Alkene
From Free net encyclopedia
| Image:Ethene.png | |
| Properties | |
|---|---|
| General formula | CnH2n |
An alkene 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, the alkenes with general formula CnH2n.
The simplest alkene is C2H4, which has the common name "ethylene" and the International Union of Pure and Applied Chemistry (IUPAC) name "ethene". Alkenes are also called olefins (an archaic synonym, but widely used in the petrochemical industry) or vinyl compounds.
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Structure of Alkenes
Shape of Alkenes
As predicted by the VSEPR model of electron pair replusion, in the molecular geometry of alkenes the bond angles about each carbon in a double bond are about 120°, although the angle may be larger 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 propene is 123.9°. The alkene double bond is stronger than a single covalent bond and also shorter with an average bond length of 133 picometre.
Molecular Geometry
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. As a consequence substituted alkenes exist as one of two isomers called a cis isomer and a trans isomer. In cis-2-butene two methyl substituents face the same side of the double bond and in trans-2-butene they face the opposite site.
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.<ref>Template:Cite journal</ref> 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.
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 an elimination reaction that removes one water molecule:
H3C-CH2-OH + H2SO4 → H3C-CH2-O-SO3H + H2O → H2C=CH2 + H2SO4 - An Elimination reaction from an amine occurs in the Hofmann elimination and the Cope reaction
- 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.
- Alkenes scramble in an olefin metathesis
- Alkenes are generated from a ketone and an alkyl halide in the Wittig reaction, from two different ketones in the Barton-Kellogg reaction and from coupling of one ketone in the Bamford-Stevens reaction or the Shapiro reaction.
- Alkenes are generated from a coupling reaction of vinyl halides
- Alkenes are generated from selective reduction of alkynes
- Alkenes rearrange in a Diels-Alder reaction and an Ene reaction
- Alkenes are generated from α-halo sulfones in the Ramberg-Bäcklund Reaction
Reactions
Alkenes serve as a feedstock for the petrochemical industry because they can participate in a wide variety of reactions.
Addition reactions
Alkenes reacts in many addition reactions.
- Catalytic addition of hydrogen: 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: 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 yield 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
- This is the mechanism for the reaction:
- Image:AlkeneAndBr2Reaction.png
- 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 like HCl or HBr to alkenes yield 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 (Markovnikov's rule).
- This is the reaction mechanism for Hydrohalogenation:
- Image:AlkeneAndHBrReaction.png
- 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 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
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.
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.
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
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CH3CH2CH2CH2CH==CH2</br> 6 5 4 3 2 1 </br> 1-Hexene
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CH3</br> |</br> CH3CH2CHCH2CH==CH2</br> 6 5 4 3 2 1 </br> 4-Methyl-1-hexene
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CH3</br> |</br> CH3CH2CHCH2C==CH2</br> 6 5 4 3 |2 1</br> CH2CH3</br> 2-Ethyl-4-methyl-1-hexene
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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: | Ethene | Propene | 2-Methylpropene |
| Common name: | Ethylene | Propylene | Isobutylene |
See also
- hydrocarbons with increasing bond order:Alkanes, alkenes and alkynes.
- Arenes are also alkenes but have very different properties due to aromaticity
References
<references/>ar:ألكين ca:Alquè da:Alken de:Alkene et:Alkeenid el:Αλκένια es:Alqueno eo:Alkeno fr:Alcène io:Alkeno it:Alcheni he:אלקן lv:Alkēni mk:Алкен nl:Alkeen ja:アルケン pl:Alken pt:Alceno ru:Алкены sr:Алкен su:Alkéna fi:Alkeeni sv:Alken vi:Anken uk:Алкени zh:烯烃