Alkenes

What an addition reaction is

• An addition reaction combines a small molecule with the C=C of an alkene to give one larger product molecule and nothing else
• Each atom of the small molecule attaches to one of the two C=C carbons; the C=C double bond becomes a C–C single bond
• Several useful addition reactions are met in the IGCSE syllabus:
  • With hydrogen (over a nickel catalyst, 150 °C) to make the corresponding alkane (the basis of hardening vegetable oils into margarine)
  • With steam (catalysed by phosphoric acid, 300 °C, 60 atm) to make an alcohol (industrial route to ethanol from ethene — see topic 25)
  • With halogens to make a dihalogenoalkane

Example — bromination of propene

• Propene reacts with bromine by addition:

CH₃–CH=CH₂(g) + Br₂(l) → CH₃–CHBr–CH₂Br(l)

propene + bromine → 1,2-dibromopropane

• The bromine atoms add across the C=C: one to each of the two carbons that used to share the double bond
• Only one product is made — there is no by-product such as HBr (contrast this with the substitution reaction of alkanes with halogens in topic 23, where HBr or HCl is given off)

Example — hydrogenation of but-1-ene

• But-1-ene reacts with hydrogen over a nickel catalyst to give butane:

CH₂=CH–CH₂–CH₃(g) + H₂(g) → CH₃–CH₂–CH₂–CH₃(g)

but-1-ene + hydrogen → butane

• The C=C double bond is now a single C–C bond; the alkene has become a saturated alkane

The bromine water test for unsaturation

• Bromine water is an orange solution of bromine dissolved in water
• It is the standard laboratory test for distinguishing an alkene from an alkane:
  • Shake a sample of the unknown hydrocarbon with bromine water in a test tube
  • If the orange colour decolourises (turns colourless), a C=C double bond is present — the substance is an alkene
  • If the orange colour stays the same, no C=C is present — the substance is an alkane (saturated)
• Why it works:
  • With an alkene, bromine atoms add across the C=C to make a colourless dihalogenoalkane in solution — the free Br₂ is consumed by the reaction, and the orange colour disappears
  • With an alkane, there is no C=C for the bromine to react with, so the bromine stays dissolved and the solution remains orange

Why this test matters

• Bromine water (an aqueous bromine solution) is safer to handle and easier to see colour-change with than liquid bromine itself
• The test is the simplest way to identify whether a hydrocarbon contains a C=C double bond — i.e. whether it is unsaturated
• The same test will give a positive result for any alkene of any chain length, because the C=C is the reactive functional group rather than the chain length being the key feature