Unit 2, Lesson 02: Alkenes and Alkynes
Alkenes are hydrocarbons that contain at least one C = C double bond
• A double bond removes two hydrogen atoms from the molecule. Alkenes are said to be unsaturated
because they contain less than the maximum number of hydrogen atoms
• The suffix “ene” indicates that the molecule contains a C = C double bond
• The general formula for an alkene with one double bond is CnH2n, where “n” is the number of carbon
atoms
• The names of the alkenes follow the same pattern as the alkanes (ethene, propene, butene etc)
• Alkenes may be straight-chain or branched
The Alkene Base Names
• The number of carbon atoms in the longest carbon chain that contains the double bond determines the
base name of the compound
• The position of the double bond is indicated with the position number of the carbon where the double
bond is found
• Alkenes are numbered so that the double bond has the lowest possible position number
Name
Molecular
Formula
ethene
C2H4
propene
C3H6
1-butene
C4H8
2-butene
C4H8
Expanded Molecular Formula
Structural Formula
Similarly,
• If the carbon chain is 5 carbons long, with one double bond- it is pentene, C5H10
• If the carbon chain is 6 carbons long, with one double bond- it is hexene, C6H12
Note:
• For the first two examples, the naming rules require that double bond will be found on the first carbon
• Because the position of the double bond is known, no position number is needed in the name
•
For the second two examples, the double bond is found in a different position. These molecules are
structural isomers of each other (they have the same molecular formulas but different structural
formulas)
If an alkene contains only one double bond, it is mono-unsaturated (mono means “one”)
If an alkene contains two or more double bonds, it is poly-unsaturated (poly means “many”)
• The name of poly-unsaturated alkenes includes the position numbers of all double bonds. The suffix at
the end of the base name is changed to indicate the number of double bonds
• 1,3-hexadiene is a six carbon chain with double bonds on C-1 and C-3
• 2,4,6-nonatriene is a nine carbon chain with double bonds on C-2, C-4 and C-6
Naming alkenes (in more detail):
1. If there is one or more double bonds in a molecule, the carbon chain must include these double bonds;
that is, the double bond(s) must be part of the longest carbon chain even if it means that the carbon
chain will be shorter than it could be
2. The double bond gets priority in naming- the carbon chain is numbered in the direction that will give
the double bond the lowest possible number (then number any side chains accordingly)
3. If the double bond is the same distance from both ends, begin with the end nearer the first branch point
so that the total position numbers will be as low as possible
4. The suffix of the base name tells us how many double bonds there are:
•
If there is one double bond, the suffix of the base name is “…ene”
eg. the five carbon chain with one double bond (shown to the
right) would be named “2-pentene”
•
If there are two double bonds, the suffix of the base name is “…diene”
eg. the four carbon chain with two double bonds (shown to the right)
would be named 1,3-butadiene
eg. the four carbon chain with two double bonds (shown to the right) would
be named 1,2-butadiene
•
If there are three double bonds, the suffix of the parent name is “…triene” etc.
5. Use the same naming rules for side chains (substituents) that were discussed for the alkanes. Combine
the numbers and names of all substituents with the parent name to form one word. Use hyphens to separate
numbers from names. Use commas to separate numbers.
examples:
Cis-trans Isomers of Alkenes
Atoms can rotate around a single bond. This means that all positions around are a
single bond are equivalent.
Atoms can not rotate around a double bond. Double bonds “lock” the molecule
into a certain arrangement.
If the largest side chains of an alkene are “locked into” the same side of the double bond, then the
molecule is called the “cis” isomer (you can remember this using alliteration “cis same side”). Most
molecules in living systems are “cis” isomers.
If the largest side chains of an alkene are “locked into” opposite sides of the double bond, then the
molecule is called the “trans” isomer.
cis-2-butene
cis-2-pentene
trans-2-butene
trans-2-pentene
Many living organisms can not use the trans-form of a molecule. For example, there is concern about the
“trans-fatty acids” in margarine and hydrogenated shortenings. “Trans fats” are created when oils are
hydrogenated, and our bodies are not very efficient at using these fats. The effects of “trans fats” on
human health is being actively researched.
The “cis” and “trans” isomers of molecules have some different physical properties.
To be absolutely correct in naming an alkene, we should specify whether the alkene is cis or trans;
however, you can not always tell unless a complete structural diagram is given. There is no way of
knowing whether an alkene is cis or trans from its molecular or expanded molecular formula. So, unless
you are asked for the cis or trans isomer specifically, you do not need to include it in the molecule’s name.
Alkynes are unsaturated hydrocarbons that contain at least one C ≡ C triple bond.
• The base name of an alkyne depends on the longest carbon chain of the molecule that contains the
triple bond(s)
• The suffix “yne” on the end of the hydrocarbon’s name tells you that the molecule contains at least one
C ≡ C triple bond
• The position of the triple bond is indicated with the position number of the carbon where the triple
bond is found
• The general formula for an alkyne with one triple bond is CnH2n-2
eg. a two carbon chain with a triple bond
eg. a three carbon chain with a triple bond
ethyne (a.k.a. ethylene or acetylene)
propyne
The rules for naming alkynes are similar to the rules for alkanes; however, the suffix “-yne” is substituted
for the “ene” ending. Number the carbon chain so that the triple bond has the lowest number possible.