces a level of knocking equivalent to a mixture of

90% isooctane and 10% heptane, is assigned an octane rating of 90. Low-quality

gasoline can be improved by adding other compounds with octane ratings that

are higher than 100, such as ethanol (octane rating 112) or toluene (octane rating

118). Brands of automobile gasoline are typically available with octane ratings in

the range of 83 to 98.

C CH

CH CH

2,2,4-Trimethylpentane (isooctane)

Heptane

12.6 Typical Reactions of the Alkanes

Globally, the most signiﬁcant reaction of the alkanes is their combustion in air. As

we have already seen, the complete combustion of alkanes generates carbon diox-

ide and water vapor and releases considerable amounts of energy. The energy can

be used to provide heat, both for warmth and for cooking, to generate electricity,

or to power vehicles.

The chemical industry also makes substantial use of alkanes in

substitution

reactions

, in which one or more hydrogen atoms are replaced by other types of

atoms, most commonly halogens such as bromine and chlorine. These substitu-

tion reactions of the alkanes are promoted by ultraviolet light, which causes the

halogen molecules to split into two atoms:

The halogen atoms generated in this process are known as free radicals, each hav-

ing an unpaired electron. The presence of the unpaired electron makes them

highly reactive. In the presence of an alkane, the free radicals can readily attack

the C—H bonds. The result is a halogenated alkane and a molecule of the hydro-

gen halide.

For example, the simplest hydrocarbon, methane, can be converted into four

different organic products and HCl by successive substitution reactions with

chlorine:

These four products can be put to a variety of uses. Chloromethane is used in the

manufacture of silicone-based polymers and as a solvent in the production of

certain types of rubber. Dichloromethane is widely used as a solvent in the chem-

ical industry and within products such as paint strippers.

Trichloromethane, also known as chloroform, was one of the ﬁrst anesthetics,

pioneered as such in 1847 by the Scottish physician Sir James Simpson. Because

of the sweetness of this molecule, it was also used as the solvent in early cough

medicines. However, this compound was found to be both toxic and carcino-

genic, and ethanol has taken its place in medicines. Chloroform is still used as an

industrial solvent. Tetrachloromethane, also known as carbon tetrachloride, once

was the solvent most commonly used for the dry cleaning of clothes and other

fabrics. It was also used as a ﬁre extinguisher agent because it is not ﬂammable.

Unfortunately, it too is toxic and carcinogenic. It has now largely been abandoned

for dry cleaning and ﬁre extinguishing in favor of less toxic compounds such as

liquid carbon dioxide (see Chapter 10). However, it is still used as a solvent in

various industrial processes.

Another important reaction of the alkanes is their

dehydrogenation to pro-

duce the corresponding alkenes. In a dehydrogenation reaction, a molecule of

+ Cl

Cl + Cl

+ Cl

CHCl

+ Cl

CCl

+ HCl

Chloromethane

Dichloromethane

Trichloromethane

(chloroform)

Tetrachloromethane

(carbon tetrachloride)

Cl Cl



510 Chapter 12 Carbon

12.7 The Functional Group Concept 511

hydrogen is removed from the starting material. The result is an unsaturated

hydrocarbon, an alkene. For example, dehydrogenation of ethane produces

ethene. The dehydrogenation of propane produces propene, and so on:

12.7 The Functional Group Concept

The processes of adding chlorine atoms or CPC double bonds to the basic struc-

ture of alkanes are examples of adding

functional groups to a hydrocarbon frame-

work. A functional group is an atom or group of atoms with a characteristic set

of chemical properties. Chemists often make sense of the inﬁnite variety of

chemical reactions in organic chemistry by treating each molecule as a hydrocar-

bon framework with a particular set of functional groups incorporated within

that framework.

A chlorine atom bonded to a hydrocarbon is an example of an

alkyl halide

functional group, a term that covers any of the halogen atoms bonded to a hydro-

carbon framework. These and many other examples of functional groups are

listed in Table 12.6. For example, the CPC double bond (the alkene functional

group) is a functional group whose properties depend on the type of bonding

between the atoms involved. The CqC triple bond (the alkyne functional group)

is another example.

Much of the chemistry done with hydrocarbons from oil involves the addi-

tion of speciﬁc functional groups. Chemists can modify the structure of the basic

hydrocarbon chain, adjusting its length and degree of branching, and then

add whatever selection of functional groups will create the molecules they are

seeking.

