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 laying 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 convention 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 conventions 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:
Common Substituents:
There are two common names for alkene substituents: a R-CH=CH2
group is a vinyl group; and a R-CH2-CH=CH2 is an
allyl group.
Next Page: Alkenes - II
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