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
11.5
manganate in water). After each sample is mixed with the test solution, evidence
of a chemical reaction (a colour change or not) is noted.
Materials
lab apron
eye protection
vinyl gloves
small test tubes with stoppers
test-tube rack
waste container, with lid, for organic substances
potassium permanganate solution in a dropper bottle
cyclohexane or hexane in dropper bottle (or propane gas)
cyclohexene or hexene in dropper bottle (or acetylene gas)
common substances, such as mineral oil, paint thinner, kerosene, liquid
paraffin, soybean oil, corn oil, margarine, butter
Procedure
Cyclohexane, cyclohexene,
acetylene, and propane are
highly flammable.
Solid potassium permanganate is an oxidant and a
toxin.
Vapours of cyclohexane,
cyclohexene, and paint
thinner are hazardous
because they are flammable
and toxic. Avoid inhaling
these vapours.
Keep test tubes stoppered
and waste containers closed.
Work in a fume hood or in a
well-ventilated area. Avoid
skin contact. Eye protection
and gloves must be worn.
1. Add 10 drops of a known saturated hydrocarbon to a clean test tube.
2. Add 1 drop of the aqueous diagnostic test solution to the test tube. Shake
the test tube gently. Repeat this procedure with up to 4 drops of the diagnostic test solution.
3. Dispose of all materials into the labelled waste container.
4. Repeat steps 1 to 3 using a clean test tube and a known unsaturated compound.
5. Repeat steps 1 to 3 using the samples provided. Use a clean test tube each time.
Analysis
(a) Answer the Question.
Geometric Isomers
In alkanes, the rotation of attached groups about the carbon–carbon single bond
is quite free. The situation is different for alkenes, where rotation about a
carbon–carbon double bond is not possible without breaking the bond.
(Molecular models are quite useful in simulating this difference in rotation
ability.) Molecular models usually provide a good 3-D representation of a molecule. When you build an alkene model with a ball-and-spring model (Figure 7),
you can see that the molecule is flat with single bonds approximately 120° from
the carbon–carbon double bond. This shape and the lack of rotation about the
double bond mean that alkenes can have geometric isomers, which differ from
each other only in the position of attached groups relative to the double bond.
Unlike structural isomers, the same atoms are bonded to the same parent chain
locations, but the molecular shape differs, depending on which side of the double
bond they are attached. These are geometric isomers; they are also known as
cis-trans isomers. The term cis means on this side, so two groups attached on
each side of the double bond are on the same side of the molecule; the term trans
means across, so two groups attached on each side of the double bond are on
opposite sides of the molecule, across from each other. For a simple example,
consider the two geometric (cis-trans) isomers of 2-butene, CH3CHCHCH3:
CH3
C
H
CH3
C
H
cis –2–butene
CH3
C
H
H
H
H
C
H
C
H
Figure 7
In this molecular model of ethene (ethylene),
notice the shape of the molecule and angles
between the bonds.
geometric (cis-trans) isomers:
Organic molecules that differ in structure
only by the position of groups attached on
either side of a carbon–carbon double bond.
A cis isomer has both groups on the same
side of the molecular structure; a trans
isomer has the groups on opposite sides of
the molecular structure.
C
CH3
trans –2–butene
Hydrocarbons 551
Practice
Understanding Concepts
13. Draw structural diagrams and write IUPAC names for the geometric
isomers of CH3CHCHCH2CH3.
14. When answering the following questions, use complete structural
diagrams to communicate your reasoning.
(a) Does 1-butene have any geometric isomers?
(b) Does 3-hexene have any geometric isomers?
(c) Can an alkene be correctly named 4-hexene?
(d) Can an alkene be correctly named 4-heptene?
15. Using diagrams, demonstrate whether it is possible to have geometric isomers of
(a) an alkane
(b) an alkene
(c) an alkyne
(d) a cycloalkane or cycloalkene
Making Connections
16. Physical molecular models are still very useful to a chemist. However,
computer molecular models, especially for large biochemically
important molecules, are now common and have become an important modelling tool. Using computer models, chemists can construct
almost any molecule, rotate it, and even simulate reactions. How do
these models appear on a computer screen? How can you manipulate the models? What are some advantages and disadvantages of
computer models? (To use computer models, you may need to install
a small free program called Chime.)