Ethane

500˚C

C H + H

Propane

Ethene

Propene

500˚C

C H + H

Chloroform

CHCl

Carbon tetrachloride

CCl

512 Chapter 12 Carbon

Selected Functional Groups

Structure Group Example 3-D Structure Name Selected Uses

Alkene CH

PCH

Alkyne HCqCH Acetylene

Alcohol CH

OH Ethanol

Ether CH

OCH

Diethyl ether

Aldehyde CH

PO Formaldehyde

Ketone

Carboxylic Formic acid

acid

Ester Methyl acetate

Amine CH

(CH

)

Butylamine

Manufacture

of rubber,

insecticides

Paint remover,

pharmaceutical

manufacture

Manufacture of

textiles, pesticides;

electroplating

Solvent in

synthetic rubber

industry

Methyl ethyl

ketone

Bactericide,

fungicide, chem-

icals production

Industrial

solvent

Liquors,

industrial solvent

Welding,

cutting, brazing

C C

Refrigerant,

production of

polyethylene

Ethene

(ethylene)

TABLE 12.6

(Continued)

C C

C OH

COH

12.7 The Functional Group Concept 513

EXERCISE 12.5 Functional Groups and You!

Circle and name each of the functional groups in cyclexanone, a pharmaceutical

agent used to prevent coughing (an antitussive agent).

Solution

Ether

Amine

lkene

Ketone

(Continued)

Structure Group Example 3-D Structure Name Selected Uses

TABLE 12.6

Nitrile Acetonitrile

Amide Formamide

Thiol CH

SH Propanethiol Herbicide, ﬂavor-

ing agent, additive

to odorless poiso-

nous gases

Manufacture

of paper, glue;

industrial solvent

Industrial solvent,

pharmaceutical

manufacture

Source: Kelter, Scott, and Carr. Chemistry, A World of Choices, 2nd Ed; McGraw-Hill, 2003: New York; and New Jersey

Department of Health & Senior Services Right-to-Know Program Fact Sheets.

C N

HNH

C SH

PRACTICE 12.5

Circle and name each of the functional groups in tyrosine,one of the building blocks

used to make proteins.

See Problems 51–54.

12.8 Ethene, the CPC Bond, and Polymers

One of the most versatile of the hydrocarbons produced by catalytic cracking is

ethene (ethylene). It serves as the central feedstock molecule that is converted

into a vast range of organic chemicals utilized in modern life. The chemistry of

ethene is dominated by the reactivity of its CPC double bond. Especially impor-

tant is the

addition reaction, in which two parts of another chemical species be-

come added to the atoms at either end of the double bond. The CPC of ethene

is converted into a single bond in the process. The pi electrons of the double bond

end up participating in the new bonds holding the groups that have become

added “across” the double bond. We’ve already examined one of the addition re-

actions with ethene, hydrogenation.

As another example, ethene can be converted into ethanol (CH

OH) by

reaction with water in the presence of phosphoric acid (H

) catalyst at 330°C.

Industrially, ethanol can be made in large quantities using this process. However,

because of the value of ethene as a feedstock for other organic molecules, much

of the ethanol that is made in the United States today comes from the fermenta-

tion of sugars found in corn. Ethanol is an example of an

alcohol, a compound

carrying the

hydroxyl group (—OH group).

Another addition reaction converts ethene into dichloroethane (ethylene

dichloride), an intermediate in the production of the seemingly ubiquitous ma-

terial polyvinyl chloride (PVC).

Ethene is the starting material for a great many of the plastics that are such a

common feature of modern life (see Chapter 13). The simplest of these plastics is

polyethene, which is made when huge numbers of ethene molecules participate in

addition reactions with themselves to generate long chain-like polyethene mole-

cules. In the reaction, one molecule of ethene adds to an adjacent molecule of

ethene, which adds to another molecule of ethene, and so on. The result is a very

long chain of carbon atoms.

H H

330C

H H

CHO

514 Chapter 12 Carbon

12.8 Ethene, the CPC Bond, and Polymers 515

Polyethene, which is more commonly known as polyethylene, is an example of a

polymer (literally,“many units”), a compound composed of large molecules made

by the repeated bonding together of smaller

monomer (literally, “one unit”) mol-

ecules, ethene in this case. Because the reaction that bonds the ethene monomers

together is an addition reaction, polyethylene is known as an

addition polymer,

formed by the process of

addition polymerization.

Different types of polyethylene can be made by varying the temperatures and

pressures of the reaction, as well as changing the type of catalysts and initiating

substances used. These different forms are classiﬁed into two main types:

high-

density polyethylene (HDPE)

, in which the molecules are long and linear with no

branch points, and

low-density polyethylene (LDPE), with shorter, branched-chain

molecules, as shown in Figure 12.14. The length of the polyethylene chain im-

parts different properties to the resulting molecule. For instance, because we

know that shorter alkanes have lower melting points, you might expect a shorter

polyethylene chain to have a lower melting point than a longer polymer.

The high-density part of HDPE is a result of the close packing of the linear

molecules. This makes HDPE a strong plastic, used to make such things as blow-

molded bottles and toys. LDPE has a more ﬂexible, less crystalline structure; it is

used to make such things as plastic trash and grocery bags, packaging ﬁlms, insu-

lation sleeves around electrical cables, and squeeze bottles. The properties of

polyethylene can be modiﬁed by incorporating some other atoms, such as chlo-

rine or sulfur, into its structure as it is formed. This can make the resulting mod-

iﬁed polyethylene more resistant to oxidation, for example, or more ﬂexible.

