Alkenes are hydrocarbons that have double bonds between carbon atoms and therefore are called unsaturated substance. The generic formula is CnH2n. The simplest alkene is ethene (also known as ethylene):

Physical properties

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.\n