Follow the links for Nelson Chemistry 11, 11.5.
GO TO
www.science.nelson.com
Activity 11.5.1
Structures and Properties of Isomers
The purpose of this activity is to use molecular models and a chemistry reference
to reveal the structures and physical properties of some isomers of unsaturated
hydrocarbons. Structures of possible isomers are determined by means of a
molecular model kit. Once each structure is named, the boiling and melting
points are obtained from a current reference, such as The CRC Handbook of
Chemistry and Physics, The Merck Index, or Lange’s Handbook of Chemistry.
Materials
molecular model kits
chemical reference
Procedure
1. Use the required atoms to make a model of C4H8.
2. Draw a complete structural diagram of the model and write the IUPAC
name for the structure.
552 Chapter 11
11.5
3. By rearranging bonds, produce models for all isomers of C4H8, including
cyclic structures. Draw a structural diagram and write the IUPAC name for
each structure before disassembling the model.
4. If your model kit creates a C=C bond as two separate bonds, test and note
the restricted rotation of groups about the bond axis, and then construct
the geometric isomer of your model.
5. Repeat steps 1 to 4 for C4H6.
6. In a reference, find the melting and boiling points of each of the compounds you have identified.
Analysis
(a) Prepare a summary table for the molecular structures and relative physical
properties of all the substances that are isomers of C4H8 and C4H6.
Investigation 11.5.2
Preparation and Properties of Ethyne (Acetylene)
Ethyne (acetylene) is the simplest alkyne, C2H2(g). The purpose of this investigation is to test the Experimental Design, Materials, and Procedure provided about
the production of ethyne. Ethyne can be prepared readily in a laboratory by
reacting the ionic compound calcium carbide, CaC2(s), with water, H2O(l), to
produce calcium hydroxide, ethyne gas, and some energy. Complete the
Prediction, Analysis, Evaluation, and Synthesis sections of the lab report.
INQUIRY SKILLS
Questioning
Hypothesizing
Predicting
Planning
Conducting
Recording
Analyzing
Evaluating
Communicating
Question
What are the products of the reaction of calcium carbide and water?
Prediction
(a) Write and balance an equation for the reaction to synthesize ethyne.
Experimental Design
The expected gaseous product of the reaction of calcium carbide and water is
collected by water displacement. The reaction mixture is tested with litmus paper
and the expected gaseous product is tested for combustion and for saturation.
Materials
lab apron
eye protection
tongs (for handling the calcium carbide)
water
calcium carbide, CaC2(s), pea size
250-mL beaker
four 18 mm 150 mm test tubes
bromine water
test-tube rack
stopper for test tube
wooden splint and matches
limewater
red and blue litmus paper
Ethyne (acetylene) is very
flammable. Work, if possible,
in a fume hood, and attempt
to ignite only small volumes.
Calcium carbide must be
kept away from water, unless
actually being used to produce ethyne. A beaker of
cold water should be handy
to slow the reaction if it
becomes too vigorous. All
calcium carbide must be
completely reacted before
the disposal of any liquids.
Hydrocarbons 553
test tube holder
water in
test tube
beaker
water
bubbles
calcium carbide
Figure 8
Gases that have low solubility in water, like
acetylene, may be collected by downward
displacement of water.
DID YOU KNOW ?
Procedure
1. Using the tongs, add one piece of calcium carbide to about 150 mL of
water in a 250-mL beaker (Figure 8). If the reaction becomes too rapid,
cold water should be added to the beaker.
2. Collect two test tubes full of ethyne by downward displacement of water
(Figure 8). Set the labelled test tubes upside down on the countertop.
3. Collect a third test tube half full of ethyne. Lift the test tube out of the
beaker, allowing air to flow in as the water flows out, so that the test tube
has an air:ethyne mixture at a roughly 1:1 ratio. Set the labelled test tube
upside down in a test-tube rack.
4. Collect in a fourth test tube 1 cm of ethyne. Lift the test tube out of the
beaker, allowing air to flow in as the water flows out, so that the test tube
has an air:ethyne mixture at a roughly 12:1 ratio. Set the labelled test tube
upside down in a test-tube rack.
