11.5
Making Connections
12. Oil spills in ocean water cause a wide variety of environmental
problems. Use your knowledge of alkanes to describe and
explain what happens physically and chemically when oil is
spilled from a tanker.
13. The density of air at SATP is about 1.2 g/L. If a pure gas is
assumed to be an ideal gas, then its density can be calculated
using its molar mass and the molar volume at SATP (24.8 L/mol).
(a) What is the density of methane gas at SATP in grams per litre?
(b) Explain why propane-powered vehicles are prohibited from
parking in underground parkades, while natural gas vehicles
are allowed.
(c) Why are gasoline vehicles allowed to park underground?
Alkenes and Alkynes
11.5
Laboratory evidence of hydrocarbon mixtures reveals that there are more kinds
of hydrocarbons than just alkanes. Molecular formula determinations reveal the
chemical formulas shown in Table 1.
Table 1: Series of Empirical Chemical Formulas of Hydrocarbons
Series 1
Series 2
Series 3
C2H6(g)
C2H4(g)
C2H2(g)
C3H8(g)
C3H6(g)
C3H4(g)
C4H10(g)
C4H8(g)
C4H6(g)
C5H12(l)
C5H10(l)
C5H8(l)
Analysis of these series of hydrocarbons reveals a pattern, not only in their
formulas, but also in their chemical properties. Series 1, which you will recognize
as alkanes, are relatively unreactive compared with the other two series. The molecules in series 1 are alkanes; those in series 2 are called alkenes, and the molecules in series 3 are called alkynes. Like alkanes, alkenes and alkynes each form a
homologous series:
CnH2n+2
CnH2n
CnH2n2
alkanes
alkenes
alkynes
To explain these series, chemists apply the theory of bonding and bonding
capacity to draw structural diagrams. The structural diagrams reveal that the
series can be explained using current theory if series 1 contains C –C single
bonds, series 2 involves one CC double bond, and series 3 is explained by one
CC triple bond. For example, consider the structural diagrams for the three
hydrocarbons with two carbon atoms per molecule:
Hydrocarbons 543
(a)
ethane
complete
C2H6(g)
(b)
H
ethene
(ethylene)
H
ethyne
(acetylene)
alkynes: a hydrocarbon family of molecules
that contain at least one carbon–carbon
triple bond; general formula, CnH2n–2
Table 2: The Alkene Family of
Organic Compounds
IUPAC name
(common name)
Molecular
formula
ethene (ethylene)
C2H4(g)
propene (propylene)
C3H6(g)
1-butene (butylene)
C4H8(g)
1-pentene
C5H10(1)
1-hexene
C6H12(1)
–ene
CnH2n
544 Chapter 11
C
C
H
H
CH3 – CH3
H
H
H
C
C
condensed
CH2 CH2
H
complete
C2H2(g)
alkenes: a hydrocarbon family of molecules
that contain at least one carbon–carbon
double bond; general formula, CnH2n
H
complete
C2H4(g)
(c)
H
condensed
H
C
C
condensed
H
CH
CH
The bonding capacity of carbon requires four covalent bonds. With insufficient
hydrogen to saturate the molecule with single bonds, a double bond is required
to explain the C2H4(g) formula, and a triple bond is required to explain C2H2(g).
All alkanes, therefore, are explained as having all carbon–carbon single bonds;
alkenes have one or more double bonds; and alkynes have one or more triple
bonds.
Qualitative and quantitative analysis of petroleum tells us that hydrocarbons
containing double or triple covalent bonds are relatively minor constituents.
However, these compounds are often formed during cracking reactions during
the refining of crude oil and are valuable components of gasoline. Hydrocarbons
containing double or triple bonds are vitally important in the petrochemical
industry because they are the starting materials for the manufacture of many
derivative compounds, including most kinds of plastics.
Hydrocarbons with carbon–carbon double bonds are members of the alkene
family (Figure 1). The names of alkenes with only one double carbon–carbon
bond have the same prefixes as the names of alkanes but take the suffix
-ene (Table 2).
H
H
H
H
C
C
H
H
C
C
C
H
H
ethene
H
H
propene
Figure 1
Ethene and propene are the simplest members of the alkene family. Ethene, called ethylene in
the petrochemical industry, is the starting material for an enormous number of consumer, commercial, and industrial petrochemical products. The double covalent bonds are shorter and
much more reactive than single carbon–carbon bonds.
