3.14 Reaction of Alkenes Flashcards
What type of hydrocarbons are alkenes?
Unsaturated hydrocarbons
Why are alkenes ‘unsaturated’?
They contain at least 1 carbon double bond, which is their functional group.
How can alkenes take place in addition reactions?
The carbon-carbon double bond will open up to leave a single bond and a new atom is added to each carbon.
Describe how ethene can become saturated in an addition reaction (hydrogen)
Hydrogen can react with the double-bonded carbons in ethene to open up the double bond and form the equivalent alkane (ethane). Now, there are no carbon double bonds and therefore it’s saturated.
What are the three types of addition reactions?
- Reactions with hydrogen
- Reactions with water
- Reactions with halogens
Describe how ethene can become saturated in an addition reaction (water)
When the alkene and steam react, the double bond on the alkene opens up and the water molecule splits up into a hydrogen atom and an O-H group, which can then bond to the carbon atoms. This will form ethanol (an alcohol). Now, there are no carbon double bonds and therefore it’s saturated.
What are the conditions for an addition reaction with water?
- Presence of a catalyst
- High temperatures (so that the water is in the form of vapour)
What do we have to do after the addition reaction of ethene when reacting with water?
- We then have to separate the ethanol from any unreacted ethene and unreacted water. The ethene is easy to separate as it has a relatively low boiling point, and so if we cool down the mixture, the ethene will stay as a gas while the water and ethanol will condense into a liquid form.
- To separate the water and ethanol though, we will have 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 boil first as it has a slightly lower boiling point than water does and so it will evaporate up through the fractionating column and then condense into a separate beaker, whereas our water will remain in the heating flask, as it didn’t evaporate.
Describe how ethene can become saturated in an addition reaction (halogens)
Alkenes can also react in addition reactions with halogens such as bromine, chlorine and iodine. The double bond opens up so that each of the C=C carbons bond to a halogen atom. For example, bromine and ethene react together to form dibromoethane. Now, there are no carbon double bonds and so it’s saturated.
Describe the colour change when bromine is added to ethene and why this happens
- If we add bromine (which has an orange colour) to ethene, and we give it a shake, it will form dibromoethane. As this uses up all of the bromine, the orange colour will disappear and the solution will be colourless.
- Due to alkenes having double carbon bonds and being more reactive, they can react with the bromine to decolourise it.
How can alkene molecules react with other alkenes?
By opening up their double bonds and forming single bonds between themselves.
What do lots of alkene molecules form when they join together?
An addition polymer.
What polymer can lots of ethene molecules make if they join together?
Poly(ethene)
What is poly(ethene) used for?
Making carrier bags
What type of reaction can an alkene be part of, because of the carbon to carbon double bond?
Addition reactions
