1. Alkenes have the general formula CnH2n. They're just made of carbon and hydrogen atoms, so they're hydrocarbons.
2. Alkene molecules all have at least one C=C double covalent bond. Molecules with C=C double bonds are unsaturated because they can make more bonds with extra atoms in addition reactions.
3. Because there's two pairs of electrons in the C=C double bond, it has a really high electron density. This makes alkenes pretty reactive.
Electrophilic addition reactions
Electrophilic addition reactions aren't too complicated...
1. The double bonds open up and atoms are added to the carbon atoms.
2. Electrophilic addition reactions happen because the double bond has got plenty of electrons and is easily attacked by electrophiles.
Electrophiles are electron pair-pair acceptors - they're usually a bit short of electrons, so they're attached to areas where there's lots of them about.
Here's a few example...
Positively charged ions, like H+, NO2 +
Polar molecules - the delta + atom is attracted to places with lots of electrons.
The double bond is also nucleophilic - it's attracted to places that don't have enough electrons.
Using Bromine Water
When you shake an alkene with orange bromine water, the solution quickly decolourises. Bromine is added across the double bond to form a colourless dibromoalkane - this happens by electrophilic addition. Here's the machanism:
Addition with Hydrogen Bromide
Alkenes undergo addition reactions with hydrogen bromide - to form bromoalkanes. This is the reaction between ethene and HBr.
1. If HBr adds to an unsymmetrical alkene, there are two possible products.
2. The amount of each product formed depends on how stable the carbocation formed in the middle of the reaction is.
3. Carbocations with more alkyl groups are more stable because the alkyl groups feed electrons towards the positive charge. The more stable carbocation is much more likely to form.
R = alkyl group
--> = electron donation
Alkyl groups are alkenes with a hydrogen removed, e.g. methyl, CH3 -
HBr and propene
This secondary (major product) carbocation's more stable because it's got two alkyl gorups. This carbocation will form most of the time.
Thi primary carbocation (minor product) is less stable as it's got one alkyl group. It forms less often.
Reaction with sulphuric acid.
Alcohols are produced industrially by hydrating alkenes in the presence of an acid catalyst, such as sulphuric acid.
1. Cold concentrated sulphuric acid reacts with an alkene in an electrophilic addition reaction.
2. If you then add cold water and warm the product, it's hydrolysed to form an alcohol.
3. The sulphuric acid isn't used up - it acts as a catalyst.
Manufacture of ethanol
1. Ethene can be hydrated by steam at 300 degrees celsius and a pressure of 60 atm. It needs a solid phosphoric (V) acid catalyst.
2. The reaction's reversible and the reaction yield is low - only about 5%. This sounds rubbish, but you can recycle the unreacted ethen gas, making the overall yield a much more profitable 95%.