CHEMISTRY 30S – MODULE 5
Organic chemistry
LESSON 3  ALKENES & ALKYNES
Alkenes and alkynes are unsaturated hydrocarbons. Alkenes and alkynes are industrial made
from breaking apart or "cracking" large alkanes found in crude oil. We will study the
naming and structure of alkenes and alkynes.
Outcomes
When you have completed this lesson, you will be able to:

Construct, illustrate and name alkenes and alkynes (up to 10 C atoms) using IUPAC
nomenclature.

Construct, illustrate and name branched alkenes and alkynes using IUPAC
nomenclature.

Compare and contrast saturated and unsaturated hydrocarbons, giving examples and
uses of each.
Saturated and Unsaturated Hydrocarbons
Note: saturated and unsaturated have different definitions than in the solutions unit:
Saturated hydrocarbons have the most hydrogens that the given number of carbons can
hold. An unsaturated hydrocarbon does not contain the maximum number of hydrogens.
What takes up the bonds with carbon? When carbons form double and triple bonds, bonding
that can occur with hydrogens is used up by these double and triple bonds.
A double carbon-carbon bond is shown by two lines, rather than one:
–C=C–
A triple bond is shown with three lines rather than one:
–C≡C–
Saturated hydrocarbons tend to have higher melting points and boiling points. Oils that are
largely saturated fats are usually solid at room temperature. Butter and lard are high in
saturated fats. These types of fats are not healthy in that they promote the body's production
of cholesterol, which is a risk factor for heart disease and stroke. The saturated
hydrocarbons, because of their higher melting point, are more likely to be solid at room
temperature and are good as a spread on bread. You will often see food products advertised
as low in saturated fats and high in polyunsaturated fats. Polyunsaturated means the fat has
more than one, or many double and/or triple bonds. Unsaturated fats are required by the
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body for making cell membranes and hormone production.
Formation of Alkenes and Alkynes
Long chain alkanes found in crude oil undergo a process called ‘pyrolysis’ (from the Greek
pyr = fire and lysis = a loosening) or thermal cracking. In the thermal cracking process,
alkanes are simply passed through a high temperature reaction chamber. During thermal
cracking long chain alkanes are broken into shorter chain alkanes and alkenes (some with
multiple double bonds).
Multiple carbon-carbon bonds can also be form by the removal of hydrogen. This process is
called dehydrogenation.
Alkanes From Alkenes and Alkynes
In contrast, however, alkenes are converted to alkanes by a less complex reaction. Hydrogen
can be added to an alkene in the presence of a nickel, platinum or palladium catalyst. This
process is called an addition reaction or catalytic hydrogenation and is a common
reaction in organic chemistry.
This hydrogenation can also be performed on alkynes:
This process is used to convert vegetable oils into margarine. ‘Trans’ fats or ‘trans’ fatty
acids (TFA’s) are formed when manufacturers hydrogenate unsaturated compounds with
hydrogen to form saturated structures. Manufacturers have found that hydrogenating
vegetable oils has many economic benefits. The process extends the shelf life, increases the
flavour stability of the product, produces a solid product, and reduces the risk of rancidity,
to name a few benefits. There are however, some studies that have suggested that TFA’s
present a greater risk of coronary artery disease than saturated fatty acids.
Naming Alkenes
An alkene has at least one double carbon-carbon bond. The general molecular formula for
and alkene is
CnH2n
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When naming alkenes, the process is similar to alkanes, except alkenes substitute the ending
–ene for –ane.
Steps to follow when naming alkenes:
1. If condensed, expand the structure.
2. Find the longest chain that contains the double bond. Number the carbons, this time
ensuring the carbon containing the double bond has the lowest number. Name the
parent chain (ending in –ene), giving the number of the carbon with the double bond
separated with a hyphen.
3. Name any alkyl group branches as you did for the alkanes.
Example 1. Name the following structure.
CH2=CHCH2CH2CH3
Solution.
Step 1: Expand the structure.
This is a straight-chained alkene, so we don't need to expand it.
Step 2: Find the longest chain that contains the double bond.
The double bond begins on carbon #1.
The compound is 1-pentene.
Example 2. Name the following structure.
CH3CH=CHCH2CH3
Solution.
Step 1: Expand the structure.
This is a straight-chained alkene, so we don't need to expand it.
Step 2: Find the longest chain that contains the double bond.
The double bond begins on carbon #2.
The compound is 2-pentene.
Example 3. Name the following structure.
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Solution.
