22.1-Alkenes

Alkenes have a C=C double bond

• Means that alkenes have two fewer hydrocarbons

• They are unsaturated as they contain the same number of hydrogen and carbon atoms, unlike alkanes which are saturated as they contain different amounts of hydrogen and carbon

• Bond can open, allowing the two carbon atoms to bond with other atoms, which makes them far more reactive than alkanes

• First four are ethene, propene, butene, pentene

Alkenes burn with a smokey flame

• Alkenes normally combust completely if there’s enough oxygen

• However, there isn’t enough oxygen in the air of this, so they undergo incomplete combustion

  • Alkenes + oxygen - carbon + carbon monoxide + carbon dioxide + water(+energy)

  • Result in a smokey yellow flame and less energy being released

22.2-Reactions of Alkenes

React via additional reaction

• The carbon-carbon double bond will open up and add additional atoms on

• New atoms are added to each carbon

Reactions with hydrogen:

• Open up the double bond and form the equivalent saturated alkane

• Needs a catalyst

Reaction with steam/water:

• Water is added across the double bond and an alcohol is formed

  • For example ethene and water makes ethanol

  • After reactions, mixture is passed from the reactor into a condenser

  • Ethanol and water have a higher boiling point then ethene so both condense whilst any unreacted ethene gas is recycled back into the reactor

  • Alcohol can then be purified through fractional distillation

Reactions with halogens

• Halogens include bromine, chlorine and iodine

• Molecules formed are saturated, with the C=C carbons each becoming bonded to a halogen atom

• Form di(halogen-ine)(alkane)

Test for alkene:

• The addition of bromine to a double bond can be used to test for alkenes

  • When orange bromine water is added to a saturated compound, like an alkene, no reaction will happen and it remains bright orange

  • If it’s added to an alkene the bromine will add across the double bond making a colourless dibromo-compound, so the bromine water is decolourised

Practice Questions:

• Test for alkene and how it works?

  • Add bromine water to the solution, if alkane present it turns from orange to colourless

  • Works as the bromine will add across the double bond making a colourless dibromoethene compound

• Reaction with halogens

  • Molecules formed are saturated with the C=C carbon each becoming bonded to a halogen atom

• Reaction with hydrogen

  • Open up the double bond and form the equivalent saturated alkane, and needs a catalyst to work

• Reaction with water

  • Water is added across the double bond, alcohol is formed this way