alkenes-1.pdf chemistry organic chemistry

Alkenes

1. Alkenes
Chapter 11

2. Overview
Important topic
Concepts
High weightage
Organic takes up around 20% of past TYS
papers

Weightage

3. What is an Alkene?
An alkene is a hydrocarbon with general formula CnH2n.
Alkenes contain C=C double covalent bonds
Alkenes are unsaturated. Unsaturated Hydrocarbons have one or more C=C
double bonds

4. Physical Property Reasoning
Melting and Boiling
Point
When the number of Carbon atoms in an alkene increases, molecules are bigger and have stronger
intermolecular forces of attraction between alkene molecules. Hence, as the number of carbon
atoms in the alkene increases, the melting and boiling points of alkenes increase.
Volatility
When number of Carbon atoms in an alkene increase, the alkene becomes less volatile. With a higher
relative molecular mass, there would be stronger intermolecular forces of attraction between alkene
molecules, hence, alkene is more stable (less volatile).
Density As number of carbon atoms increases, density increases.
Viscosity As number of carbon atoms increases, viscosity increases (more difficult to flow)
Flammability As number of carbon atoms increases, flammability decreases (more difficult to burn)
Solubility Alkenes are insoluble in water, but soluble in organic solvents.
Properties of Alkanes

5. Combustion
Hydrogenation (Addition of Hydrogen)
Chemical Reactions of Alkenes
For all organic reactions, we must memorise the reagents and conditions.

6. Reagents: Excess Oxygen
Conditions: High temperature
(burning)
Complete Combustion
(Example equation)

7. Reagents: Limited Oxygen
Conditions: High temperature (burning)
Water and Carbon Monoxide produced
If extremely limited Oxygen is used, Carbon (soot) is produced instead of Carbon Monoxide
Incomplete Combustion
(Example equation)

8. Reagents: H2 gas
Conditions: 200C and Nickel
Catalyst
Hydrogenation

9. Reagents: Aqueous Bromine
Conditions: Room temperature and pressure
Bromination
This is used as a test to distinguish alkanes and alkenes.
For alkenes, the reddish-brown solution decolourises. No change for
alkanes.

10. Reagents: Steam, H2O (g)
Conditions: 300C, 60atm, Phosphoric(V) Acid
Hydration
This reaction is used to turn an alkene into an alcohol.

11. Polymers are formed when multiple identical
Alkene molecules (monomers) are joined
together to form a large molecule.
Polymerisation

12. Catalytic Cracking is a process where long-chain
hydrocarbons from petroleum are broken down into shorter-
chain hydrocarbons in the presence of a catalyst.
The process is used to produce fuels for Vehicles, production
of Alkenes etc.
Cracking

13. Cracking
Conditions: Aluminium Oxide Catalyst, Silicon (V) Oxide, at
temperature of 600C
At least one of the products formed will be an alkene.

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