Properties of Alkenes (at least 1 double bond) and
alkynes (at least 1 triple bond)
Physical properties dependent on


The size of the molecule
The number of double or triple bonds present
As the size increases - State moves from gas to liquid to solid
just like alkanes
As the number of double or triple bonds increases the
 Melting point
 Boiling point
 Density decreases
Melting, boiling and density differences here can be explained
by the fact that larger molecules with double and triple bonds
have kinks in their shape causing poor packing.
 Not miscible in water
Chemical properties dependent on the fact that alkenes and
alkynes are not fully saturated. They are unsaturated.
 Most reactions of these molecules are faster than in alkanes
Alkenes and Alkynes


Chemists often group these two families together as unsaturated
hydrocarbons.
Do you remember what alkanes were called if alkenes and
alkynes are unsaturated?

Alkenes have the general formula CnH2n, and end in “ene”

Alkynes have the general formula CnH2n-2, and end in “yne”.

What is the general formula for alkanes?


The position of the multiple bonds is indicated by a number in
front of the name. Treat double and triple bonds as branches.
Greek prefixes are used before the ending if there are more than
one double or triple bond (i.e. 2 doubles bonds = a “diene”).
Formula
CH2=CH2
CH2=CHCH3
CH2= CH CH2CH3
CH3CH=CHCH3
CH2= CH CH=CH2
Name
Ethene
Propene
1-butene
2-butene
1,3-butadiene
Structure
2 carbon atoms
3 carbon atoms
Double bond at carbon 1
Double bond at carbon 2
Double bonds at carbon 1
and 3.
 Notice the number of hydrogens around each carbon. Remember
carbon can only form FOUR bonds. A double bond counts as two
and a triple bond counts as three.
 The same rules apply for naming alkynes you just use the “yne”
ending instead of “ene.”
 Notice that we put an “a” after But in the last example…just so that
it can be pronounced.
?
Why don’t we need to indicate the position of the
double bond in examples 1 and 2?
?
Examples:
Name the following hydrocarbons.
1.
C3H4
3.
2.
C5H10
4.
Draw structural, condensed and line formulas for the following
alkanes.
1.
2-butene
2.
3-octyne
What are the general formulas for alkanes, alkenes and alkynes?