5. Add 10 drops of bromine water to test tube 1, stopper the test tube, then
shake it. Record your observations. Wash any bromine from your hands.
6. Ignite the ethyne in test tubes 2 to 4, one at a time, and record your observations for each.
7. Add a few millilitres of limewater to test tube 4 and shake. Record your
observations.
8. Use litmus paper to test the solution in the beaker.
9. Dispose of any extra ethyne in a fume hood and any extra bromine water
in the waste container provided. Wash any remaining bromine water and
limewater from the test tubes down the sink with lots of water.
Discovery of Acetylene
Analysis
Acetylene was discovered by notable Canadian
scientist Thomas Leopold Willson (1860–1915),
who was born in Princeton, Ontario, and
attended high school at Hamilton Collegiate
Institute. An active inventor and entrepreneur,
Willson is credited with more than 60 inventions, from electric arc lighting (which he
patented at age 21) to gas navigational buoys
and beacons. He is best known for his discovery
in 1892 of a more efficient process for making
calcium carbide and its byproduct, acetylene
gas. Willson’s small aluminum smelting furnace
in North Carolina produced a slag, which he
threw into the nearby stream until a large pile
accumulated. One day, upon dumping red-hot
slag into the stream, there was a dazzling burst
of flame. Willson investigated and discovered
that by adding water to the smelting furnace
slag he produced a gas that he could ignite with
a match. Willson had produced calcium carbide
and acetylene gas. Willson’s discovery of a
method to economically make calcium carbide
led to his nickname “Carbide” Willson. The discovery of acetylene helped to establish the
automotive industry: The acetylene, with
oxygen, is used as fuel in the oxyacetylene
torch, an invaluable tool in metal cutting and
welding.
(b) Answer the Question by listing the products together with the key Evidence
that identifies each product.
554 Chapter 11
Evaluation
(c) Is your Prediction supported by the Evidence gathered in this investigation? How certain are you about the evidence collected?
(d) Are the Experimental Design, Materials, and Procedure adequate for the
synthesis of ethyne? Include pros and cons, complete with your reasoning.
Synthesis
(e) Why can ethyne be collected by the displacement of water?
(f) In the Procedure, you were asked to keep the test tube with ethyne inverted
until ready for use. What does this suggest about the density of ethyne?
(g) How does the Evidence you collected illustrate incomplete and complete
combustion of ethyne?
(h) According to the Evidence you collected, what is the best ratio of air:ethyne
for complete combustion?
(i) Write a balanced chemical equation for the combustion of ethyne if the
products are
(i) carbon and water vapour;
(ii) carbon monoxide and water vapour;
(iii) carbon dioxide and water vapour;
(iv) carbon, carbon dioxide, carbon monoxide, and water vapour.
11.5
(j) How does the ratio you found for complete combustion compare with the
ratio of oxygen:ethyne in i(iii) above? (Remember that air is about 20%
oxygen.)
The Diversity of Organic Molecules
You have studied relatively small alkanes, alkenes, alkynes, and their corresponding cyclic compounds. You have also seen examples of both structural and geometric isomers. Now consider that there can easily be hydrocarbons with
hundreds of thousands of carbon atoms. There are also numerous hydrocarbon
derivatives containing carbon, hydrogen, and other nonmetal atoms. And there
are also many other kinds of isomers. So you can imagine that there must be a
staggering number of organic molecules. Of the more than 10 million compounds known, at least 90% are molecular compounds of the element carbon.
The number of known compounds of carbon far exceeds the number of compounds of all other elements combined. This observation is explained by
chemists as resulting from the combination of several properties of carbon:
• Carbon is a small atom that can form four bonds, more than atoms of
most other elements.
• Carbon atoms have the special property of being able to bond together to
form chains, rings, spheres, sheets, and tubes of almost any size (Figure 9).
• Carbon can form multiple combinations of single, double, and triple covalent bonds with itself and with atoms of other elements.
Polymers
Polymers are substances whose molecules are made up of many similar small
molecules (monomers) linked together in long chains. Polymerization is the formation of polymers from many monomers. These compounds have long existed
in nature but were only synthesized by technological processes in the 20th century. They have molar masses up to millions of grams per mole.
