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

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Branched-Chain Alkanes: Isomers of the Normal Alkanes

Crude oil also contains a signiﬁcant proportion of alkanes with “branched”

chains. Each branched-chain alkane has the same formula as a corresponding

straight-chain (normal) alkane, so each one can be regarded as a

structural isomer

of a normal alkane. Structural isomers always have the same formula but differ in

the way the atoms are attached. We say that they share the same molecular for-

mula but have different molecular structures. Structural isomers do not have the

same properties. For instance, the structural isomers pentane and 2,2-dimethyl-

propane have the same formula (C

). Their boiling points, however, are much

different: 36°C for pentane and 10°C for 2,2-dimethylpropane.

EXERCISE 12.1 Finding the Isomers

How many structural isomers of pentane (C

) exist? How many structural iso-

mers are there for hexane (C

First Thoughts

Isomers must have the same atoms arranged in different ways. Isomers of alkanes

differ in their skeleton of bonded carbon atoms. We can ﬁnd all the isomers by

drawing the possible different carbon skeletons then adding the hydrogen atoms

needed to ensure that each carbon atom has four bonds.

Solution

Isomers of pentane:

Isomers of hexane:

498 Chapter 12 Carbon

2-Methylbutane (isopentane)

CH CH

2-Methylpentane (isohexane)

CH CH

2,4-Dimethylpentane

2,2,4-Trimethylpentane (isooctane)

CH CH

CH CH CH

CH CH

CH CH CH

CH CH

Further Insights

Why are the two structures below not counted as different isomers of hexane?

Although at ﬁrst glance they look different, they are actually the same molecule just

ﬂipped over.

12.3 Hydrocarbons 499

Here are two more examples of structures that initially appear different but are

in fact identical. You can tell that they’re identical by noting the attachments of the

carbon atoms. In each case, there are four carbons in a row, with the branched

carbons emanating from the middle two carbon atoms. You need to be careful to

eliminate such identical structures when searching for isomers. Working with three-

dimensional models, rather than two-dimensional drawings, can make this easier to

follow.

PRACTICE 12.1

Draw all of the isomers of heptane (C

See Problems 13, 14, 21, and 22.

Naming the Alkanes

With over 16 million known organic chemicals, it is useful to have an interna-

tionally agreed-upon set of nomenclature rules. We hinted at such a system with

the ﬁrst 10 normal alkanes in Table 12.4. These names form the basis of the

more complex names given to branched-chain alkanes. This molecule is called

3-methylheptane. Why?

1. We identify and name the longest straight chain in the molecule, which in this

case is derived from heptane.

3-Methylheptane

2. We name the branch, using the name of the alkane with the same number of

carbon atoms as are in the branch (methane, in this case). However, we replace

the -ane ending with -yl to indicate that the group is a branch. The word

methyl then is added to the front of our molecule’s name, which becomes

methylheptane.

3. We identify the location of the branch by associating it with the numbered

carbon atom of the main chain to which it is attached. We number the main

chain from the end that gives the branch point the lowest number. The ﬁnal

name is 3-methylheptane.

Various other rules guide us in naming more complex alkanes. These rules,

collectively, are known as IUPAC nomenclature rules after the International

Union of Pure and Applied Chemistry, whose members developed the naming

system. All compounds in the world can be named using IUPAC nomenclature,

but many compounds have less structured names. For instance, as we learned in

Chapter 2, dihydrogen monoxide is commonly known as water. The common

names, unfortunately, have few rules—and many exceptions. It is best to learn the

common names as you are exposed to the compound, though with all but the

most common compounds, we will use the IUPAC nomenclature.

Cyclic Alkanes

Carbon atoms can be bonded into rings, to form cyclic alkanes. The four simplest

cyclic alkanes are shown below. Cyclohexane is used in the preparation of nylon

ﬁber and is a nonpolar solvent. Cyclopentane is increasingly employed as a

foaming agent—a substance that is needed to create foam—in the preparation of

insulation for use in refrigerators. It replaces some of the chloroﬂuorocarbons

implicated in global warming.