Step 1: Expand the structure.
This hydrocarbon has a double bond so it is an alkene. It is already in an expanded form.
Step 2: Find the longest chain that contains the double bond.
The double bond begins on carbon #2. The compound is a 2-pentene.
Step 3: Name the alkyl branches.
There is a methyl group on carbon #4.
The name of the compound is 4-methyl-2-pentene.
(Note, the lowest number goes to the double bond)
Example 4. Name the following structure.
CH3CH2CH(CH3)CH=C(C2H5)CH2CH3
Solution.
Step 1: Expand the structure.
Step 2: Find the longest chain that contains the double bond.
We number this 7-carbon alkene from right to left to give the double bond the lowest
number, 3. The compound is a 3-heptene.
Step 3: Name the alkyl branches.
There is a methyl group on carbon #5 and an ethyl on carbon #3.
The name of the compound is 3-ethyl-5-methyl-3-heptene.
Note:
When naming compounds like ethene, CH2=CH2 , and propene, CH3CH=CH2 , we do not
use the number 1 to indicate the number of the double bond. Since the double bond cannot
be on any other carbon, but the number 1 carbon, we do not write the number. We do not
write the number for these ONLY, all others must have a number for the double bond.
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Drawing Alkenes
The process of drawing alkenes is very similar to drawing alkanes.
1. Draw the carbons for the parent chain and number the carbons and insert the double
bond.
2. Draw in the alkyl groups.
3. Fill in hydrogens so every carbon has 4 bonds. A reminder that the double bond
counts as 2 bonds.
4. In your assignments, you will probably want to condense the structural formula at
this point. Putting the alkyl groups in brackets following the carbon to which it is
attached.
Example 5. Draw the structural formula for 1-butene.
Solution.
Step 1: Draw the carbons for the parent chain and number the carbons and insert the
double bond.
The ending is –ene, so the structure is an alkene. The parent chain is butene, 4 carbons. The
double bond goes on the first carbon (that is, between carbons 1 and 2)
Step2: Draw in the alkyl groups.
There are no alkyl groups.
Step 3: Fill in hydrogens so every carbon has 4 bonds.
CH2=CH–CH2–CH3
Step 4: Condense (optional).
This is sufficiently condensed.
The structural formula of 1-butene is
CH2=CH–CH2–CH3
Sometimes, when the double bond is on the first carbon, to further accentuate that the
carbons are double bonded, we can lead the structural formula with the hydrogens as below:
H2C=CH–CH2–CH3
Example 6. Draw the structural formula for 2,5,5-trimethyl-2-hexene.
Solution.
Step 1: Draw the carbons for the parent chain and number the carbons and insert the
double bond.
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The parent chain is hexene, 6 carbons. The double bong goes on carbon #2.
Step2: Draw in the alkyl groups.
There three methyl groups: two on carbon #5 and one on carbon #2.
Step 3: Fill in hydrogens so every carbon has 4 bonds.
Since carbon #2 has a double bond and two carbon-carbon single bonds, it does not need
any hydrogens. Carbon #3 has a double bond and a single bond, so it needs one more
hydrogen to give it four bonds.
Step 4: Condense (optional).
CH2–C(CH3)=CH–CH2–C(CH3)2–CH3 or
CH2C(CH3)=CHCH2C(CH3)2CH3
Notice that we show the double bond in the condensed structural formula.
Common Alkenes
Ethene, commonly called ethylene, is the gas released by fruit that hastens ripening. It is
usually released by the maturing seeds in the fruit. Fruit producers will often place crates of
fruit into vaults and expose them to ethene (ethylene) gas to speed up ripening. We can do
this at home to speed up the ripening of store bought fruit. If you place unripe fruit into a
paper bag containing a ripe apple, the ethene gas released by the apple will speed up the
ripening of the fruit.
Propene is commonly called propylene. When many molecules of an organic molecule are
joined together, we call this type of molecule a polymer. Polyproylene, a plastic, is a
polymer of propene molecules.
Alkynes from Alkenes
Acetylene, C2H2, is the simplest of all alkynes and can be used as a starting molecule for the
production of larger alkyne molecules. The manufacture of acetylene can be accomplished
by the reaction of water on calcium carbide (CaC2).
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Another method of producing alkynes is the further dehydrogenation of alkenes:
The reverse process is much less complex. An alkyne can readily be transformed into an
alkene by catalytic hydrogenation, as seen earlier in this lesson.
Naming Alkynes
Alkynes are unsaturated hydrocarbons with one or more triple carbon-carbon bonds. Since
two more hydrogens, as compared to alkenes, are replaced by the extra bond, the general
molecular formula for alkynes is
CnH2n-2
Naming alkynes is similar to naming alkanes, except alkynes end in –yne.
The steps in naming alkynes are:
1. If condensed, expand the structure.
