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Alkenes - Coggle Diagram
Alkenes
Structure and Shape
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- Ethene is the smallest alkene as you can't have a C=C double bond with only one carbon.
- Unsaturated hydrocarbon - hydrogen and carbon only.
C=C Bond
- Two orbitals overlap to form an orbital with cloud of electron density above and below the single bond.
- It is called a π-orbital and means the bond can't rotate.
- As well as a C-C single bond, there is a p-orbital on each carbon.
- This is sometimes called a restricted rotation.
- Double bond cannot rotate due to existence of overlapping P orbitals above and below single bond.
Structural Isomers
- Can form position isomers where the C=C bond is in a different place, e.g. but-2-ene and but-1-ene.
- The longer the carbon chain, the more isomers it can have.
- Need more than three carbons.
Geometric Isomers
- Have some structural formula but bonds are arranged differently in space.
- Only occur around C=C bond.
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E.g. but-2-ene
- Z-but-2-ene is on same side of C=C.
- E but-2-ene is on different sides of C=C.
Physical Properties
- Physical properties are similar to alkanes.
- The melting/boiling points increase with a longer carbon chain.
- Van der Waals forces are the only intermolecular forces.
- They are not soluble in water.
- C=C bond doesn't affect the boiling/melting points too much.
Reactivity
- Electrophiles are electron-pair acceptors.
- As they are unsaturated, they can undergo addition reactions.
- C=C bond forms electron-rich area which can be attacked easily by positively charged reagents called electrophiles.
- The double bond makes a huge difference to the reactivity.
- Most reactions are electrophilic reactions.
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Disposal of Polymers
Current Problems
- A lot of these plastics end up in landfill or pollute the oceans as large gyres floating in the sea.
- The plastics can degrade into microplastics that will be consumed by organisms and enter the food chain.
- Addition polymers are non biodegradable as they are long chain saturated alkane molecules and very unreactive.
- Not all plastics can currently be recycled into new products.
Mechanical Recycling
- Plastics are separated into different types, then washed and ground into pellets.
- These pellets are then melted and remoulded into a new plastic product.
- Advantages - the plastic can be reused in a new form and reduces the need for new raw materials.
- Disadvantages - the collection and separation of different plastics takes time and requires energy. The melting and remoulding of plastics will also require energy and will releas greenhouse gases. Furthermore, some plastics can only go through this process a certain amount of times before they lose their properties.
Feedstock Recycling
- Plastics are heated to a high temperature to break them down into monomers ready to be used in new plastic products.
- Advantages - can recover raw material from waste materials and conserves monomer resources.
- Disadvantages - can be expensive as requires a large amount of energy, and may result in net loss per year. The quality of plastic is reduced over time.
Biodegradable polymers
- E.g. starch, cellulose, lignin, wool, gelatin.
- These can be broken down using water over a long period of time, or sped up using acid catalysts and heat.
- Condensation polymers are biodegradable, as they are made from natural raw materials and renewable resources.