B3 - Alkene Reactions

Question 1

Why are alkenes much more reactive than alkanes?

Answer 1

Because of the presence of the pi-bond.

Question 2

How does the pi-bond that’s present in alkenes make alkenes more reactive than alkanes?

Answer 2

• The pi-electron density is concentrated above and below the plane of the sigma-bond
• Being on the outside of the double bond, the pi-electrons are more exposed than the electrons in the sigma-bond
• A pi-bond readily breaks

Question 3

What are the different types of addition reactions in alkenes (with any essential conditions) ?

Answer 3

Reactions with:
- Hydrogen in the presence of a nickel catalyst
- Halogens
- Hydrogen halides
- Steam in the presence of an acid catalyst

Question 4

What conditions are needed for the hydrogenation of alkenes?

Answer 4

• Nickel catalyst
• 423 K

Question 5

How can you test for unsaturation? Explain why this happens.

Answer 5

Add bromine water to a sample of an alkene, colour change from orange to colourless. This is because bromine is added across the double bond.

Any compound containing a C==C bond will decolourise bromine water.

Question 6

What molecules do alkenes react with to form haloalkanes?

Answer 6

Gaseous hydrogen halides.

Question 7

What temperature do alkenes react with gaseous hydrogen halides at to form haloalkanes?

Answer 7

Room temperature

Question 8

What is formed from the hydration of alkenes?

Answer 8

Alcohols.

Question 9

What state does the H2O have to be in for the hydration of alkenes?

Answer 9

Gas, so in the form of steam.

H2O(g)

Question 10

What other condition is required for the hydration of alkenes?

Answer 10

The presence of a phosphoric acid catalyst (H3PO4).

Question 11

Where does the steam add on to during the hydration of alkene?

Answer 11

Across the double bond.