Alkene
|
The chemical structure of ethylene, the simplest alkene. |
An
alkene, olefin, or olefine, in
organic chemistry is an
unsaturated hydrocarbon containing at least one
carbon to
carbon double bond. The simplest alkenes, with only one double bond, form a
homologous series with general formula
CnH2n.
The simplest alkene is
ethylene (C
2H
4), which has the
International Union of Pure and Applied Chemistry (IUPAC) name ethene. Alkenes are also called
olefins (an archaic synonym, widely used in the
petrochemical industry) or
vinyl compounds.
Shape of alkenes
As predicted by the
VSEPR model of
electron pair replusion, the
molecular geometry of alkenes includes
bond angles about each carbon in a double bond of about 120°. The angle may vary because of
steric strain introduced by
nonbonded interactions created by
functional groups attached to the carbons of the double bond. For example, the C-C-C bond angle in
propylene is 123.9°. The alkene double bond is stronger than a single
covalent bond and also shorter with an average
bond length of 133
picometres.
Molecular geometry
Like single
covalent bonds, double bonds can be described in terms of overlapping atomic orbitals, except that unlike a single bond (which consists 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. As a consequence substituted alkenes may exist as one of two
isomers called a
cis isomer and a
trans isomer. For example, in
cis-2-butylene the two
methyl substituents face the same side of the double bond and in
trans-2-butylene they face the opposite side.
It is certainly not impossible to twist a double bond. In fact, a 90° twist requires an energy approximately equal to half the strength of a
pi bond. The misalignment of the
p orbitals is less than expected because
pyridalization takes place.
trans-Cyclooctene is a stable strained alkene and the orbital misalignment is only 19° with a
dihedral angle of 137° (normal 120°) and a degree of pyramidalization of 18°. This explains the
dipole moment of 0.8
D for this compound (
cis-isomer 0.4 D) where a value of zero is expected.
The
trans isomer of
cycloheptene is only stable at low temperatures.
The physical properties of alkenes are comparable with
alkanes. The
physical state depends on
molecular mass. The simplest alkenes,
ethylene,
propylene and
butylene are gases. Linear alkenes of approximately five to sixteen carbons are liquids, and higher alkenes are waxy solids.
Alkenes are relatively stable compounds, but are more reactive than
alkanes. This is compatible with the idea that the carbon-carbon double bond in alkenes is stronger than the carbon-carbon single bond in alkanes, however, as the majority of the reactions of alkenes involve the rupture of this bond to form two new
single bonds.
*The most common industrial synthesis path for alkenes is
cracking of
petroleum.
*Alkenes can be synthesized from
alcohols via
dehydration that eliminates water. For example, the dehydration of
ethanol produces ethylene::CH
3CH
2OH + H
2SO
4 â†' CH
3CH
2OSO
3H + H
2O â†' H
2C=CH
2 + H
2SO
4:Other alcohol eliminations are the
Chugaev elimination and the
Grieco elimination in which the alcohol group is converted to a short-lived intermediate first.
* An
elimination reaction from an alkyl
amine occurs in the
Hofmann elimination and the
Cope reaction to produce alkenes.
*
Catalytic synthesis of higher α-alkenes can be achieved by a reaction of ethylene with the
organometallic compound triethylaluminium in the presence of
nickel,
cobalt or
platinum.
*Alkenes scramble in an
olefin metathesis.
*Alkenes can be generated from
carbonyl compounds, such as an
aldehyde or
ketone, by a variety of reactions.
**Reaction with
alkyl halides in the
Wittig reaction**Reaction with a
phenyl sulfone in the
Julia olefination **Reaction of two different ketones in the
Barton-Kellogg reaction **Coupling of one ketone in the
Bamford-Stevens reaction or the
Shapiro reaction*Alkenes can be generated from
coupling reactions of
vinyl halides.
*Alkenes can be generated by the selective reduction of
alkynes.
*Alkenes
rearrange in the
Diels-Alder reaction and an
Ene reaction.
*Alkenes are generated from α-halo sulfones in the
Ramberg-Bäcklund Reaction.
Alkenes serve as a feedstock for the
petrochemical industry because they can participate in a wide variety of reactions.
Addition reactions
Alkenes react in many
addition reactions.
*
Catalytic addition of hydrogen:
Catalytic hydrogenation of alkenes produces 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 nickel is often employed. This is an
alloy of
nickel and
aluminium. An example of this reaction is the catalytic hydrogenation of
ethylene to yield
ethane: :CH
2=CH
2 + H
2 â†' CH
3-CH
3*
Electrophilic addition: Most addition reactions to alkenes follow the mechanism of
electrophilic addition. An example is the
Prins reaction where the electrophile is a
carbonyl group.
*
Halogenation: Addition of elementary
bromine or
chlorine to alkenes yields
vicinal dibromo- and dichloroalkanes, 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 â†' BrCH
2-CH
2Br
This is the mechanism for the reaction::
:The reaction works because the high electron density at the double bond causes a temporary shift of electrons in the Br-Br bond causing a temporary induced dipole. This makes the Br closest to the double bond slightly positive and therefore an electrophile.
*
Hydrohalogenation: Addition of
hydrohalic acids such as
HCl or
HBr to alkenes yields the corresponding
haloalkanes.:CH
3-CH=CH
2 + HBr â†' 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).:This is the reaction mechanism for hydrohalogenation::
*Addition of a
carbene or
carbenoid yields the corresponding
cyclopropane.
Oxidation
Alkenes are
oxidized with a large number of
oxidizing agents.
*In the presence of
oxygen,
alkenes burn with a bright flame to produce
carbon dioxide and water.
*
Catalytic oxidation with oxygen or the reaction with
percarboxylic acids yields
epoxides
*Reaction with ozone in
ozonolysis leads to the breaking of the double bond, yielding two
aldehydes or
ketones:R
1-CH=CH-R
2 + O
3 â†' 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.
Polymerization
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.
IUPAC Names
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 that 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
CH3CH2CH2CH2CH==CH26 5 4 3 2 11-Hexene CH3 |CH3CH2CHCH2CH==CH26 5 4 3 2 14-Methyl-1-hexene CH3 |CH3CH2CHCH2C==CH26 5 4 3 |2 1 CH2CH32-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:
¦¦ CH2="CH2"| CH3CH="CH2" | CH3C(CH3)="CH2" |
| IUPAC name: | Propene | 2-Methylpropene |
| Common name: | Propylene | Isobutylene |
*
Alkane*
Alkyne*
Arenes are also alkenes but have very different properties due to
aromaticity*
Alpha-olefin*
Olefin fiber