11.5
The alkyne family has chemical properties that can be explained by assuming
the presence of a triple bond between carbon atoms (Figure 2). Like alkenes,
alkynes are unsaturated and react immediately with small molecules such as
hydrogen or bromine in an addition reaction; that is, the reaction allows new atoms
to add. Alkynes are named like alkenes, except for the -yne suffix. The simplest
alkyne, ethyne (acetylene), is commonly used as a high-temperature fuel
(Figure 3). In fact, C2H2(g) is the only commercially important alkyne, with huge
amounts being produced annually as fuel for welding and cutting torches and as
starting material for the manufacture of ethanoic acid (acetic acid) and many types
of plastics, as well as synthetic rubber for the tire industry. Table 3 lists the first five
members of the alkyne family. Isomers exist for all alkynes larger than propyne.
Table 3: The Alkyne Family of
Organic Compounds
H
H
C
C
H
H
C
C
C
H
ethyne
propyne
H
Figure 2
Ethyne and propyne are the simplest members of the alkyne family. Ethyne, called acetylene in
industry, is the fuel used in welding torches. Triple bonds are the shortest and most reactive of
all carbon–carbon bonds.
IUPAC name
(common name)
Molecular
formula
ethyne (acetylene)
C2H2(g)
propyne
C3H4(g)
1-butyne
C4H6(g)
1-pentyne
C5H8(1)
1-hexyne
C6H10(1)
–yne
CnH2n–2
Figure 3
The flame of an oxyacetylene torch is hot
enough to melt metals easily. Pure oxygen
reacts extremely rapidly with the triple bonds
of ethyne, releasing large quantities of
energy in a very short time.
Naming Alkenes and Alkynes
Since the location of a multiple bond affects the chemical and physical properties of a compound, IUPAC decided that an effective naming system should
specify the multiple bond location. Alkenes and alkynes are named much like
alkanes, with two additional points to consider:
• The longest or parent chain of carbon atoms must contain the multiple
bond, and the chain is numbered from the end closest to the multiple bond.
• The name of the parent chain of the compound is preceded by a number
that indicates the position of the multiple bond on the parent chain.
Hydrocarbons 545
• The name of any branch (e.g., alkyl group) is preceded by a number that
indicates the position of the branch on the parent chain. This is the same
procedure used with alkanes.
For example, there are two possible butene isomers: 1-butene and 2-butene. (The
isomers can also be named but-1-ene and but-2-ene, but we will not use that
system in this book.)
CH 2
1
CH
CH 2
2
CH 3
CH 3
3
4
CH
2
1
1-butene
CH
3
CH 3
4
2-butene
Sample Problem 1
Name the hydrocarbon petrochemicals (a) and (b).
(a)
CH3
CH3 — CH — CH
CH2
(b)
CH3
CH3 — CH
CH — CH2 — CH — CH3
Solution
The IUPAC name for (a) is 3-methyl-1-butene.
The IUPAC name for (b) is 5-methyl-2-hexene.
In the following branched alkyne structure, the parent chain is pentyne and
there is only one branch, a methyl group:
CH 3
CH 3
1
C
2
C
3
CH
4
CH 3
5
4-methyl-2-pentyne
The location of the multiple bond in an alkene or alkyne takes precedence
over the location of the branches in numbering the carbon atoms of the parent
chain. The IUPAC name 4-methyl-2-pentyne follows the same format as that
used for alkanes. Branches are listed in alphabetical order. Branched alkynes are
rare.
Structural Diagrams from Alkene and Alkyne Names
Whenever you need to draw a structural diagram for any hydrocarbon, you
should always look at the end of the name to find the parent chain. You draw the
parent alkene or alkyne first and then add the branches listed in the name. Be
sure to finish the structure with sufficient hydrogen atoms to complete four
bonds of each carbon atom. The following sample problem shows some typical
examples of alkenes and alkynes.
Sample Problem 2
Draw structural diagrams for the following alkyne petrochemicals:
(a) 4-methyl-1-pentyne
(b) 3,3-dimethyl-1-butyne
546 Chapter 11
11.5
Solutions
(a)
CH3
CH3 — CH — CH2 — C
(b)
CH
CH3
CH
C — C — CH3
CH3
Cycloalkenes and cycloalkynes are classes of hydrocarbons without many
members. Chemists explain this low membership by the stress put on the double
and triple bonds by creating a cyclic hydrocarbon. However, there are such molecules as cyclohexene, a six-carbon cyclic molecule with one double bond
(Figure 4). It does not matter where the double bond is shown and no number
is necessary. There is only one cyclohexene. Cycloalkanes are isomers of alkenes
with the same number of carbon atoms, both with the general formula CnH2n.
