Properties of Alkenes:
Alkenes, often called olefins, are hydrocarbons with carbon-carbon double bonds. Alkenes have three Sp2 hybridized orbitals and a p orbital, and have bond angles of about 120 degrees.
 Kekule Structure |
 Lewis Structure |
|
Unlike carbon-carbon single bonds, double bonds do not have free bond rotation, and have shorter bond lengths. This is due to the rigidity of the double bond.
Examining the bond dissociation energies, we find that the dissociation energy of a single bond is much higher than that of a double bond, about 20kcal/mol more. This means that double bonds are significantly more reactive than single bonds, classifying it as a functional group.
Nomenclature of Alkenes:
Naming of alkenes is very similar to the naming of alkanes, using the root name of the longest carbon chain. The ending, however, is changed from -ane to -ene, and a number is given to show the location of the double bond for molecules of more than three base carbons. |
 |
 |
|
ethene (There
is no need for a number. A one is implied. )
|
Propene (one is implied) | 2-Butene |
Cis-Trans Isomerism:
Due to the rigidity of the double bond and lack of rotation around the double bond, alkenes give rise to a cis-trans isomerism. When two similar substituents are bonded to the same side of the double bond, it is called the cis isomer. When two similar substituents are bonded to opposite sides of the double bond, it is called the trans isomer. |
 |
| cis-2-butene | trans-2-butene |
Information provided by: http://www.chemhelper.com