4.7.2.2 Reactions of alkenes

Question 1

What is the functional group of alkenes?

Answer 1

• C = C

Question 2

What determines the reactions of organic compounds?

Answer 2

• It’s the generality of reactions of functional groups

Question 3

Alkenes react with oxygen in combustion reactions in the same way as other hydrocarbons, but what is different?

Answer 3

• They tend to burn in air with smokey flames because of incomplete combustion

Question 4

What do alkenes react with?
- and what happens to the structure of them?

Answer 4

• Alkenes react with hydrogen, water and the halogens, by the addition of atoms across the carbon-carbon double bond so that the double bond becomes a single carbon-carbon bond

Question 5

Describe the reaction and condition for the addition of hydrogen to alkenes.

Answer 5

Condition:
- Catalyst
Reaction:
- The double bond between the carbon
atoms will break apart
- The hydrogen atoms will bond to the
carbons
Product:
- Alkene will no longer have a doubles
bond and is saturated (now and alkane)

Question 6

Describe the reaction, condition and product for the addition of water to alkenes.

Answer 6

Condition:
- Catalyst
- High temperatures
Reaction:
- Water is steam and will split into H and
OH
- The H and OH will bond to the carbon
Product:
- The product is and alcohol and is used a
lot in industry

Question 7

How do we separate ethanol from unreacted ethene and water?
- give detailed explanation

Answer 7

• The ethene is easy to separate because it has a relatively low boiling point and so if cool down the mixture the ethene will stay as a gas while the water and ethanol will condense into liquid form
• To separate the water in ethanol we need to use fractional distillation (this works by taking the mixture of ethanol and water and placing it in a round bottom heating flask) then as we heat it up with a bunsen burner the ethanol will burn first because it has a low boiling point than water does and so will evaporate further up the fractionating column and then condense into a separate beaker
• Whereas our water will remain in the heating flask because it didn’t evaporate

Question 8

Describe the reaction, condition and product for the addition of halogens to alkenes.

Answer 8

• It’s the same as the hydrogen reaction except no catalyst is needed

Question 9

Describe the test used to distinguish alkenes from alkanes?
- in detail.

Answer 9

• Add bromine water (has an orange colour) to an alkene (e.g. ethene) and give it a shake it will react (e.g. to form dibromoethane) and because this uses up all of the bromine the orange colour would disappear (turn colourless)
• Because alkenes have double bonds they are more reactive and can react with bromine water and change the colour
• In contrast alkenes are saturated so they don’t have any double bonds for the bromine to be added to which means if we add bromine water to a solution of alkenes the solution will remain orange because bromine would not be reacted