 |
Basic Organic
Nomenclature
|
|
Alkenes
Family name: suffix: -ene
In contrast to alkanes where the carbon atom will form 4 single bonds (or s
(sigma) bonds) to 4 different atoms, a carbon atom of an alkene will form bonds
to three other atoms. Two of these bonds will be single bonds while the third
is a double bond (which consists of both a s (sigma)
bond and a p (pi) bond to the same atom). The alkenes
are also referred to as unsaturated hydrocarbons (as they
contain multiple bonds), or olefins. The geometry of these unsaturated or sp2
hybridized carbon atoms is trigonal planar (bond angles of approximately
120o with all four atoms lying in a plane). The general formula for
a alkene is CnH2n where n is the number of carbon atoms
present in the molecule. The loss of a pair of hydrogen atoms indicates the presence
of the double bond between two carbon atoms.
The unsaturated hydrocarbons are named using a different suffix to the root
name of the hydrocarbon, -ene. Since there must be a minimum of two carbon atoms
for a multiple bond to occur, the simplest alkene is ethene.
Comparison of simple alkanes and alkenes:
NOTE: The convension in drawing alkenes is: the double bond is indicated
by a pair of parallel lines between the two carbon atoms, and the bond
angles should be approximately 120o.
The naming of the alkenes is similar to alkanes with the following addition.
-
The double bond will be higher in priority than any simple alkyl or halide
group, therefore the root name is derived by finding the longest chain
(or ring) which contains both atoms of the double bond.
-
All other groups will be substituents on this parent chain (or ring).
-
The location of the double bond is indicated by the number of the first
carbon atom of the double bond, and the bond is given the lowest possible
number, i.e. 1-propene not 2-propene. There are two convensions in placing
the number: the less common method is to put the number between the root
and suffix, i.e. prop-1-ene (example four above); the more common method
(and the one we will use exclusively) is to place the number before the
root name, i.e. 1-propene. It is common to leave out the number when the
double bond is between atoms 1 and 2, i.e. propene is more commonly used
than 1-propene.
A few more examples of naming alkenes are:
Next Page: Alkenes - II
Return to Index