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-to-carbon double

bond (CPC). The members of this class of compounds are important to the

chemical industry as starting materials in the manufacture of a host of organic

compounds. Only tiny amounts of alkenes are found in crude oil, but certain

alkenes are made from crude oil in huge quantities, in processes that we will

consider shortly. The simplest three alkenes are shown at the top of the next page.

500 Chapter 12 Carbon

Cyclopropane

Cyclobutane

Cyclopentane

Cyclohexane

Video Lesson: Alkenes and

Alkynes

FIGURE 12.9

Photograph of oxy-acetylene welding.

High temperatures are obtained during

the combustion of acetylene. This melts

the metals, allowing a strong weld to form.

12.3 Hydrocarbons

501

Ethene

Propene

1-Butene

Ethene Ethane

Alkynes

Hydrocarbons with a carbon-to-carbon triple bond are known as alkynes. Those

with only one triple bond form a homologous series with the general formula

(2n–2)

. The ﬁrst three members of this series are shown below. Ethyne (acety-

lene) is burned with oxygen to release the energy that generates very high

temperatures in oxy-acetylene welding, shown in Figure 12.9.

Ethyne

H C HC

Propyne

H C CC

1-Butyne

H C CC

The names of the alkenes are derived from the names of the corresponding alka-

nes, but with -ene at the end instead of -ane. The simple alkenes, those with only

one double bond, form a homologous series with the general formula C

This general formula is also the same as that of the cyclic alkanes.

Alkenes are known as

unsaturated hydrocarbons because, unlike the alkanes,

they are not “saturated” with hydrogen. Each CPC bond in an alkene can react

with one hydrogen molecule, under suitable conditions, to generate the corre-

sponding alkane. For example, the

hydrogenation (addition of a molecule of hy-

drogen) of ethene gives ethane. This is a general reaction of the alkenes. Platinum

metal is a catalyst in this reaction and therefore is indicated over the arrow in the

chemical equation that represents the reaction.

EXERCISE 12.2 Care with General Formulas

We have seen that the general formula for the alkenes is C

. Does this mean that

every hydrocarbon with that general formula must be an alkene?

Solution

Can you draw any hydrocarbon structure that has twice as many hydrogen atoms as

carbon atoms, but no double bonds? It cannot be done using straight chains or

branched chains of carbon atoms, but it can be done if the carbon chain bonds back

on itself, to form a cyclic hydrocarbon. As you can verify from the examples below,

cyclic alkanes share the general formula C

with alkenes.

PRACTICE 12.2

Provide a structural drawing for a cyclic hydrocarbon that would be an isomer of

hexene.

See Problems 23 and 24.

The general formula C

is not unique to alkenes. We saw in Exercise 12.2 that

each alkene with three or more carbon atoms will have a cyclic alkane as one of

its isomers. For example, cyclobutane is an isomer of butene:

Isomers do not always have to belong to the same homologous series; they just

have to have the same atoms bonded in different ways.

The naming of alkenes and alkynes is complicated by the need to distinguish

exactly where the double or triple bond lies within the molecule. This is handled

by attaching a number in front of the name of the molecule. The number corre-

sponds to the location of the ﬁrst carbon in the double or triple bond. Just as in

naming branched alkanes, the number we use must be the lowest number that

can be generated by numbering the longest carbon chain. For example,

CHPCHCH

is 2-pentene, not 3-pentene. In this way, we see that there

are only two alkenes with the formula C

: 1-butene (CH

PCHCH

) and

2-butene (CH

CHPCHCH

Geometric Isomers

Geometric isomers are common in the alkenes. What is a geometric isomer? Look-

ing at the boiling points of two of the isomers of C

shown at the top of the

next page will help us arrive at an answer to this question.

1-Butene

H H

Cyclobutane

C H

Cyclobutane

Cyclopropane

502 Chapter 12 Carbon

Video Lesson: Isomers

We see that two cyclic molecules, cyclobutane (A) and methylcyclopropane (B),

are the only alkanes we can draw. The remainder of the molecules with the for-

mula C

are alkenes, including 1-butene (C), 2-methylpropene (D), and two

different 2-butenes (E and F). Are the two 2-butenes really different? We could

imagine that if we rotated the CPC bond around, we would have the same struc-

tures. However, the CPC bond doesn’t allow rotation to occur, so these two mole-

cules cannot be redrawn to show the same structure. In fact, their physical prop-

erties indicate to us that they are completely different. This is where the boiling

points prove important. Molecule (F) has a boiling point of 3.7°C, whereas mol-

ecule (E) has a boiling point of 1°C, giving evidence for two different molecules.

