Overview
This lecture covers the structure, general formulas, reactions, uses, and distinguishing tests for alkanes and alkenes, including their isomerism.
Alkanes: Structure and Properties
- Alkanes have the general formula CₙH₂ₙ₊₂.
- They are saturated hydrocarbons with only single carbon-carbon bonds.
- Examples: Methane (CH₄), Ethane (C₂H₆), Propane (C₃H₈), Butane (C₄H₁₀).
- Displayed formulas show all atoms and bonds explicitly.
Alkanes: Reactions and Uses
- Undergo combustion in oxygen to produce CO₂ and H₂O with a clean blue flame.
- React with chlorine in UV light (substitution reaction), replacing hydrogen atoms with chlorine.
- Used as fuels (e.g., propane, butane for cooking) and in manufacturing other compounds (e.g., methane for ammonia).
Alkenes: Structure and Properties
- Alkenes have the general formula CₙH₂ₙ.
- They are unsaturated hydrocarbons with at least one carbon-carbon double bond.
- The first member is ethene (C₂H₄), followed by propene (C₃H₆), but-1-ene (C₄H₈).
- Displayed formulas highlight the double bond.
Alkenes: Reactions and Identification
- Undergo combustion with a smokey yellow flame due to higher carbon content.
- React with hydrogen (hydrogenation) in presence of a nickel catalyst at 200°C to form alkanes.
- React with halogens (e.g., bromine, chlorine) in addition reactions, rapidly decolorizing bromine solution (test for alkenes).
- Undergo hydration with steam and phosphoric acid catalyst (300°C, 60 atm) to form alcohols.
- React with acidified potassium manganate(VII), decolorizing the purple solution (test for alkenes).
Comparing Alkanes and Alkenes
- Alkanes: saturated, substitution reactions, no reaction with bromine water or acidified potassium manganate(VII).
- Alkenes: unsaturated, addition reactions, rapid decolorization with bromine water and potassium manganate(VII).
Isomerism
- Isomers have the same molecular formula but different structural formulas.
- Examples: Butane and 2-methylpropane (C₄H₁₀); but-1-ene, but-2-ene, and 2-methylpropene (C₄H₈).
Key Terms & Definitions
- Alkane — saturated hydrocarbon; only single carbon-carbon bonds; general formula CₙH₂ₙ₊₂.
- Alkene — unsaturated hydrocarbon; contains at least one double carbon-carbon bond; general formula CₙH₂ₙ.
- Saturated — containing only single bonds between carbon atoms.
- Unsaturated — containing double or triple bonds between carbon atoms.
- Substitution reaction — an atom or group in a molecule is replaced by another atom or group.
- Addition reaction — atoms are added to a molecule at a double or triple bond.
- Isomerism — molecules with the same molecular formula but different structural arrangements.
Action Items / Next Steps
- Memorize the general formulas and displayed structures of alkanes and alkenes.
- Practice drawing isomers for C₄H₁₀ and C₄H₈.
- Review reaction conditions for hydrogenation and hydration of alkenes.