Alkene

An alkene is a hydrocarbon that has a double bond between two carbon atoms.

Alkenes are unsaturated substances.

The generic formula is C_nH_2n. The simplest alkene is ethene (also known as ethylene):

File: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.

Reactions

Synthesis

1. The most common industrial synthesis path for alkenes is cracking of petroleum.
2. Alkenes can be synthesized from alcohols via an elimination(chemistry) reaction that removes one water molecule: H₃C-CH₂-OH + H₂SO₄ → H₃C-CH₂-O-SO₃H + H₂O → H₂C=CH₂ + H₂SO₄
3. 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.

Hydrogenation

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

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

CH₂=CH₂ + Cl₂ → ClCH₂-CH₂Cl

Electrophilic addition

1. hydrohalogenation
   • alkene + hydrogen halide → alkyl halide

• 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

Oxidation reactions : ozonolysis reaction

Oxidation reactions : arranged oxidation

Oxidation reactions : brutal oxidation

See also: alkane, alkyl.