To distinguish between these two different molecules, we use the designations

cis and trans to describe their structures, as illustrated in Figure 12.10. These pre-

ﬁxes are always given in italics. In the

cis form, the groups on either side of the

CPC are on the same side of the molecule (Figure 12.10). (As a remembering de-

vice, think of cis as “sisters.”) The molecule with the 3.7°C boiling point is cis-2-

butene. In the

trans form, the groups on either side of the CPC are on opposite

sides of the molecule (Figure 12.10). (Think about the groups having to transfer

from one side to the other.) This molecule is trans-2-butene.

In general, geometric isomers are isomers that differ in the location of the

atoms only because of the geometry of the carbon to which they are attached. Cis

and trans isomers are geometric isomers because the carbons of the alkene group

have a ﬁxed geometry.

12.3 Hydrocarbons 503

C H

cis trans

FIGURE 12.10

Cis and trans isomers of the alkenes. The cis isomer contains groups on the same side

of the alkene. The trans isomer contains groups on opposite sides of the alkene.

The side-to-side overlap of adjacent

p orbitals within the C

PC bond gives

rise to the pi bond between the carbon

atoms. As an atom on one end of the

bond begins to rotate the p orbital over-

lap decreases. Thus, the pi bond restricts

rotation of the atoms at either end.

EXERCISE 12.3 Cis and Trans

Consuming trans fatty acids is considered bad for your health. They arise when

vegetable oil is hydrogenated. The resulting partially hydrogenated vegetable oils con-

tain a small amount of trans fats. Is the compound below a cis or a trans fatty acid?

Solution

This compound is a trans fatty acid. The groups on either end of the CPC are on

opposite sides of the molecule

PRACTICE 12.3

Draw all of the alkenes with the formula C

and indicate whether each is cis or

trans.

See Problems 25 and 26.

Aromatic Hydrocarbons

Aromatic hydrocarbons contain one or more aromatic rings, which were discussed

in Chapter 9. A common aromatic compound is benzene. This compound and its

derivatives are used in the production of a vast array of consumer products such

as polystyrene and other plastics, nylon, detergents, and pharmaceuticals. The

molecule consists of three alternating carbon-carbon double bonds in a six-

carbon ring. Benzene and several more complex aromatic compounds are shown

on page 505.

Alkyl Groups

Many hydrocarbons can be regarded as being composed of a main chain of car-

bon atoms with various hydrocarbon branches attached. These branches are

known as

alkyl groups, and many have a name derived from the name of the

alkane on which they are based. The names of other structures are derived from

common names. The names of alkyl groups are placed in front of the parent

name in alphabetical order.

—CH

is a methyl group.

—C

is an ethyl group.

—C

is a propyl group.

—C

is a butyl group.

—C

is a pentyl group.

CHO

HH H

504 Chapter 12 Carbon

Phenyl group

—C

is a hexyl group.

—C

is a heptyl group.

—C

is an octyl group.

—Ph (C

) is a phenyl group.

Video Lesson: Aromatic

Hydrocarbons

CHH

Benzene

CHH

Naphthalene

Pyrene Phenanthrene

CHH

Anthracene

12.3 Hydrocarbons

505

EXERCISE 12.4 Naming with Alkyl Groups

Fill in the blanks, corresponding to the type of alkyl group, to complete the names

of the hydrocarbons shown.

CH CH

4-[ ] octaneb.

CH CH

2-[ ] butanea.

CH C CH

]-2,2-[3-[ ] pentaned.

CH CH CH

2,3-[ ] butanec.

506 Chapter 12 Carbon

Solution

a. 2-methylbutane c. 2,3-dimethylbutane

b. 4-ethyloctane d. 3-ethyl-2,2-dimethylpentane

PRACTICE 12.4

Draw the structure of 2,2-dimethyl-4-propyloctane.

See Problems 17 and 18.

