Alkenes are unsaturated hydrocarbons with a general formula of CnH2n.
Carbon=Carbon bonds consist of one sigma and one pi bond, pi bonds are exposed and have high electron density which makes them vulnerable to attack by electrophiles.The pi bond is formed by sideways overlap of p orbitals. The pi bond is weaker than the sigma bond and rotation cannot occur.
Stereoisomers have the same structural formula but different spatial arrangement of atoms. Alkenes can exhibit a type of isomerism called E-Z sterioisomerism, forming due to the restricted rotation of the carbon=carbon bond and when there are different atoms attached to both ends of the double bonds.
- Determine priority groups on both sides of the double bond – atom with the bigger atomic number.
- If the priority group is on the same side of thedouble bond the molecule is a Z isomer (Z=same). If the priority groups are on opposite sides it is an E isomer.
- Cis-trans isomerism is a type of E-Z isomerism.They can only be called cis-trans isomers if the priority groups are the same.
Reactions of Alkenes
- where two molecules react together to produce one.
- Alkenes can react with hydrogen to become an alkane with a nickel catalyst under addition/reduction.
Electrophilic addition reactions:
Electrophile: electron pair acceptor
Conditions: room temperature
As the bromine molecule approaches the alkene the electrons in the pi bonds repel the electron pair in the Br-Br bond forming an induced dipole and giving the molecule a partial charge.
Heterolytic fission allows bromine to attach to the carbon chain. The Bromine ion is then attracted to the partial charge on the carbon as it has lost an electron.
When Hbr or Hcl is the electrophile there are two products formed. In most cases the halogen ion will add to the carbon with the fewest hydrogens.
There will be a major product (90%) and a minorproduct. The major product is formed via the more stable carbocation which is any carbon furthest from either end. The more stable carbocation is stabilised because the methyl groups on either side are electron releasing and reduce the charge on the ion. If both carbocations are secondary they will both be equally stable.
Reactions with steam:
- This reaction can be called hydration as water is added to a molecule.
- The reagent required is steam.
- Conditions: high temperature (300-600 degreescentigrade); high pressure (70 atm), catalyst of concentrated concentrated H₃PO₄.
It is preferable to complete the reaction industrially as there are high pressures required for the reaction.There are no waste products produces with high atom economy.
- Addition polymers are formed from alkenes in addition polymerisation. Polyalkenes are unreactive due to strong covalent bonds.
- Polymers have major industrial usage, such as the production of margarine by catalytic hydrogenation of unsaturated vegetable oils using a nickel catalyst.
- Liquid vegetable oils are generally polyunsaturated alkenes. Hydrogenation by reaction of hydrogen with a nickel catalyst converts the double bond to saturated single bonds thus increasing the melting point of the oil making it harder and more solid.
Separation – this involves sorting the waste of each type of polymer (PTFE, PVC, PET) and then each type can be melted and remoulded.
Feedstock for cracking – waste polymers can beused for cracking which allows the production of plastics and other chemicals.
Combustion for energy production – polymers can be incinerated and the energy released can generate electricity. Combustion of halogenated plastics can lead to the formation of toxic acidic waste products like HCl – this can be averted by removing the HCl fumes formed from the combustion process.
Chemists are also developing a range of biodegradable polymers, compostable polymers, soluble polymers and photo-degradable polymers.
Drafted by Eunice (Chemistry)