Year 12 Shut down work part 2 – Alkenes Supporting Videos Alkenes https://www.youtube.com/watch?v=3QGSShSKYeQOr Alkenes https://www.youtube.com/watch?v=bDyhetwQWus

Additional work - Polymers Research Project Success Criteria • What are High and low density polymers? • How are they different? Think about the conditions used to

make them, Give a use for each explaining why they are suitable for that job

• Define biodegradability • How can we solve the plastic problem???

• Alkenes• Structure, bonding and

reactivity

• Addition reactions

• Addition polymers

LearnIT!KnowIT!Alkenes

• Structure, bonding and reactivity

• Addition reactions• Addition polymers

Structure, bonding and reactivity

Alkenes are unsaturated hydrocarbons; containing a carbon-carbon double bond. Bonding involves a double covalent bond between

carbon, a centre of high electron density.

It is possible to break the π-bond in alkenes and form σ-bonds with other species. The π-bond in an alkene is an area of high electron

density. It can attract electrophiles and undergo heterolytic fission.

An electrophile is a species which can accept a pair of electrons from a

species with a high electron density.

Heterolytic fission is the breaking of a

covalent bond which results in both

electrons going to the same atom.

Addition reactions of alkenes

Electrophilic addition reactions of alkenes – making halogenoalkanes

The H in the hydrogen bromide (H-X bond) has a

positive dipole and is attacked by the pair of electrons on the C=C

bond, which undergoes heterolytic fission.

The halide ion then attacks the carbocation to

form a dative covalent bond and halogenoalkane.This reaction is fairly quick and takes

place readily at room temperature.

Addition reactions of alkenes

Electrophilic addition reactions of alkenes – making alcohols

Alkenes will undergo an electrophilic addition reaction with cold concentrated sulphuric acid (H2SO4).

If the mixture is then warmed and water added, the H2SO4 group will be replaced by an OH group and an alcohol will be formed.

Addition reactions of alkenes

Electrophilic addition reactions of alkenes – making dihalogenoalkanes

The Br-Br molecule is non-polar. In the presence of

alkenes the electrons move to one side of the molecule and it acquires a temporary

dipole. The δ+ Br is attacked by the alkene.

The bromide ion with lone pair of electrons act as a nucleophile and attacks

carbocation forms dative covalent bond.

Orange/brown bromine water

Turns colourless when alkene is added

Testing for unsaturation

Alkenes are unsaturated hydrocarbons; containing a carbon-carbon double bond. Bromine is added across the double bond by

electrophilic addition, forming a dibromoalkane.

• Double bond repels electrons in Br2 – it becomes polarised

• Brδ+ attracted to pair of electrons in double bond. Br-Br bond breaks.

• Carbocation intermediate created. Br-

attracted to carbocation.• Dative covalent bond formed between

Br- and carbon.

Addition reactions of alkenes

Formation of major and minor products in addition reactions of unsymmetrical alkenes.

Unsymmetrical alkenes are those in which the two carbon atoms in the double bond are not attached to the same groups.

If an alkene with 3 or more carbon atoms is unsymmetrical it will produce a mix of products. Unequal amounts of these products will be formed because of the relative stability of the carbocation intermediates.

Secondary carbocations (two R groups) are more stable than primary carbocations (one

R group). Tertiary carbocations (three R groups) are even more stable than

secondary cations.

R group= alkyl group

Addition polymers

Addition polymerisation to make polymers

Alkenes can be made to join together in the presence of high pressure and a suitable catalyst. This is known as polymerisation

addition.

The product of this addition process is a very long hydrocarbon chain, known as an addition polymer.

Addition polymers are saturated and are therefore unreactive.

Addition polymers

Repeating units

You need to be able to:• Draw a repeating unit from a monomer structure.

• Draw the repeating unit from a section of polymer.

• Draw the structure of a monomer from a section of the polymer.

Addition polymers

Properties and production

Knowledge and understanding of the production and properties of polymers has developed over time. The properties of addition polymers are dependent upon their structure:• They are very unreactive as they are saturated molecules and non-polar.• The chains are held together by van der Waals intermolecular forces

between molecules. The stronger the van der Waals forces the higher the melting point.

• The longer the chain the stronger the van der Waals forces• The less branched the chain the stronger the van der Waals forces.

Strong and rigid polyalkenes tend to be made of long, straight chains.

Weaker, flexible polyalkenes tend to be made of short, branched chains.

Addition polymers

Poly(chloroethene) – PVC

The rigid form of PVC can be made more flexible by adding a plasticiser. This pushes the polymer chains apart, meaning they can move around more, making

the polymer more easy to bend and flex.

PVC comes in two basic forms: • rigid – used in construction and pipes• Flexible – electrical cable insulation and

inflatable products.

Questions

Alkenes

• Structure, bonding and reactivity

• Addition reactions• Addition polymers

Structure, Bonding & reactivity & Addition Reactions Questions

1. What are alkenes?

2. What is the arrangement and bond angle of the C=C bond?

3. What is meant by addition?

4. Define heterolytic fission.

5. What is an electrophile?

6. What are the conditions needed for the electrophilic addition of alkenes to produce alcohols?

Addition Reactions continued Questions

7. What conditions are needed for addition polymerisation?

8. How are alkenes converted into halogenoalkanes?

9. Explain how to test for unsaturation.

10. Draw a curly arrowed diagram for the reaction between alkanes and bromine water.

11. Why are major and minor products formed in addition reactions of alkenes?

Polymerisation Questions

12. How are polymers made from alkenes?

13. Explain why polymers are unreactive.

14. How can rigid PVC be converted into flexible PVC?

15. Describe the forces between molecules of polyalkenes and why they are significant.

16. Draw the repeating unit for the following: