1. Alkenes By C Hughes

2. At the end of this lesson you should know: That bonding in alkenes involves a double bond. That the arrangement C=C is planar Understand that the double bond in an alkene is a centre of high electron density Know that alkenes decolourise in Bromine water Know that alkenes can be hydrogenated catalytically and understand the use of this process in the manufacture of margarine.

3. Saturated or Unsaturated The main difference between and alkene and alkane is that an alkene has a double bond. This makes it an unsaturated hydrocarbon. A saturated hydrocarbons, eg, alkanes have the maximum number of Hydrogens.

4. The Double bond….. Example – Ethene Each Carbon has three atoms attached to it. They are attached by strong sigma bonds using three of the electrons from the carbon atom. The sigma bonds are most stable when they have 120 between them. They are therefore planar molecules.

5. So what has happened to the fourth electron from each carbon in the double bond? They are shared between the two atoms. They do not form a sigma bond. They form a pi bond in which the electrons are delocalised

6. What are delocalised electrons? The delocaised electrons are held in the molecule less tightly than electrons in a sigma bond. This makes it a good target for positively charged molecules who are after some negative electrons! Allkenes are very reactive due to the fact above

7. What happened hen you added Bromine to the alkene? What happened when you add Bromine to an alkane? Why is there a difference?

8. NameState (room temp) MrMr Density / gMelting pt / ◦C Boiling point / ◦C But-1-eneG56.10.595-185.26.2 Trans-but-2-eneG56.10.604-105.41.0 Cis-but-2-eneG56.10.621-138.83.8 1)Why do all of the alkenes above have the same molecular mass? 2)Why do the isomers have different densities? 3)Why is the melting point of cis- but-2-ene lower than the melting point of trans-but-2-ene? 4) How could you distinguish between the cis and trans isomers of but-2-ene?

9. Hydrogenation of alkenes This reaction is very important in the manufacture of margarine. It is use to convert vegetable oils into margarine.

10. At normal temperatures and pressures, alkenes do not react with hydrogen. The conditions required for hydrogen to react with an alkene is a nickel catalyst and a temperature of 150◦C

11. The Hydrogen molecule splits into two atoms on the surface of the catalyst and the two atoms then add to the same side of the double bond in the alkene. Diagram:

12. Vegetable oils  Margarine Carbon double bonds - some double Lower melting point so - bonds eliminated Liquid at room temp - Higher mpt so - solid at room temp Hardening using Hydrogenation Saturated butter is solid at room temp so why bother? Vegetable oils are unsaturated and are healthier