Cycloalkenes, similarly, are isomers of alkynes, both with the general formula
CnH2n–2.
Isomers of alkenes and alkynes exist for different locations of the double or
triple bond and by changing a straight-chain hydrocarbon into a branched
hydrocarbon or into a cyclic hydrocarbon. If you find that several structures have
the same formula but different names, then the structures are isomers.
Understanding Concepts
1. Classify each of following hydrocarbons as an alkane, alkene, or
alkyne and/or as a cycloalkane or cycloalkene.
(a) C2H4(g)
(b) C3H8(g)
(c) C4H6(g)
(d) C5H10(l)
C
H
H
C
C
C
C
H
H
H
H
C
H
H
Practice
H
H
C
H
C
H
H
H
H
C
C
C
C
H
H
H
H
Figure 4
Cyclohexene is a cycloalkene and an isomer
of the alkyne hexyne. Both have the formula
C6H10(l).
2. Draw a structural diagram and write a chemical formula for each of
the following.
(a) propane
(b) propene
(c) propyne
(d) cyclopropane
3. Draw structural diagrams for each of the following
petrochemicals.
(a) propene
(b) 2-butene
(c) 2,4-dimethyl-2-pentene
(d) 1-butyne
4. Why are no numbers required for the location of the multiple bonds
in propene or propyne?
Hydrocarbons 547
H
5. Write IUPAC names for each of the following structures:
(a)
CH3
CH2
CH — C — CH2 — CH3
CH3
(b)
CH3
CH3 — C — CH
CH2
CH2 — CH3
(c)
CH3
CH3 — C
CH3
C — CH — CH — CH2
CH2 — CH3
6. Draw structural diagrams and write the IUPAC names for the four
structural isomers of C4H8(g). (Remember alicyclic compounds.)
7. Alkenes and alkynes are the starting materials in the manufacture of
a wide variety of organic compounds. Draw structural diagrams for
the following starting materials that are used to make the products
named in parentheses.
(a) propene (polypropylene)
(b) methylpropene (synthetic rubber)
Properties of Alkenes and Alkynes
Table 4: Boiling Points of Alkanes
and Alkenes
Alkane
name
Boiling
point
(°C)
Alkane
name
Boiling
point
(°C)
ethane
–88.6
ethene
–103.7
propane
–42.1
propene
–47.4
butane
–0.5
1-butene
–6.3
pentane
36.1
1-pentene
30.0
unsaturated hydrocarbon: a reactive
hydrocarbon whose molecules contain double
and triple covalent bonds between carbon
atoms; for example, alkenes and alkynes
548 Chapter 11
Hydrocarbons with molecules containing one or more carbon–carbon double
bonds (alkenes) or triple bonds (alkynes) have very similar physical properties to
alkanes of the same molar mass. Melting points, boiling points, solubilities, and
densities are all very much like those of comparable alkanes. A change of two or
four hydrogen atoms and their electrons is usually a small change in the total
number of electrons and, therefore, only a small change in London forces; however, it is sometimes measurable. For example, with two fewer hydrogen atoms,
the alkenes have a slightly lower boiling point than the alkanes (Table 4).
However, double or triple bonds between carbon atoms in the molecules
dramatically affect the chemical properties of the substance. For example, hydrocarbons with double bonds react quickly at room temperature with bromine,
compared with alkanes, which react extremely slowly (Figure 5). Organic compounds with carbon–carbon double and triple bonds are said to be unsaturated
because fewer atoms are attached to the carbon atom framework than the
number that could be attached if all the bonds were single.
Note that the reaction of a double bond allows two new atoms to add, and
the reaction of a triple bond allows up to four new atoms to add. Both of these
reactions require only a rearrangement of the electrons involved in the double
and triple bonds leaving those forming the single carbon–carbon bond unaffected. These reactions—addition reactions—are generally very fast.