Figure 9
The soccer-ball-shaped C60 molecule has
pentagons of carbon atoms surrounded by
hexagons of carbon atoms. This structure of
carbon, called buckminsterfullerene, was discovered in 1985. Common soot contains this
molecule.
Addition Polymers
Many plastics are produced by the polymerization of alkenes. For example, polyethene (polyethylene) is made by polymerizing ethene molecules in a reaction
known as addition polymerization. Polyethylene is used to make plastic insulation
for wires and containers such as plastic milk bottles, refrigerator dishes, and laboratory wash bottles. Addition polymers are formed when monomers join each other
in a process that involves the rearranging of electrons in double or triple bonds in
the monomer. In addition polymerization, the polymer is the only product formed.
H
H
C
C
H
H
+
H
H
H
H
C
C + C
C
H
H
H
H
→
H
H
C
H
H
H
C + C
C + C
C
H
H
H
H
H
H
H
→
ethylene
Using tetrafluoroethene instead of ethene in an addition polymerization
reaction produces the substance polytetrafluoroethene, commonly known as
Teflon. Teflon has properties similar to polyethylene, such as a slippery surface and
a nonreactive nature. But Teflon has a much higher melting point than polyethylene, so it is used to coat cooking utensils. Polypropene (polypropylene),
polyvinyl chloride, Plexiglas, polystyrene, and natural rubber are also addition
polymers (Figure 10, page 556).
polymers: a long chain molecule made up
of many small identical units (monomers)
monomers: the smallest repeating unit of
a polymer
H
H H
H H
H
C
C
C
C C
C
H
H H
H H
H
part of polyethylene
polymerization: a type of chemical reaction involving the formation of very large molecules (polymers) from many small molecules
(monomers)
addition polymerization: a reaction in
which unsaturated monomers combine with
each other to form a polymer
Hydrocarbons 555
The Manufacture of Polypropylene
crude oil
oil refined to naphtha
extrusion into pellets
Polypropylene powder
is purified.
purified propylene
molecule
polypropylene pellets
shipped to processor
Polymerization: chain formation
is assisted by catalyst.
Figure 10
Polypropylene is one of many chemicals
derived from crude oil.
Practice
Understanding Concepts
17. Polyethene (polyethylene) is a very common plastic.
(a) The starting material for polyethylene is ethane, which is
obtained from natural gas under high pressure and low temperature. Write a chemical equation for the condensation of ethane
gas.
(b) Write a chemical equation using complete structural diagrams for
the synthesis of ethene from ethane.
(c) Write a chemical equation using complete structural diagrams for
the synthesis of polyethylene.
18. List some technological products that are made from polyethylene.
Making Connections
19. Polypropylene and polybutylene are two other common hydrocarbon
polymers. Research and list some of the main uses of each of these
polymers. Identify some benefits and risks to society and the environment of our use of polymers.
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556 Chapter 11
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Careers in the Petrochemical Industry
Many areas of industry and commerce begin with the production of
petrochemicals from petroleum; only a few of the many associated
careers are shown here.
Plastics Technologist
Plastics technologists work with a wide range
of different materials, matching properties of
different plastics to specialized applications.
They are concerned with recycling technology,
measurement, testing, and fabrication techniques controlled largely by the chemistry of,
and bonding structure within, any of the
myriad types of plastic materials.
Petrochemicals supply the raw materials for
the manufacture of most types of plastics.
Refinery Laboratory Technician
Lab technicians are generally concerned with testing and
analysis, both to monitor and control refinery processes and to
do testing for projects involving research into improvements and
changes. Familiarity with reaction and bonding theory is essential, as are the ability to operate a wide variety of technological
equipment and the skills to perform analytical techniques.
Petroleum Geologist
Geologists who work in the petrochemical industry require
specialized knowledge about the geological formations associated with underground petrochemical deposits. They work
extensively with rocks and minerals in test drilling cores and
with charts from seismic exploration of underground strata.
They must also be familiar with characteristics of fossil fuels
under extreme pressure and temperature conditions. Their
work may extend into such interesting and exotic areas as
examining the increased likelihood of earthquakes in regions
where large amounts of materials are removed from underground deposits.
