Alkene Reactions


Alkenes react with a mechanism called electrophilic addition. This involves the breaking of the weak pi bond and a molecule being added in an addition reaction. All four of these reactions occur using this mechanism.

1. Hydrogenation (addition of hydrogen)
Conditions: Nickel catalyst, high temperature and pressure

CH2=CH2 + H2 CH3CH3

Uses: Hardening of Vegetable oils in the manufacture of margarine (converts unsaturated vegetable oils into solid fats)


2. Hydration (reaction with water)
Conditions: Phosphoric acid catalyst (H3PO4) and high temperature and pressure

CH2=CH2 + H2O CH3CH2OH

Uses: manufacture of ethanol

Advantages compared to fermentation: Pure product formed, continuous process.
Disadvantages: Corrosive catalyst, non-renewable sources, expensive (high tech)


3. Halogenation (addition of halogen)
Conditions: None

CH2=CH2 + Br2 CH2BrCH2Br

Uses: Test for unsaturation (alkenes decolourise bromine water).


4. Reaction with halogen halides to form halogenalkanes
Conditions: Make HBr is situ. (in situation) by reacting:

H2SO4 + KBr KHSO4 + HBr

The HBr that’s formed can then react with CH2=CH2 as follows:

CH2=CH2 + HBr CH3CH2Br

Uses: No specific uses

Note: With these reactions it is possible that isometric products can be formed. E.g. Propene and steam can react to form either or these two different products:


Mechanism - Electrophilic addition

Reactions 1-4 all use the same mechanism called electrophilic addition. A mechanism basically means the reaction occurs in a series of steps.


Definitions

Electrophile
An electron pair acceptor.

Heterolytic fission (not to be confused with homolytic fission)
A covalent bond breaks unevenly to form a positive and negative ion.


Step 1
The Br2 molecule becomes polarized as it approaches the alkene (a dipole is induced due to the electrons at the double bond). The d+Br atom is known as an electrophile and accepts the electrons from the pi bond.

Step 2
The Br molecule undergoes heterolytic fission and the Br+ ion forms a dative covalent bond. The carbocation (positive carbon ion) that forms then forms a further covalent bond with the Br- ion.

Step 3
The molecule is formed.


Additional polymerisation
This is when alkene molecules join together to form a long chain molecule. As usual the pi bond breaks and forms a bond with another molecule. Note that the monomer always consists of two carbon atoms; any more are displayed attached to a hydrogen atom.



The previous two polymers only show one repeat unit. Two repeat units may be requested in a question, two repeat units look like this:

Disposal
The problem: Plastics are non-biodegradable.

Solution:
· They can be recycled but it is relatively expensive and time consuming.
· They can be burnt as fuels but this can produce toxic gas (e.g. PVC produces chlorine gas). Incinerators can, however be used to remove these pollutants.
· Polymers can also be heated and cracked to produce other hydrocarbons.