Alkene

From Wikinfo

Alkenes are hydrocarbons that have double bonds between carbon atoms and therefore are called unsaturated substance. The generic formula is C_nH_2n. The simplest alkene is ethene (also known as ethylene):

Image:Ethene.png

Physical properties

• The same as alkanes.
• Physical state depends on molecular mass.

Chemical properties

Alkenes are relatively stable compounds, but are more reactive than alkanes.

Obtainment of alkene

Mainly by cracking reaction of an alkane.

alkane → light alkane + alkene

Hydrogenation reaction

This reaction is done under a pressure of about 50 atm. We use a catalyst called Raney's nickel who is an alloy of nickel and aluminium.

here is an example with ethylene :

CH₂=CH₂ + H₂ → CH₃-CH₃

Halogenation reaction

In the case of alkene, halogenation is an addition reaction.

CH₂=CH₂ + Cl₂ → ClCH₂-CH₂Cl

Hydrohalogenation reaction

It's also a reaction of addition, an electrophilic addition.

   δ+  δ-  δ+ δ-  

CH₃-CH=CH₂ + H-Cl → CH₃-CH-CH₃

                      |     
                      Cl

• Slow step : this step determine the velocity of the reaction

This is an electrophilic addition with formation of a carbocation. The attacker is the H⁺, he will search for a source of electrons, the ones from the pi-bonding.

   δ+  δ-  

CH₃-CH=CH₂ + H⁺ → CH₃-CH⁺-CH₃

• Fast step :

This is also an electrophilic addition.

CH₃-CH⁺-CH₃ + Cl^- → CH₃-CH-CH₃

                      |     
                      Cl

Polymerisation

here is the case of the ethylene

• Activation step

Under a thermic source of energy, peroxide will decompose into two free radicals.

RO - OR → 2 RO*

• Initiation step

the alone electron of the RO* will bond with an electron from the pi-bonding.

RO* + CH₂=CH₂ → ROCH₂-CH₂*

• Propagation step

There is the addition of n monomers to obtain a polymer.

ROCH₂-CH₂* + n CH₂=CH₂ → RO(CH₂-CH₂)_n-CH₂-CH₂*

• Breaking step

Recombinaison of two free radicals.

R* + *R → R-R

Oxydation reactions : ozonolysis reaction

Oxydation reactions : arranged oxydation

Oxydation reactions : brutal oxydation

See also: alkane, alkyl.[[fr:Alc�ne]]

References

• Adapted from the Wikipedia article, "Alkene" http://en.wikipedia.org/wiki/Alkene, used under the GNU Free Documentation License