Alkene
In
organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one
carbon-to-
carbon double bond. The simplest alkenes, with only one double bond and no other
functional groups, form a homologous series of
hydrocarbons with the general formula CnH2n.
The simplest alkene is
ethylene , which has the
International Union of Pure and Applied Chemistry name
ethene. Alkenes are also called
olefins or
vinyl compounds.
Encyclopedia
In
organic chemistry, an
alkene,
olefin, or
olefine is an unsaturated chemical compound containing at least one
carbon-to-
carbon double bond. The simplest alkenes, with only one double bond and no other
functional groups, form a homologous series of
hydrocarbons with the general formula
CnH2n.
The simplest alkene is
ethylene , which has the
International Union of Pure and Applied Chemistry name
ethene. Alkenes are also called
olefins or
vinyl compounds.
Structure of Alkenes
Shape of alkenes
As predicted by the
VSEPR model of
electron pair repUlsion, 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 , 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 , 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° and a degree of pyramidalization of 18°. This explains the
dipole moment of 0.8 D for this compound where a value of zero is expected. The
trans isomer of
cycloheptene is only stable at low temperatures.
Physical properties
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.
Chemical properties
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.
Synthesis
- 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:
- CH3CH2OH + H2SO4 ? CH3CH2OSO3H + H2O ? H2C=CH2 + H2SO4 + H2O
- Other alcohol eliminations are the Chugaev elimination and the Grieco elimination in which the alcohol group is converted to a short-lived intermediate first.
...
or
ketone, by a variety of reactions.
...
sulfone in the
Julia olefinationReactions
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.
- CH2=CH2 + H2 ? CH3-CH3
- 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:
- CH2=CH2 + Br2 ? BrCH2-CH2Br
- It is also used as a quantitive test of unsaturation, expressed as the bromine number of a single compound or mixture.
- 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.
- CH3-CH=CH2 + HBr ? CH3-CHBr-CH3
- 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 .
- This is the reaction mechanism for hydrohalogenation:
Oxidation
Alkenes are oxidized with a large number of
oxidizing agents.
...
s or
ketones
- R1-CH=CH-R2 + O3 ? R1-CHO + R2-CHO + H2O
- 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.
Nomenclature of Alkenes
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
CH2
6 5 4 3 2 1
1-Hexene |
CH3
>
CH3CH2CHCH2CHCH2
6 5 4 3 2 1
4-Methyl-1-hexene |
CH3
>
CH3CH2CHCH2C
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:
| CH2="CH2" | CH3CH="CH2" | CH3C="CH2" |
| IUPAC name: | Ethene | Propene | 2-Methylpropene |
| Common name: | Ethylene | Propylene | Isobutylene |
See also
References
