Alkene

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