Double bonds don't rotate
1. Carbon atoms in a C=C double bond and the atoms bonded to these carbon all lie in the same plane (they're planar). Because of the way they're arranged, they're actually said to be trigonal planar - the atoms attached to each double-bond carbon are at the corners of an imaginary triangle.
2. Ethene, C2H4, is completely planar but in larger alkenes only the >C=C< unit is planar.
3. Another important thisng about C=C double bonds is that atoms can't rotate around single bonds. In fact, double bonds are fairly rigid - they don't bend much either.
4. Even though atoms can't rotate about the double bond, things can't rotate about any single bonds in the molecule - like pent-2-ene.
5. The restricted rotation around the C=C double bond is what causes E/Z isomerism.
1. Stereoisomers have the same structural formula but a different arrangement in space.
2. Because of the lack of rotation around the double bond, some alkenes each have different atoms or groups attached to them. Then you get and 'E-isomer' and 'Z-isomer'.
1. The double bonds in alkenes can open up and join together to make long chains called polymers. It's kind of like they're holding hands in a big line. The individual, small alkenes are called monomers.
2. This is called addition polymerisation. For example, polyethene is made by the additiona polymerisation of ethene.
3. To find the monomer used to form an addition polymer, take the repeating unit and add a double bond.
Addition Polymers continued
4. Because of the loss of the double bond, polyalkanes, like alkanes are unreactive.
5. Different polymer structures have different properties, which means they're suited to different uses.
Some typical uses of polyethene and polypropene are shown below:
Polyethene - (properties) soft, flexible (uses) plastic bags, squeezy bottles
Polyproene (properties) tough, strong (uses) bottle crates, rope
6. Many polymers are difficult to dispose of and are made from non-recyclable oil fractions, so it makes sense to recycle them. For example, polypropene is recycled - it can be melted and remoulded.