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Nomenclature
Compounds which contain only carbon and hydrogen atoms, but which contain a double
bond, are called alkenes, instead of alkanes. The functional group of an
alkene is the pair of carbon atoms that are bonded together with a double bond.
In addition to changing the ending of the name, a number is shown in the name (when
necessary) to indicate the position of the double bond. As with side groups in alkanes,
the lowest numbers are used. However, in alkenes, having the lowest number for the
position of the double bond takes precedence over any side groups. So you find and number
the longest chain which contains the double bond, and you number that
chain from the end that is closest to the double bond.
There is a practice program for organic nomenclature on the computers
in the lab for your use. It is intended for students taking an organic chemistry course
and goes beyond the level that you will need, but it can be very helpful.
Examples
Let's look at the alkenes listed below (which are also listed in Example 4 of your
workbook). Note that we cannot have an alkene with only one carbon because an alkene
involves a carbon-carbon double bond. There must be at least two carbons in the chain.
| Therefore, the shortest possible alkene has two carbons in it. Again, we
will use eth- to represent those two carbon atoms. Because we have a
double bond in the molecule, we have to use the -ene ending for the name.
This compound is named ethene. It also has a common name, ethylene. |
| CH2=CH2
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ethene (ethylene) |
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| Next is the structural formula for a three-carbon alkene. Because it has
three carbons, we have prop- and because it has a double bond, we use the
ending -ene. The complete name is propene. It also has
the common name of propylene. |
| CH2=CH-CH3
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propene (propylene) |
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| The next three examples have four carbon atoms, and the double bond is in
a different place in each one (or so it would appear). All of these will be butenes. But-
because there are four carbons, and -ene because there is a double bond.
The first of these is called 1-butene because the double bond connects
the first and second carbon atoms. Keep in mind we want to use the lowest numbers
possible, so we use 1- as part of the name. In the second of these
butenes, the double bond is attached to carbon number two, the second carbon.
Consequently, the name of this compound is 2-butene.
In the third of these examples, the double bond is closest to the right end of the
chain, so we have to number from the right-hand side. Numbering in this direction, the
double bond is attached to the first carbon atom. Therefore, we call this compound 1-butene,
and it is the same compound as the 1-butene drawn above. |
| CH2=CH-CH2-CH3
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1-butene |
| CH3-CH=CH-CH3
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2-butene |
| CH3-CH2-CH=CH2
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1-butene |
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| Of course, other functional groups and alkyl groups can be
attached to a carbon chain which contains a double bond, and when this happens, you name
the compound, using the IUPAC method. You start by finding the longest carbon chain with
the double bond in it, and you number the carbons, starting at the end closest to the
double bond. |
| In the case of the last compound shown here, the longest carbon chain
containing the double bond has five carbon atoms. Naming that five carbon chain, we would
have to call it 1-pentene. We use that as a base for the name of the
whole molecule. There is still a methyl group and a chloro group
attached to this chain that need to be added to the name. The chloro group is attached to
the third carbon atom. The methyl group is attached to the fourth carbon atom. Remember,
the numbering is from the end closest to the double bond, the right in this case. The
complete name for this compound would be 3-chloro-4-methyl-1-pentene. |
Cl
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CH3-CH-CH-CH=CH2
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CH3
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| 3-chloro-4-methyl-1-pentene |
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Practice
I'd like you to take time now to practice naming alkenes and drawing their structural
formulas by using the examples shown here (and in Exercise 5 in your workbook). If the
structure is shown, give the name, and vice versa. Then check your answers below before
continuing.
C-C-C=C-C
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C |
C-C-C
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C-C |
| 2-ethyl-1-butene |
| 4-methyl-2-hexene |
| 4-isopropyl-3-octene |
Answers
C-C-C=C-C
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C
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4-methyl-2-pentene |
C-C-C
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C-C
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2-methyl-2-butene |
| 2-ethyl-1-butene |
C-C
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C=C-C-C
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| 4-methyl-2-hexene |
C-C=C-C-C-C
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C |
| 4-isopropyl-3-octene |
C-C-C=C-C-C-C-C
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C-C-C
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E-mail instructor:
Eden Francis
Clackamas Community College
©2001, 2003 Clackamas Community College, Hal Bender
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