|
Alkenes nomenclature
Alkenes are hydrocarbons that contain
a carbon–carbon double bond. We give alkenes IUPAC names by replacing
the -ane ending of the corresponding alkane with
-ene. For example, a
two-carbon alkene is called ethene and a
three-carbon alkene is called propene.
Ethene also is frequently called by its common name: ethylene
Structure and
bonding in Alkenes
Each of the carbon atoms is sp2-hybridized, and the double bond
possesses a σ component and a π component. The σ component results when an sp2
orbital of one carbon, oriented so that its axis lies along the internuclear
axis, overlaps with a similarly disposed sp2 orbital of the other
carbon. The π bond contributes the other two electrons and is formed by a
overlap of singly occupied p orbitals of the two carbons
Stabilities of Akenes
In
general, alkenes with more highly substituted double bonds are more stable than
isomers with less substituted double bonds.
Alkenes are more stable
when large substituents are trans to each other than when they are cis.
Preparation of
Alkenes
The dehydration
of alcohols and the dehydrohalogenation of alkyl halides are the
principal methods by which alkenes are prepared.
Dehydrohalogenation is the loss of a hydrogen and a halogen from an alkyl halide.
It is one of the most useful methods for preparing alkenes by β elimination.
In the dehydration of
alcohols, the H and OH are lost from adjacent carbons. An acid catalyst is
necessary.
|