Ch. 13.3 : Reactions of Alkenes

Question 1

Why are alkanes more reactive than alkanes? Use bond enthalpy

Answer 1

They have a pi bond. The pi electrons are more exposed than the electrons in a sigma bond, and so it breaks more readily.

The pi bond has a lower bond enthalpy than sigma bonds, and it is therefore weaker and broken more readily.

Question 2

How do addition reactions happen?

Answer 2

The pi bond breaks and a new bond form allowing a small molecule to be added across the double bond.

Question 3

What are the addition reactions alkanes can undergo?

Answer 3

Hydration - addition of water / steam
Hydrogenation - addition of hydrogen
Halogenation - addition of halogen
Hydrohalogenation - addition of hydrogen halides

Question 4

What are the conditions required for hydrogenation?

Answer 4

Catalyst - nickel

Temperature - 423K

Question 5

What are the conditions required for halogenation and hydrohalogenation?

Answer 5

Room temperature

Question 6

How do you test for unsaturation?

Answer 6

Add bromine water to the sample. If it stays orange then then it means it is saturated and therefore an alkane. If it changes colour from orange to clear, then it is unsaturated and therefore an alkene. This is because when it becomes colourless it means there is a c=c bond present as the bromine was added across it as it was broken.

Question 7

Describe how hydrohalogenation takes place. Product?

Answer 7

If the alkene is a gas then the reaction takes place when the t o gases are mixed.

If the alkene is a liquid then the hydrogen halide is bubbled through it.

Product = halo alkanes

Question 8

Describe the conditions required for hydration. What is the product?

Answer 8

Phosphoric acid catalyst
Heat

Product = alcohol

Question 9

What does halogenation produce ?

Answer 9

Dihaloalkanes eg 1,2 - dibromopropane