that have double bonds
atoms and therefore are called unsaturated
substance. The generic formula is CnH2n
The simplest alkene is ethene
(also known as ethylene
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
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
In the case of alkene, halogenation is an addition reaction.
CH2=CH2 + Cl2 → ClCH2-CH2Cl
It's also a reaction of addition, an electrophilic addition.
δ+ δ- δ+ δ-
+ H-Cl → CH3
- 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.
This is also an electrophilic addition.
CH3-CH+-CH3 + Cl- → CH3-CH-CH3
here is the case of the ethylene
Under a thermic source of energy, peroxide
will decompose into two free radicals.
RO - OR → 2 RO*
the alone electron of the RO* will bond with an electron from the pi-bonding.
RO* + CH2=CH2 → ROCH2-CH2*
There is the addition of n monomers
to obtain a polymer
ROCH2-CH2* + n CH2=CH2 → RO(CH2-CH2)n-CH2-CH2*
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