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Basic Organic
Nomenclature
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Alkynes
Family name: suffix: -yne
In contrast to alkanes where the carbon atom will form 4 single bonds, or alkenes
where the carbon atom will form bonds to three other atoms, alkynes form bonds
to only two other atoms. One of these bonds will be single bond while the second
is a triple bond (which consists of a s (sigma) bond
and two p (pi) bonds to the same atom). The alkynes
are also referred to as unsaturated hydrocarbons as
they contain multiple bonds. The geometry of these unsaturated or sp
hybridized carbon atoms is linear (bond angles of approximately 180o
with all four atoms laying in a line). The general formula for an alkyne
is CnH2n-2 where n is the number of carbon atoms present
in the molecule. The loss of the two pairs of hydrogen atoms indicates the presence
of the triple bond between two carbon atoms.
The unsaturated hydrocarbons are named exclusively using suffixes to
the root name of the hydrocarbon. Since there must be a minimum of two
carbon atoms for a multiple bond to occur, the simplest alkyne is ethyne.
Comparison of simple alkanes and alkynes:
NOTE: The convention in drawing alkynes is: the triple bond is indicated
by three parallel lines between the two atoms, and the bond angle to the
other two atoms should be approximately 180°.
The naming of the alkynes is similar to alkenes with the following addition.
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The triple 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 triple bond. All other groups
will be substituents on this parent chain (or ring).
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The location of the triple bond is indicated by the number of the first
carbon atom of the triple bond.
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There are two conventions in placing the number: the less common is to
put the number between the root and suffix, i.e. prop-1-yne (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-propyne. It is common
to leave out the number when the triple bond is between atoms 1 and 2,
i.e. propyne is more commonly used than 1-propyne.
A few more examples of naming alkenes are:
Next Page: Alkynes - II
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