Many of the polyethylene-based materials around us are actually composed of

such modiﬁed polyethylenes.

We can prepare polymers that have properties different from those of poly-

ethylene, and yet are closely related to it in chemical structure, by using

monomers in which one or more of the hydrogen atoms of ethene are replaced

with other atoms or larger chemical groups. Table 12.7 lists some of these.

Polymerize

Polyethene (polyethylene)

H H

FIGURE 12.14

HDPE and LDPE. The molecular structures

of HDPE and LDPE are similar, but their

processing gives them unique properties

for use in different materials.

Video Lesson: Organic Polymers

516 Chapter 12 Carbon

12.9 Alcohols

As the alcohol found in alcoholic beverages, ethanol is the best known of the al-

cohols. Ethanol can be made by hydrating ethene. However, because of the value

of ethene as a feedstock for other organic molecules, and because of the toxicity

of some of the by-products of this process, much of the ethanol that is typically

used for human consumption is made by the biological activity of yeast cells. In

this reaction, the yeast consumes glucose in a process known as

fermentation (see

Chapter 14). The waste products of the process are ethanol and carbon dioxide.

These waste products are toxic to the yeast and are excreted from the cell. Fortu-

nately, the industrial chemist can easily harvest the ethanol by distilling it from

the solution. The chemist also can capture the CO

gas given off in the reaction in

order to make other products.

Yeast

H2C

H  2CO

Polymerization of Some Common Monomers

Monomer 3-D Structure Polymer Uses

Vinyl chloride Polyvinylchloride (PVC)

Styrene Polystyrene

Propene Polypropylene

Acrylonitrile

Polyacrylonitrile

H C

C……

H C

NNN

H CH

C……

H CH

H C

C……

H C

H Cl

C……

H Cl

TABLE 12.7

Indoor plumbing, toys,

plastic wrap, vinyl

siding

Outdoor furniture,

insulation, packing

“peanuts”

Contained in

indoor–outdoor

carpeting

Orlon, Acrilon clothing,

yarns, wigs

Video Lesson: Alcohols,

Ethers, and Amines

12.10 From Alcohols to Aldehydes, Ketones, and Carboxylic Acids 517

12.10 From Alcohols to Aldehydes,

Ketones, and Carboxylic Acids

In addition to determining the rate of reaction, the distinction between primary,

secondary, and tertiary alcohols is important in determining what types of mol-

ecules can be made from the alcohol. For example, primary alcohols can undergo

oxidation to form

aldehydes, which carry a carbonyl functional group (CPO) with

at least one H atom bonded to the carbonyl carbon atom. The oxidation is usu-

ally performed with a highly oxidized metal such as chromium (VI). In the lab,

however, special reactions must be performed to prevent the overoxidation of

the aldehyde. Methanol, for example, can be oxidized to the aldehyde methanal

CPO), also known as formaldehyde. The solution formed when form-

aldehyde is dissolved in water is most commonly known as formalin, used to pre-

serve biological specimens.

Oxidation

HHC

Methanol Methanal

Primary alcohol Secondary alcohol Tertiary alcohol

The ethanol that is distilled from the fermentation mixture unavoidably con-

tains a small amount (roughly 5%, by volume) of water. For beverages, this water

is not a problem. However, if the ethanol is to be used as an additive to fuel used

in automobiles, the water must be removed. The process used to make 100%

ethanol by fermentation often leaves behind traces of toxic compounds, which

render the pure ethanol undrinkable.

Other alcohols of note include methanol, 2-propanol, the “dihydroxy alcohol”

1,2-ethanediol, and the “trihydroxy alcohol”1,2,3-propanetriol, whose structures

and uses can be found in Table 12.8. Note the way in which numbers are used

to indicate which carbon atom carries the —OH (hydroxy) group or groups in

alcohol structures. We can also use the structures themselves to explain the way

in which alcohols are classiﬁed into categories known as primary, secondary, or

tertiary alcohols, a type of classiﬁcation that can be applied to other functional

groups as well.

Primary alcohols have only one carbon atom bonded to the carbon atom that

carries the hydroxyl group. This means that ethanol is a primary alcohol.

Sec-

ondary alcohols

have two carbon atoms bonded to the carbon atom that carries

the hydroxyl group, so 2-propanol is a secondary alcohol.

Tertiary alcohols have

three carbon atoms bonded to the carbon atom that carries the hydroxyl group,

so 2-methyl-2-propanol is a tertiary alcohol. This classiﬁcation system is well cor-

related with the reactivity of the alcohols. For instance, primary alcohols undergo

many reactions more rapidly than do secondary or tertiary alcohols. For exam-

ple, primary alcohols react fastest, and tertiary alcohols slowest, in the formation

of esters.

Video Lesson: CarbonyI-

Containing Functional Groups