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 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.

Addition reactions

Catalytic addition of hydrogen

Catalytic hydrogenation of alkenes produce the corresponding alkanes. The reaction is carried out under pressure in the presence of a metallic catalyst. Common industrial catalysts are based on platinum, nickel or palladium, for laboratory syntheses, Raney's nickel is often employed. This is an alloy of nickel and aluminium.

This is the catalytic hydrogenation of ethylene to yield ethane:

CH₂=CH₂ + H₂ → CH₃-CH₃

Electrophilic addition

Most addition reactions to alkenes follow the mechanism of electrophilic addition.

1. Halogenation: Addition of elementary bromine or chlorine to alkenes yield vicinal Dibromo- and dichloroalkenes, respectively. The decoloration of a solution of bromine in water is an analytical test for the presence of alkenes:
   CH₂=CH₂ + Br₂ → BrCH₂-CH₂Br
2. Hydrohalogenation: Addition of hydrohalic acids like HCl or HBr to alkenes yield the corresponding haloalkanes.
   CH₃-CH=CH₂ + HBr --> CH₃-CHBr-CH₃
   If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with less hydrogen substituents (Markovnikov's rule).

Oxidation

1. In the presence of oxygen, alkenes burn with a bright flame to carbon dioxide and water.
2. Catalytic oxidation with oxygen or the reaction with percarboxylic acids yields epoxides
3. Reaction with ozone leads to the breaking of the double bond, yielding two aldehydes or ketones
   R₁-CH=CH-R₂ + O₃ → R₁-CHO + R₂-CHO
   This reaction can be used to determine the position of a double bond in an unknown alkene.

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

See also: alkane, alkyl.