Alkene

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File:Ethylene-3D-vdW.png

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 (C2H4), 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.

Structure

Shape

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. It is a common error, among materials science teachers especially, to group the molecule Benzene (C6H6) with the alkenes due to its "double" bonds.

Molecular geometry

cis-2-Butylene
trans-2-Butylene

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. This is exactly why methene does not exist, simply because of the absence of the C=C double 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 (See: pyramidal alkene). 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.[1] 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 (gases from ethene to butene - liquids from pentene onwards). 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 due to their double carbon-carbon bond. Although stronger than the single carbon-carbon bond in alkanes, the majority of the reactions of alkenes involve the rupture of this double bond, forming two new single bonds.

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, which occur by opening up the double-bond.

CH2=CH2 + H2 → CH3-CH3
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. 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.
CH3-CH=CH2 + HBr → CH3-CHBr-CH2-H
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 fewer hydrogen substituents (Markovnikov's rule).
This is the reaction mechanism for hydrohalogenation:
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Oxidation

Alkenes are oxidized with a large number of oxidizing agents.

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.

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 ethene:
CH3CH2OH + H2SO4 (conc. aq) → 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.

For unsymmetrical products the more substituted carbons (those with fewer hydrogens) tend to form more stable sites for double bonds (see Saytzeff's rule).

Nomenclature

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:

  1. 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.
  2. Indicate the location of the double bond by the location of its first carbon
  3. Name branched or substituted alkenes in a manner similar to alkanes.
  4. Number the carbon atoms, locate and name substituent groups, locate the double bond, and name the main chain
File:Alkene nomenclature.png
Naming substituted hex-1-enes

The E,Z notation

When an alkene has more than one substituent, the double bond geometry is described using the labels E and Z. These labels come from the German words "entgegen" meaning "opposite" and "zusammen" meaning "together". Alkenes with the higher priority groups on the same side of the double bond have these groups together and are designated Z. Alkenes with the higher priority groups on opposite sides are designated E.

File:EZalkenes2.png
The difference between E and Z isomers

See also

References

  1. Barrows, Susan E. (2005). "Understanding Rotation about a C=C Double Bond". J. Chem. Educ. 82: 1329. Unknown parameter |coauthors= ignored (help)
 

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