Alkanes, Alkenes & Alkynes

Building on our introduction to organic chemistry, this lesson dives deeper into the three main families of hydrocarbons. Understanding their properties, reactions, and preparation methods is essential for Grade 11 chemistry.

Alkanes (Saturated Hydrocarbons)

Alkanes contain only single bonds (C-C and C-H). General formula: CnH₂n+₂.

Properties:

• Non-polar, insoluble in water
• Boiling point increases with chain length
• Relatively unreactive (hence called "paraffins" meaning "little affinity")

Key Reactions:

• Combustion: CH₄ + 2O₂ --> CO₂ + 2H₂O (complete combustion releases energy)
• Substitution (halogenation): CH₄ + Cl₂ --> CH₃Cl + HCl (in UV light)

Alkenes (Unsaturated -- One Double Bond)

Alkenes contain one C=C double bond. General formula: CnH₂n.

Properties:

• More reactive than alkanes due to the double bond
• Can undergo addition reactions (the double bond opens up)

Key Reactions:

• Addition of hydrogen (hydrogenation): C₂H₄ + H₂ --> C₂H₆ (with Ni catalyst)
• Addition of halogen: C₂H₄ + Br₂ --> CH₂Br-CH₂Br (decolourises brown bromine water -- a test for unsaturation)
• Addition of HBr: C₂H₄ + HBr --> CH₃-CH₂Br (Markovnikov's rule applies)
• Polymerization: n(C₂H₄) --> (-CH₂-CH₂-)n (polyethylene)

Preparation: Dehydration of alcohols: C₂H₅OH --> C₂H₄ + H₂O (with conc. H₂SO₄ at 170 C)

Alkynes (Unsaturated -- One Triple Bond)

Alkynes contain one C≡C triple bond. General formula: CnH₂n-₂.

Properties:

• Most reactive among the three types
• Can undergo addition reactions (two rounds possible)

Key Reactions:

• Addition of hydrogen: C₂H₂ + 2H₂ --> C₂H₆ (with Ni catalyst)
• Addition of halogen: C₂H₂ + 2Br₂ --> CHBr₂-CHBr₂
• Combustion: 2C₂H₂ + 5O₂ --> 4CO₂ + 2H₂O (very hot flame, used in oxy-acetylene welding)

Preparation: CaC₂ + 2H₂O --> C₂H₂ + Ca(OH)₂ (calcium carbide method)

Isomerism

Isomers are compounds with the same molecular formula but different structural arrangements.

Example: C₄H₁₀ has two isomers:

• n-butane: CH₃-CH₂-CH₂-CH₃ (straight chain)
• isobutane (2-methylpropane): CH₃-CH(CH₃)-CH₃ (branched)

Nepal Connection

LPG (liquefied petroleum gas) used for cooking in Nepal contains propane (C₃H₈) and butane (C₄H₁₀) -- both alkanes. Acetylene (C₂H₂) is used in welding workshops across Nepal for metal fabrication.

Key Takeaways

• Alkanes undergo substitution; alkenes and alkynes undergo addition reactions
• Bromine water test distinguishes unsaturated from saturated hydrocarbons
• Reactivity order: alkynes > alkenes > alkanes
• Isomers have the same formula but different structures