halogenation

1. Reactions of alkenes

2. Alkenes structure
C2H4
C3H6
C4H8

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
• 150⁰C
• 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. Br2
• 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, H2
• 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, NelsonThornes.
Chapter 14, 247-248
• https://www.slideshare.net/lerangeline/alkenes-ppt-slides