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Addition reactions of alkenes
A C=C double bond is made up of two different single bonds a sigma and Pi bond. The electrons are more tightly held in a sigma bond between the two carbons. This is stronger than the Pi bond.
Reactions of alkenes mostly occur when the double bond becomes a single bond. The pi bond being the weaker of the two is the one that forms new bonds. As the new product will contain sigma bonds instead of pi the product bonds are more stronger and therefore more stable.
A common addition reaction is the reaction between ethene and bromine.
Reactions of alkenes mostly occur when the double bond becomes a single bond. The pi bond being the weaker of the two is the one that forms new bonds. As the new product will contain sigma bonds instead of pi the product bonds are more stronger and therefore more stable.
A common addition reaction is the reaction between ethene and bromine.
Hydrogenation
This is an addition reaction where hydrogen is added to an alkene.
This needs to be done using heat and a nickel catalyst
Halogenation
Reactions between alkenes and halogens are examples of halogenation. The products are dihalogenalkanes. When chlorine is involved we call it chlorination.
Reactions between alkenes and halogens are examples of halogenation. The products are dihalogenalkanes. When chlorine is involved we call it chlorination.
Hydration
As the word dehydration means the loss or removal of water, hydration is the adding of water. The way water is added to an alkene is H and OH on the C=C bond. The reaction is usually done by heating an alkene with steam and passing the mixture over a catalyst of phosphoric acid.
As the word dehydration means the loss or removal of water, hydration is the adding of water. The way water is added to an alkene is H and OH on the C=C bond. The reaction is usually done by heating an alkene with steam and passing the mixture over a catalyst of phosphoric acid.
The hydration of ethene forms ethanol. This is a very useful product and like with reactions that make propanol is used extensively in industry.
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Hydrogen halides
The addition of a hydrogen halide is another example of an addition reaction which involves alkenes. Normally hydrogen bromide or hydrogen chloride is used to form a bromoalkane or chloroalkane.
This reaction is very similar to the addition bromine. However, this reaction cannot be used to test for a C=C bond as all the reactants and products are colourless, meaning no observable colour change.
The addition of a hydrogen halide is another example of an addition reaction which involves alkenes. Normally hydrogen bromide or hydrogen chloride is used to form a bromoalkane or chloroalkane.
This reaction is very similar to the addition bromine. However, this reaction cannot be used to test for a C=C bond as all the reactants and products are colourless, meaning no observable colour change.
Oxidation
Alkenes can undergo both addition and oxidation. This will result in two alcohol groups being produced (diol).
Ethene can react with water to form an alcohol. But when an oxidising agent is induced along with the addition of water we get a diol. The oxidising agent is normally potassium manganate (VII) which provides the oxygen atom for oxidation. The water molecule provides the other oxygen and the two hydrogen atoms.
This reaction can also be used to test for alkenes with the changing of the purple potassium manganate and ethene to the colourless alcohol