12.4 Separating the Hydrocarbons

by Fractional Distillation

Crude oil contains a wide variety of carbon-based compounds. Separating those

compounds so that they can be used to make fabrics, medicines, and other com-

pounds we need everyday is an important procedure. The ﬁrst step in this process

is to separate the oil into several “fractions” using fractional distillation, as shown

in Figure 12.11. Each of the fractions contains a selection of hydrocarbons with

generally similar physical properties. Because

a hydrocarbon’s boiling point is broadly re-

lated to its molecular size, each fraction con-

tains molecules that are similar in molecular

size. The oil is heated to around 400°C, caus-

ing much of it to vaporize. The tarry residue

that remains is a mixture of very large hydro-

carbon molecules. This residue is collected as

one of the basic fractions produced by the

fractional distillation process.

The vaporized portion of the oil rises up

the fractionating column (Figure 12.11),

which can be around 60 m in height. The

vapor cools as it rises, causing the various

fractions of the oil to condense back into

liquid form at different heights up the col-

umn, depending on their boiling points. At

selected levels, the hydrocarbons that are in

liquid form gather on collecting trays and are

piped away. The smallest (and lowest-boiling)

hydrocarbons do not condense into liquid at

all but emerge at the top of the column as

“reﬁnery gas.”

This primary fractional distillation pro-

cess is typically used to separate the crude

oil into six major fractions, summarized in

Table 12.5. These fractions can be subjected to

secondary fractional distillation or can be fed

Residue, asphalt

and up

Hot petroleum

(crude oil)

Gasoline

40–200°C

to C

Petroleum gas

<40°C

to C

Kerosene, jet fuel

200–250°C

to C

Heating oil

250–300°C

to C

Lubricating oil

300–370°C

and up

Condenser

FIGURE 12.11

Fractional distillation—a

diagram of the process and the

main fractions.

12.4 Separating the Hydrocarbons by Fractional Distillation 507

On March 24, 1989, the oil tanker Exxon Valdez ran

aground in Prince William Sound, Alaska, spilling over 40 million liters

of crude oil into the sea. Despite the massive clean-up effort, small

amounts of crude oil are still found

along the shores near the accident.

Toward the end of 1990, samples of

these oil residues were collected as

part of research to monitor the fate

of the oil as the environment slowly

recovered. It turned out, however,

that some of the samples could not have come from

the Exxon Valdez, because they contained the wrong

mixture of hydrocarbons. The debris from older oil

spills was complicating the picture. How was this

determined? It was done using a technique called

gas chromatography (GC), one of the three most

common methods for analyzing the hydrocarbon

content of crude oil.

In gas chromatography, a sample such as crude

oil that is to be analyzed is converted into vapor by

heating and then carried by a ﬂow of inert gas (such

as He) through a heated column packed with solid

powder such as silica, as shown in Figure 12.12. In

some cases, the particles of solid in the column may

be coated with a nonvolatile liquid, a technique

known as gas–liquid chromatography (GLC). As the

sample ﬂows through the column, the components

of the mixture travel at different speeds because of

the differing extents to which they are adsorbed

onto the solid phase or are soluble in the liquid

phase (in GLC). The components emerge from

the column separately and are detected,

often by their effect on a detecting ﬂame.

The apparatus will have previously been

calibrated using a range of known hydro-

carbons, so speciﬁc hydrocarbons in the

sample can be identiﬁed by comparison

with the results of the calibration.

An example of the results of an analysis

of one type of crude oil is shown in Fig-

ure 12.13. The technique is very powerful,

particularly in providing a GC “ﬁngerprint”

of different samples from different origins,

but it has signiﬁcant limitations. In particu-

lar, it cannot readily distinguish between

some of the different structural isomers of

hydrocarbons. For this reason, more thor-

ough analysis of the hydrocarbons in crude

oil also makes use of the techniques of mass

spectrometry and nuclear magnetic reso-

nance spectroscopy.

How do we know?

Which hydrocarbons are in crude oil?

FIGURE 12.13

A gas chromatogram showing the

components found in crude oil.

Pressure

regulator

Carrier

gas tank

Detector

Sample

injection

chamber

Flow

meter

Column

Oven

Amplifier Recorder

Time

Response of detector

FIGURE 12.12

The operating principles of gas chromatography and a sample

readout (a gas chromatogram).