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Basic Organic
Nomenclature
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Alkenes - Multiple Double Bonds
Family name: suffix: -ene
There are of course compounds which contain two or more double bonds. The general
formulas for these alkenes are: CnH2n-2, CnH2n-4,
CnH2n-6, ..., for two, three, four, ... double bonds. Again,
the loss of each pair of hydrogen atoms indicates the presence of an additional
double bond between two carbon atoms.
The naming of the poly-alkenes is similar to that of a simple alkene.
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Locate the longest chain (or ring) which contains all
of the carbon atoms of all double bonds.
- The chain is then numbered from the end with a double bond closet to that
end of the chain. Substituents are then located according to this numbering
scheme.
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If there is no preferred numbering scheme due to the double bonds (i.e.
the double bonds are the same distance from the ends of the chain), pick
the one that will give the first substituent from either end the lower
number.
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Rings are treated the same way, but note, one of the double bonds will
always start at carbon 1, then number around the ring, the shortest distance,
to the other double bond(s).
The presence of the multiple bonds are indicated in the name by: first,
the number locating each double bond present; second, by adding the di,
tri, tetra, ... prefixes before the -ene ending. NOTE: you need to add
an "a" before the di, tri, etc, i.e. 1,3-butadiene, 1,3,5-hexatriene.
Examples of naming poly-alkenes are:
There are a number of terms used to describe the relationship of multiple
double bonds to one another. The first are isolated double bonds. These
occur when there are two or more single bonds between the double bonds.
The second are conjugated double bonds. This occurs when there is only
one single bond between the double bonds. This arrangement allows the double
bonds to interact with one another, and the entire system will tend to
be planar. Finally, there are what are termed cumulated double bonds. These
occur when a central carbon atom forms double bonds to two carbon atoms,
one on each side. This results in a linear arrangement of the three carbon
atoms. For example:
NOTE: In the line drawing the space between the double bonds in the cumulated
double bond indicates the central carbon atom of the two double bonds.
Tutorial Quiz: Alkenes
Next Page: Alkenes - III
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