Alkenes

Alkenes are hydrocarbons with a carbon–carbon double bond, C=C, so they are unsaturated. Their general formula is CnH2n. The double bond makes them much more reactive than alkanes: other atoms can add across it. This gives a simple test, bromine water, and lets alkenes be made into alcohols and polymers. Alkenes are made by cracking.

Key facts

  • Alkenes have a C=C double bond and the general formula CnH2n: ethene C2H4, propene C3H6, butene C4H8.
  • The C=C double bond is the functional group of the alkenes.
  • Test: alkenes turn orange bromine water colourless; alkanes leave it orange.
  • Addition reactions: the atoms add across the double bond. With hydrogen (nickel catalyst) an alkane forms; with bromine, a dibromoalkane; with steam (phosphoric acid catalyst, about 300 °C, 60–70 atm), an alcohol.
  • Alkenes burn with smoky flames, because there is often not enough oxygen for complete combustion.
  • Alkenes can join together into long chains: addition polymers such as poly(ethene).

Worked examples

Example 1

Draw the displayed formula of propene and give its molecular formula.

  1. "Prop-" means 3 carbons; "-ene" means one C=C double bond.
  2. Each carbon makes four bonds. The double bond counts as two.
  3. Count the hydrogens: there are 6. CnH2n: 2 × 3 = 6, so C3H6

Answer: C3H6

Example 2

Ethene reacts with bromine. Name the product and say what you would see with bromine water.

  1. The C=C double bond opens up and one bromine atom adds to each carbon. C2H4 + Br2 → C2H4Br2
  2. The product is 1,2-dibromoethane: a saturated compound.
  3. With bromine water the orange colour disappears: the test for a C=C bond.

Answer: 1,2-dibromoethane; orange to colourless

Example 3

How is ethanol made from ethene? Give the conditions.

  1. Steam adds across the double bond: this is called hydration. C2H4 + H2O → C2H5OH
  2. Conditions: a phosphoric acid catalyst, about 300 °C and 60–70 atmospheres.
  3. It is fast and continuous and makes pure ethanol, but ethene comes from crude oil, which will run out.

Answer: ethene + steam with a phosphoric acid catalyst, about 300 °C and 60–70 atm