An
alkene is an unsaturated hydrocarbon containing at least one carbon-carbon double bond. Alkenes form a
homologous series,
the alkenes with general formula
CnH2n.
The alkenes is one of three homologous series that contain carbon-carbon double bonds; the other two being the
alkynes and the
arenes.
The simplest alkene is C
2H
4, which has the common name "ethylene" and the IUPAC name "ethene".
Structure of Alkenes
Shape of Alkenes
As predicted by the VSEPR model of
electron pair replusion (see
covalent bond), the bond angles about each carbon in a double bond are about 120°, although the angle may be larger because of strain introduced by
nonbonded interactions created by groups attached to the carbons of the double bond. For example, the C-C-C bond angle in propene (
propylene) is 123.9°.
See also:
molecular geometry
Molecular Geometry Carbon-Carbon Double Bond
Like single
covalent bonds, double bonds can be described in terms of overlapping atomic orbitals, except that unlike a single bond (which consist of a single sigma bond), a carbon-carbon double bond consists of one sigma bond and one pi bond.
Each carbon of the double bond uses its three
sp2 hybrid orbitals to form sigma bonds to three atoms. The unhybridized
2p atomic orbitals, which lie perpendicular to the plane created by the axes of the three
sp2 hybrid orbitals, combine to form the pi bond.
Because it requires a large amount of energy to break a pi bond (264 kJ/
mol in ethylene), rotation about the carbon-carbon double bond is very difficult and therefore severely restricted.
See also:
molecular geometry
Physical properties
Chemical properties
Alkenes are relatively stable compounds, but are more reactive than
alkanes.
Reactions
Synthesis
#The most common industrial synthesis path for
alkenes is
cracking of
petroleum.
#'''Alkenes''' can be synthesized from
alcohols via an
elimination reaction that removes one water molecule:
H
3C-CH
2-OH + H
2SO
4 → H
3C-CH
2-O-SO
3H + H
2O → H
2C=CH
2 + H
2SO
4
#Catalytic synthesis of higher α-alkenes can be achieved by a reaction of ethene with triethylaluminium, an organometallic compound in the presence of
nickel,
cobalt or
platinum.
Addition reactions
Catalytic hydrogenation of
alkenes produce the corresponding
alkanes. The reaction is carried out under pressure in the presence of a metallic
catalyst. Common industrial catalysts are based on
platinum,
nickel or
palladium, for laboratory syntheses, Raney's nickel is often employed. This is an
alloy of
nickel and
aluminium.
This is the catalytic hydrogenation of
ethylene to yield
ethane:
CH
2=CH
2 + H
2 → CH
3-CH
3
Most addition reactions to alkenes follow the mechanism of
electrophilic addition.
#Halogenation: Addition of elementary
bromine or
chlorine to alkenes yield vicinal Dibromo- and dichloroalkenes, respectively. The decoloration of a solution of bromine in water is an analytical test for the presence of alkenes:
CH
2=CH
2 + Br
2 → BrCH
2-CH
2Br
#Hydrohalogenation: Addition of hydrohalic acids like
HCl or HBr to alkenes yield the corresponding
haloalkanes.
CH
3-CH=CH
2 + HBr → CH
3-CH'''Br'''-CH
3 If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with less hydrogen substituents (
Markovnikov's rule).
#Addition of a carbene or
carbenoid yields the corresponding
cyclopropane
Oxidation
#In the presence of
oxygen, alkenes burn with a bright flame to
carbon dioxide and water.
#Catalytic oxidation with oxygen or the reaction with percarboxylic acids yields
epoxides
#Reaction with ozone leads to the breaking of the double bond, yielding two
aldehydes or
ketones R
1-CH=CH-R
2 + O
3 → R
1-CHO + R
2-CHO + H
2O
This reaction can be used to determine the position of a double bond in an unknown
alkene.
Polymerisation
Polymerization of
alkenes is an economically important reaction which yields
polymers of high industrial value, such as the plastics
polyethylene and
polypropylene. Polymerization can either proceed via a free-
radical or an ionic mechanism. For detail regarding the reaction mechanisms, see the
polymerization article.
Nomenclature of Alkenes
To form the root of the IUPAC names for alkenes, simply change the -an- infix of the parent to -en-. For example,
CH3-CH3 is the
alkane ethANe. The name of
CH2=CH2 is therefore
ethENe.
In higher alkenes, where isomers exist that differ in location of the double bond, the following numbering system is used:
#Number the longest carbon chain the contains the double bond in the direction that gives the carbon atoms of the double bond the lowest possible numbers.
#Indicate the location of the double bond by the location of its first carbon
#Name branched or substituted alkenes in a manner similar to
alkanes.
#Number the carbon atoms, locate and name substituent groups, locate the double bond, and name the main chain
Common-Names
Despite-the-precision-and-universal-acceptance-of-the-IUPAC-naming-system,-some-alkenes-are-known-almost-exclusively-by-their-common-names:
">CH2
6 5 4 3 |2 1
CH2CH3
2-Ethyl-4-methyl-1-hexene
Common Names
Despite the precision and universal acceptance of the IUPAC naming system, some alkenes are known almost exclusively by their common names:
See also:==
Category:Hydrocarbons
Category:Alkenes
ca:Alquè
da:Alken
de:Alkene
es:Alqueno
fr:Alcène
nl:Alkeen
ja:アルケン
it:Alcheni
pl:Alken
ru:Алкены
fi:Alkeeni
su:alkéna
sv:Alken
zh:烯烃