11.5
H
H
H
H
H
C
C
C
C
H
H
H + H
H
→ H
H
H
C
C
C
H +
2H
H
→ H
H
H
H
H
H
C
C
C
C
H
H
H
H
H
H
H
C
C
C
H
H
H
H
H
A diagnostic test for the presence of multiple bonds is the bromine water test
(Figure 5): If bromine water is added to a hydrocarbon and the orange bromine
colour disappears instantly, then a multiple bond is likely present. If bromine is
added and the orange colour remains, then the hydrocarbon is likely saturated,
for example, an alkane. The reaction explaining this diagnostic test is as follows,
using ethylene as an example:
CH2CH2 + Br—Br → CH2Br—CH2Br
colourless
orange
(fast)
colourless
This is a very fast reaction compared to the substitution reaction that saturated hydrocarbons undergo:
Figure 5
Bromine water (a saturated aqueous solution of
bromine) is used in a diagnostic test for
unsaturated organic compounds. When an
equal amount of bromine water is added simultaneously to cyclohexane and cyclohexene, the
unsaturated cyclohexene reacts with the
bromine water instantaneously, decolourizing
the orange solution. In the saturated cyclohexane, there is no immediate colour change,
which is interpreted as no reaction.
CH3— CH3 + Br—Br → CH3—CH2Br + H—Br
colourless
orange (slow)
colourless
turns moist blue litmus red
The two compounds with the empirical formulas C6H12(l) and C6H10(l) have
very similar physical properties. Physical properties alone cannot be used to
identify separate samples of the two chemicals. A chemical diagnostic test that
can be used to differentiate these chemicals is the reaction with bromine water or
aqueous potassium permanganate. The slow reaction of C6H12(l) with either of
these reactants indicates the presence of single bonds, that is, a saturated compound. The rapid reaction of C6H10(l) indicates the presence of multiple (double
or triple) bonds, an alkyne or cycloalkene. The simplest interpretation of these
results is that C6H12(l) is cyclohexane and C6H10(l) is cyclohexene (Figure 6).
(a)
(b)
H H
H
H C
H
C
C
H
H
H
H C
C
C
H
C
H
H
or
H C
H
C
C
H
H
H H
or
H C
H
C
H H
cyclohexane
cyclohexene
Figure 6
The structural diagram of cyclohexane (a)
shows that all bonds are single bonds. The
cyclohexene structure (b) indicates one
carbon–carbon double bond. The second
structure for diagrams (a) and (b) represents
the same molecules with simpler line
(polygon) diagrams.
From a theoretical perspective, cyclohexane and cyclohexene are believed to
be almost identical, except for the presence of a double bond between two carbon
atoms in cyclohexene. These compounds illustrate a relationship between
structure and reactivity: Cyclohexene reacts rapidly with bromine water or
aqueous potassium permanganate but cyclohexane does not. The reaction is
Hydrocarbons 549
indicated by the disappearance of the orange colour of the bromine or the purple
(pink) of the potassium permanganate.
SUMMARY
Hydrocarbon
Diagnostic Test Results for Saturated
and Unsaturated Hydrocarbons
Br2(aq)
KMnO4(aq)
Rate
saturated
orange
purple
slow
unsaturated
colourless
brown
fast
Practice
Understanding Concepts
8. Write a generalization describing the trend in boiling points for
(a) an increasing size of aliphatic hydrocarbon molecules
(b) alkanes and alkenes with the same number of carbon atoms per
molecule
9. Provide theoretical definitions for saturated and unsaturated
hydrocarbons.
10. Describe two diagnostic tests for saturated and unsaturated
hydrocarbons.
11. Draw condensed structural diagrams for cylcohexane and
cyclohexene.
Applying Inquiry Skills
12. Due to the potential hazards of doing diagnostic tests for cyclohexane
and cyclohexene with bromine, these tests are available for viewing
on the Internet. How does the reaction of cyclohexane with bromine
compare with that of cyclohexene?
Follow the links for Nelson Chemistry 11, 11.5.
GO TO
INQUIRY SKILLS
Questioning
Hypothesizing
Predicting
Planning
Conducting
Recording
Analyzing
Evaluating
Communicating
www.science.nelson.com
Investigation 11.5.1
Evidence for Multiple Bonds
The purpose of this investigation is to use the bromine or potassium permanganate diagnostic test to identify which of the samples provided are saturated
and which are unsaturated. Cyclohexane and cyclohexene are provided as
optional examples of saturated and unsaturated compounds to model the reaction with bromine water. You will complete the Analysis section of the lab report.
Question
Which of the common substances tested are saturated and which are
unsaturated?
Experimental Design
The unknown samples and two controls (e.g., cyclohexane and cyclohexene) are
tested by adding a few drops of a diagnostic test solution (e.g., potassium per550 Chapter 11