Alkenes and Alkynes
Compounds those contain double or triple bond are said to be unsaturated. The hydrocarbons with double bonds are called alkenes and the ones with triple bonds are called alkynes. The general formula for alkenes is
CnH2n and that of alkynes is CnH2n-2.
Alkenes and alkynes are similar to alkanes in physical properties. Alkenes those containing two to four carbon atoms are gases, those with five to eighteen are liquids and the ones with more than 18 carbon atoms are solids at room temperature. Both alkenes and alkynes are lighter than water and almost insoluble in it. Below are the basic properties of some selected alkenes and alkynes.
Name |
Formula |
Density (g/mL) |
Mp(oC) |
Bp(oC) |
Ethene |
|
0.384 |
-169 |
-104 |
Ethyne |
|
- |
-84 |
-80.8 |
Propene |
|
0.514 |
-185 |
-47 |
Propyne |
|
- |
-101.5 |
-23 |
1-Butene |
|
0.595 |
-185 |
-6.3 |
1-Butyne |
|
- |
-32.3 |
27 |
1-Pentene |
|
0.641 |
-165 |
30 |
1-Pentyne |
|
0.695 |
-90 |
39.3 |
1-Hexene |
|
0.673 |
-140 |
63 |
1-Hexyne |
|
0.715 |
-132 |
71 |
Nomenclature of Alkenes and Alkynes:
Structure |
Molecular |
Common |
IUPAC |
|
C2H4 |
Ethylene |
Ethene |
|
C2H2 |
Acetylene |
Ethyne |
Many alkenes and alkynes have common name in addition to their systematic names. The common names of ethene and ethyne are ethylene and acetylene respectively. Common names for these compounds are widely used.
The systematic nomenclature of alkenes and alkynes are very similar to nomenclature of alkanes. Let’s start with the rules for alkenes:
- Choose the longest continuous carbon chain containing the double bond. This chain will determine the parent name. In the parent name, the root will be the same with the alkanes of identical length but the suffix changes from –ane to -ene.
Ethene Propene
- Determine the longest carbon chain containing the double bond. This bond will determine the parent name. Then number the carbon atoms starting from the end nearer to the double bond.
2-Pentene 3-Heptene
- Indicate the substituents and their locations on the main chain.
2-Methyl-1-butene 4-Chloro-5-methyl-2-hexene
(not 3-Chloro-2-methyl-5-butene)
The rules for naming alkynes are almost the same as those for naming alkenes. The suffix for the parent names of alkynes is –yne.
2-Pentyne 3-Bromo-4-methyl-1-hexyne 5-Ethyl-4,6-dimethyl-2-heptyne
Isomerism:
Three common types of structural isomerism might be seen in unsaturated hydrocarbons: Chain isomerism, position isomerism and functional group isomerism. In chain isomers, the arrangements of the carbon atoms are different from one another. But in position isomers the location of the functional group is different. In functional group isomers the compounds are belong to different homologous series.
and 
Position
isomers
and 
Chain
isomers
and 
Functional Group Isomers
Alkenes exhibit another type of isomerism called geometric (or cis-trans) isomerism. The isomers that exhibit geometric isomerism have different positions of groups with respect to the double bond. In such compounds if two identical groups are on the same side of the double bond, the compound is cis; but if they are on opposite sides, it is trans. 2-Butene has two isomers one is cis and the other one is trans.
............ 
... 
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cis-2-Butene trans-2-Butene cis-1,2-Dichloroethene trans-1,2-Dichloroethene