Overview
This lecture covers the reactions of alkenes (unsaturated hydrocarbons), focusing on addition reactions with hydrogen, water, and halogens, and explains how these reactions alter the properties and classification of alkenes.
Alkenes and Their Properties
- Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond as their functional group.
- The double bond in alkenes allows them to undergo addition reactions by opening up and bonding with other atoms.
Addition Reaction with Hydrogen (Hydrogenation)
- When hydrogen gas is added to an alkene (e.g., propene) with a catalyst, the double bond breaks and hydrogen atoms bond to the carbons.
- The product is an alkane (e.g., propane), which is saturated and has only single bonds.
Addition Reaction with Water (Hydration)
- Alkenes react with water vapor (steam) in the presence of a catalyst at high temperatures.
- The double bond opens; hydrogen and OH from water attach to the carbons, forming an alcohol (e.g., ethanol from ethene).
- Ethanol can be separated from the reaction mixture using fractional distillation because it boils before water.
Addition Reaction with Halogens
- Alkenes react with halogens (like bromine) without a catalyst; the double bond opens, and halogen atoms add to the carbons.
- Adding bromine to ethene forms dibromoethane and removes the orange color of bromine (decolorizes it).
- This reaction is used to distinguish alkenes (which decolorize bromine water) from alkanes (which do not).
Key Terms & Definitions
- Alkene — an unsaturated hydrocarbon with at least one carbon-carbon double bond.
- Addition Reaction — a reaction where atoms are added to a molecule at a double or triple bond.
- Hydrogenation — addition of hydrogen to an alkene, converting it into an alkane.
- Hydration — addition of water to an alkene, producing an alcohol.
- Halogenation — addition of a halogen to an alkene.
- Saturated Hydrocarbon (Alkane) — hydrocarbon with only single bonds.
- Functional Group — a group of atoms responsible for the characteristic reactions of a molecule.
Action Items / Next Steps
- Review the process of fractional distillation for separating ethanol and water.
- Understand and practice the tests for distinguishing alkenes from alkanes using bromine water.