Alkenes are hydrocarbons which contain a carbon-carbon double bond. The simplest alkene is ethene, C2H4.
Ethene is a flat molecule with bond angles of about 120o.
The four electrons in ethene's double bond give the region between the two carbon atoms a high density of negative charge. Positive ions or molecules with a partial positive charge on one of the atoms, will be attracted to this negatively charged region. We call these attacking species electrophiles. Alkenes take part mainly in electrophilic addition reactions.
The addition reactions of alkenes can be summarised by the general equation:
1. Reaction with Bromine
If ethene gas is bubbled through bromine, the bromine is decolourised. This is used as a test for alkenes. With ethene the product is 1,2-dibromoethane.
CH2 = CH2 + Br2 CH2Br - CH2Br
In the mechanism for this reaction, chemists believe that the bromine molecule becomes polarised as it approaches the alkene. The bromine atom nearest the alkene becomes slightly positively charged and the bromine atom furthest from the alkene becomes slightly negatively charged. The positively charged bromine atom now acts as an electrophile and reacts with the double bond.
The carbocation (positively charged ion) produced now reacts quickly with the bromide ion.
2. Reaction with Hydrogen
In the presence of a nickel or platinum catalyst, hydrogen adds across the double bond to form a saturated compound. With platinum room temperature can be used, with nickel 150oC and 5 atm pressure need to be used.
|CH2=CH2 + H2||C2H6|
3. Reaction with Water
Ethene and water undergo an electrophilic addition reaction in the presence of a catalyst, phosphoric acid on silica at 300oC and 60 atm pressure. This reaction is used for the manufacture of ethanol.
|CH2=CH2 + H2O||CH3CH2OH|
4. Reaction with Hydrogen Halides
Alkenes react with concentrated aqueous solutions of hydrogen halides in the cold.
|CH2=CH2 + HBr||CH3CH2Br|
When propene reacts with hydrogen halides, there are two possible products.
|CH3CH=CH2 + HBr||CH3CH2CH2Br||A|
It has been found that B is the major product although some A is also formed. Markovnikov's Rule states that the major product is the one in which the hydrogen atom attaches itself to the carbon atom already carrying the greater number of hydrogen atoms.
5. Addition Polymerisation
When ethene is polymerised the product is polyethene or polythene. The reaction requires a catalyst, often an organic peroxide which provides radicals. There are three stages to the reaction: initiation, propagation and termination.
Initiation: This involves the formation of a radical from the catalyst. The O - O bond in the peroxide breaks.
R - O - O - R R - O· + R - O·
Propagation: A radical now combines with one of the electrons from the double bond in the alkene.
R - O· + CH2=CH2 R - O - CH2 - CH2·
The new radical now goes on to react with more of the alkene.
R - O - CH2 - CH2· R - O - CH2 - CH2 - CH2 - CH2·
The chain grows in length rapidly. Chains containing up to 10,000 monomer units can be produced per second.
Termination: The reaction ends when the radicals are used up. This can happen by the radicals simply joining together.
R - O - (CH2)n - CH2· + R - O - (CH2)m - CH2·
R - O - (CH2)n - CH2 - CH2 - (CH2)m - O - R
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