Reactions of alkenes

halogenation

1. Reactions of alkenes

2. Alkenes structure C 2 H 4 C 3 H 6 C 4 H 8

3. Covalent Bonding in Alkenes

4. Combustion of alkenes Alkenes react with oxygen in combustion reactions in the same way as alkanes, but they tend to burn in air with a smoky flame The reactions are exothermic; as a large amount of energy is produced More oxygen is required for alkenes than alkanes because of the higher C:H ratio STEAM

5. Addition Reactions – Halogens + Br Br → Ethene Bromine Dibromoethane Colourless It's the addition of a halogen to the C=C double bond of an alkene to produce a haloalkane . The reaction rapid

6. Addition Reactions – Halogens How do we know halogens and alkenes undergo addition reactions? Bromine water turning colourless Alkene + Bromine water (turns colourless) Alkenes will also react with other halogens: Chlorine Bromine Iodine

7. Practice What is the product when propene reacts with bromine?

8. Did you get?

9. Addition Reactions – Hydrogenation Alkenes are unsaturated so we can add more hydrogen to make them saturated In hydrogenation, hydrogen is added to the alkene 150 ⁰ C Nickel catalyst Pressure of 5 atms + H H → Ethene Hydrogen Ethane

10. All Addition Reactions In all addition reactions, only one molecule of halogen, hydrogen or water is needed Only one double bond to open up Halogens, e.g. Br 2 One bromine from the bromine molecule bonds to one carbon from the double bond, then the other Br bonds to the other carbon from the double bond Hydrogen, H 2 Same concept, but with H-H

11. Practice What is the product when pentene undergoes hydrogenation?

12. Did you get?

13. References https:// socratic.org/questions/what-is-halogenation-of-alkenes Tindale, Ritchie et al, 2014, Chemistry for CSEC 2nd Edition , Nelson Thornes. Chapter 14, 247-248 https://www.slideshare.net/lerangeline/alkenes-ppt-slides