57. Addition Reactions of Alkenes

1. Why Addition Reactions Occur

Alkenes are unsaturated hydrocarbons, meaning they contain at least one carbon-carbon double bond (C=C). This double bond serves as the functional group. Because it can break and "open up," alkenes can add other molecules to their structure, a process called an addition reaction.

2. Reaction with Hydrogen (Hydrogenation)

• Conditions: Requires the presence of a catalyst.

• Process: The double bond breaks, and hydrogen atoms bond to the remaining spots on the carbon atoms.

• Product: The alkene is converted into a saturated alkane.

• Example: Propene + Hydrogen → Propane

3. Reaction with Water (Hydration)

• Conditions: Requires a catalyst and high temperatures (so that water is in the form of steam).

• Process: The water molecule (H2O) splits into a hydrogen atom (H) and an OH group, which both bond to the carbon atoms.

• Product: An alcohol.

• Example: Ethene + Water → Ethanol

• Separation: After the reaction, unreacted ethene is removed by cooling the mixture (it stays as a gas). The remaining water and ethanol are then separated using fractional distillation.

4. Reaction with Halogens

• Conditions: No catalyst is required; it simply involves mixing/shaking the reactants.

• Process: Halogen atoms (like Bromine, Chlorine, or Iodine) add across the double bond.

• Example: Ethene + Bromine → Dibromoethane

5. Testing for Alkenes

The reaction with bromine is the standard test to distinguish alkenes from alkanes:

• Alkenes: Being reactive and unsaturated, they react with orange bromine water and decolorize it (turning it from orange to colorless).

• Alkanes: Being saturated, they do not react with bromine water, so the solution remains orange.

Summary of Addition Reactions