2. Find the longest chain that contains the triple bond. Number the carbons, this time
ensuring the carbon containing the triple bond has the lowest number. Name the
parent chain (ending in –yne), giving the number of the carbon with the triple bond
separated with a hyphen.
3. Name any alkyl group branches as you did for the alkanes.
Example 7. Name the following structure
CH3–C≡C–CH2–CH3
Solution.
There is a triple bond, so the structure is an alkyne.
Step 1: Expand the structure.
This is a straight-chained alkyne, so we don't need to expand it.
Step 2: Find the longest chain that contains the triple bond.
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The triple bond begins on carbon #2. The compound is a 2-pentyne.
Step 3: Name the alkyl branches.
There are no branches
The name of the compound is 3-pentyne.
Example 8. Name the following structure
HC≡CCH2C(C2H5)2CH2C(CH3)CH(CH3)CH3
Solution.
There is a triple bond, so the structure is an alkyne.
Step 1: Expand the structure.
Step 2: Find the longest chain that contains the triple bond.
The triple bond begins on carbon #1 and the parent chain is 8 carbons long. The compound
is a 1-octyne.
Step 3: Name the alkyl branches.
There are two ethyl groups on the #4 carbon and one methyl on the #6 carbon and another
on the #7 carbon.
The name of the compound is 4,4-diethyl-6,7-dimethyl-1-octyne.
Drawing Alkynes
As with alkenes, when drawing alkynes
1. Draw the carbons for the parent chain and number the carbons and insert the triple
bond.
2. Draw in the alkyl groups.
3. Fill in hydrogens so every carbon has 4 bonds. A reminder that the triple bond
counts as 3 bonds.
4. In your assignments, you will probably want to condense the structural formula at
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this point. Putting the alkyl groups in brackets following the carbon to which it is
attached.
Example 9. Draw the structural formula for 1-butyne.
Solution.
Step 1: Draw the carbons for the parent chain and number the carbons and insert the
triple bond.
The ending is –yne, so this is an alkyne. The parent chain is butyne, 4 carbons. The triple
bond goes on the first carbon (that is, between carbons 1 and 2)
Step2: Draw in the alkyl groups.
There are no alkyl groups.
Step 3: Fill in hydrogens so every carbon has 4 bonds.
CH≡C–CH2–CH3
Step 4: Condense for assignments.
This is sufficiently condensed.
The structural formula of 1-butyne is
CH≡C–CH2–CH3
Sometimes, when the triple bond is on the first carbon, as with the alkene structures, we can
lead the structural formula with the hydrogen as below:
HC≡C–CH2–CH3
Example 10. Draw the structural formula for 2,2,5,5-tetramethyl-3-hexyne.
Solution.
Step 1: Draw the carbons for the parent chain and number the carbons and insert the
triple bond.
The ending is –yne, so this is an alkyne. The parent chain is hexyne, 6 carbons. The triple
bond goes on the third carbon (that is, between carbons 3 and 4)
Step2: Draw in the alkyl groups.
The "tetramethyl" indicates there are four methyl groups: two on the #2 carbon and two on
the #5 carbon.
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Step 3: Fill in hydrogens so every carbon has 4 bonds.
Step 4: Condense for assignments.
The condensed structural formula of 2,2,5,5-tetramethyl-3-hexyne would be
CH3C(CH3)2C≡CC(CH3)2CH3
Common Alkynes
The common name for ethyne is acetylene. Acetylene is very important to manufacturing
and construction due to the use of this gas in the process of oxy-acetylene welding.
Dissolved under pressure in acetone contained in tanks, it is sold as a fuel for welding.
Acetylene burns very clean and hot in the presence of pure oxygen.
Acetylene is also the organic starting material for the large–scale synthesis of a number
of important organic compounds including acetic acid (also known as vinegar) and a
number of unsaturated compounds that are used in the manufacture of plastics and
synthetic rubber.
Many of the synthetic uses of acetylene resulted from the work done in Germany before,
during, and after the Second World War. Research with this compound was accelerated
by the lack of crude petroleum resources in Germany. The hope was that this compound
might replace petroleum as a fuel. Much of the research that was done revolutionized the
industrial chemistry of acetylene.
Lesson Summary
In this lesson we have learned

Alkenes have at least one double carbon-carbon bond. The molecular formula for
alkenes is CnH2n. The name of alkenes ends in –ene.

Alkynes have at least one triple carbon-carbon bond. The molecular formula for
alkynes is CnH2n-2. The name of alkynes ends in –yne.

Naming unsaturated hydrocarbons is similar to saturated hydrocarbons, except the
parent chain is numbered to give the double or triple bond the lowest number.

Draw the parent chain unsaturated hydrocarbons with the double or triple bonds
first. Add in alkyl groups and then hydrogens so each carbon has 4 bonds.
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