Alkanes, alkenes, alkynes: properties, reactions

1. Introduction to Hydrocarbons

1. Hydrocarbons are organic compounds composed entirely of carbon and hydrogen atoms.
2. They are classified into three main types:
   • Alkanes: Saturated hydrocarbons with single bonds only.
   • Alkenes: Unsaturated hydrocarbons with one or more double bonds.
   • Alkynes: Unsaturated hydrocarbons with one or more triple bonds.

2. Alkanes

1. Alkanes have the general formula C_nH_2n+2.
2. Common examples include methane (CH₄), ethane (C₂H₆), and propane (C₃H₈).
3. Properties:
   • Nonpolar molecules with weak van der Waals forces.
   • Insoluble in water but soluble in organic solvents.
   • Lower alkanes are gases, while higher alkanes are liquids or solids.
4. Reactions:
   • Combustion: Alkanes burn in oxygen to form CO₂ and H₂O.
   • Substitution: Undergo halogenation in the presence of UV light (e.g., CH₄ + Cl₂ → CH₃Cl + HCl).
   • Cracking: Breaking down larger alkanes into smaller alkanes and alkenes.

3. Alkenes

1. Alkenes have the general formula C_nH_2n.
2. Common examples include ethene (C₂H₄) and propene (C₃H₆).
3. Properties:
   • Nonpolar and insoluble in water.
   • Lower alkenes are gases; higher alkenes are liquids or solids.
4. Reactions:
   • Addition: React with H₂, halogens, and acids (e.g., HCl, HBr) across the double bond.
   • Polymerization: Form long-chain polymers (e.g., ethene → polyethylene).
   • Oxidation: Oxidized by KMnO₄ to form diols or acids.

4. Alkynes

1. Alkynes have the general formula C_nH_2n-2.
2. Common examples include ethyne (C₂H₂) and propyne (C₃H₄).
3. Properties:
   • Nonpolar molecules, insoluble in water.
   • High melting and boiling points compared to alkenes and alkanes.
4. Reactions:
   • Addition: Undergo addition reactions with H₂, halogens, and acids (e.g., HCl).
   • Hydration: Form aldehydes or ketones when treated with H₂O in the presence of catalysts.
   • Combustion: Burn in oxygen to produce CO₂ and H₂O.

5. Key Differences Between Alkanes, Alkenes, and Alkynes

1. Bonding: Alkanes have single bonds, alkenes have double bonds, and alkynes have triple bonds.
2. Reactivity: Alkanes are less reactive compared to alkenes and alkynes.
3. Uses:
   • Alkanes: Fuels, solvents, and lubricants.
   • Alkenes: Production of polymers and chemicals like alcohols.
   • Alkynes: Welding gases (e.g., acetylene), synthesis of organic compounds.