Alkenes

Alkenes are unsaturated hydrocarbons because they each contain one carbon-carbon double bond. The presence of this double bond as their functional group makes alkenes more reactive than alkanes.

Reactions of alkenes

• Alkenes burn easily in air or oxygen

Alkenes burn in air or oxygen to form carbon dioxide and water as steam. They burn with smoky yellow flames because they have a higher ratio of carbon to hydrogen atoms in their molecules than alkanes. Not all the carbon is converted to carbon dioxide and the unreacted carbon remains, giving the flames a yellow, smoky appearance. The reactions are exothermic. e.g. C,H, (g) + 30,(g) -

• 2CO,(g) + 2H,0(g) AH-ve

• Alkenes undergo addition reactions
  Alkenes undergo addition reactions with other small molecules in which the alkene and the other molecule react to form one molecule. One bond in the double bond is broken and the compound formed is saturated (it has no double bonds).

Figure 16.1 Summary of addition reactions of alkenes