Petrochemical Engineer
Industrial engineering is fundamentally concerned with making
processes more efficient and dependable. A petrochemical engineer
must understand the chemistry of the reactions, the physical changes
that occur, and the technology of the equipment and power requirements for those processes occurring in the industrial workplace.
Engineers are expected to operate in a supervisory capacity, directing
the efforts of teams of other employees. Engineers are also expected
to report findings and procedures in written reports, published papers,
and audiovisual presentations to interested groups.
Practice
Making Connections
20. Use the Internet to research any career connected with the petrochemical industry, and write a brief summary that describes
(a) the type of work and how petrochemicals are involved in it;
(b) the education required to qualify for employment in this field;
(c) the current working conditions, opportunities, and salary for an
employee in this field.
Follow the links for Nelson Chemistry 11, 11.5.
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Hydrocarbons 557
Section 11.5 Questions
Understanding Concepts
1. Alkanes, alkenes, and alkynes are the three main families of
aliphatic hydrocarbons.
(a) What is the general molecular formula for each family?
(b) What is the main structural feature of each family?
(c) Why does the number of hydrogen atoms in the molecular
formula decrease by 2 as you go from alkanes to alkenes and
then to alkynes?
2. Why are there more possible isomers of an alkene than an alkane
with the same number of carbon atoms?
3. Compare the physical properties of alkanes, alkenes, and alkynes.
4. Compare the chemical properties of alkanes, alkenes, and
alkynes.
5. State one major use of the first member of the alkene and alkyne
families.
6. Explain what is meant by the term “unsaturated” as applied to a
hydrocarbon.
7. Draw structural diagrams and write IUPAC names for the five
acyclic (non-ring) structural isomers of C5H10.
8. Write IUPAC names for the following hydrocarbons. Draw a structural diagram of and name any geometric isomers formed by
these compounds.
(a) CH3 — CH2 — CH2 — CH2 — CH3
(b) CH3 — CH
CH — CH2 — CH3
(c) CH
C — CH2 — CH2 — CH3
(d) CH2
CH — CH2 — CH3
(e) CH3 — CH — CH
CH — CH3
CH3
9. Draw a structural diagram and write the IUPAC name for an alicyclic hydrocarbon that is a structural isomer of 1-butyne.
10. Draw structural diagrams, labelled with IUPAC names, for all the
acyclic isomers of C4H6(g).
11. Draw a structural diagram for each of the following hydrocarbons:
(a) 3-ethyl-4-methyl-2-pentene
(b) 5-ethyl-2,2,6-trimethyl-3-heptyne
12. List three reasons why there are more molecular compounds of
carbon than compounds of all other elements combined.
13. What is the monomer from which polypropene (polypropylene) is
made?
14. Polyvinyl chloride, or PVC plastic, has numerous applications.
Write a chemical equation to represent the polymerization of
chloroethene (vinyl chloride), C2H3Cl(g).
558 Chapter 11
11.5
Applying Inquiry Skills
15. Fats and oils for cooking and consumption vary in structure.
Some edible products are said to be high in polyunsaturated fats.
Describe a possible chemical test for multiple bonds in polyunsaturated fats, explaining how to test, what the results might be,
and what the possible results would indicate about any substances tested.
16. Using a labelled diagram, describe how the gaseous products of
a chemical reaction may be collected. Title the diagram and indicate the kind of gases for which this process is suitable.
Making Connections
17. Ethyne (acetylene) is used in extremely large quantities by industrial processes. Normally, gaseous substances are liquefied under
high pressure and stored in steel cylinders in order to provide a
reasonably large quantity for use; cylinders of propane are a typical example. Research to find out and report why it is not advisable to highly compress acetylene and how solubility is used to
store C2H2(g) in cylinders.
18. As with most consumer products, the use of polyethylene has
benefits and problems. What are some beneficial uses of polyethylene and what problems result from these uses? Suggest
alternative substances for each application.
19. Hydrocarbons can be burned (as 95% currently are) or used in
the production of petrochemicals such as polymers. Is it right for
your generation to be burning a finite, nonrenewable resource?
Write one pro and one con statement from economic, social,
environmental, and ethical perspectives.
Hydrocarbons 559