Lecture on Alkanes

Introduction to Alkanes

• Also called paraffins or harafins
• Saturated hydrocarbons: Each carbon in alkanes is SP3 hybridized.
• General molecular formula: $C_nH_{2n+2}$
• Simplest member: Methane ($CH_4$)

Examples and Structural Formulas

• Methane: $CH_4$
• Ethane: $C_2H_6$
• Propane, Butane, etc.
• Structural representations: Using strokes for showing bonds.

Naming Compounds

• Example 1: 9-carbon chain
  • Longest continuous chain: 9 carbons ($C_9$)
  • Named “nonane” (Single-bonded/n-alkane)
  • Substituents: Methyl and ethyl groups considered
  • Example name: 4-ethyl-5-methyl-nonane
• Example 2: Compound with bromine ($Br$)

Preparation of Alkanes

1. Hydrogenation of Alkenes

• Addition of hydrogen ($H_2$) to alkenes
• Catalysts: Platinum (Pt), Palladium (Pd), Nickel (Ni)
• Example: Propene ($C_3H_6$) + Hydrogen -> Propane ($C_3H_8$)

2. Reduction of Alkyl Halides

• Halogen substituted with hydrogen
• Methods:
  1. Hydrolysis of Grignard Reagents ($RMgX$): Important for exams
  2. Reduction by Metal and Acid (e.g., Zinc and Hydrochloric Acid)

Physical Properties of Alkanes

• State of matter:
  • First four (C1 to C4) are gases.
  • Next thirteen (C5 to C17) are liquids.
  • 18 carbons and above are solids.
• Solubility:
  • Soluble in non-polar solvents like benzene.
  • Insoluble in water.

Reactions of Alkanes

1. Halogenation

• Example: Methane ($CH_4$) + Chlorine ($Cl_2$) -> Chloromethane ($CH_3Cl$) + Hydrogen Chloride ($HCl$)
• Further chlorination results in di-, tri-, and tetra-chloromethane.
• Most reactive: Fluorine, Chlorine, Bromine.
• Iodine does not react.

2. Combustion

• Burning in the presence of oxygen.
• General formula: $C_nH_{2n+2} + (3n+1)/2 O_2 -> n CO_2 + (n+1) H_2O$
• Example: Methane combustion yields Carbon Dioxide ($CO_2$) and Water ($H_2O$).

3. Pyrolysis (Cracking)

• Decomposition by heat (600°C)
• Large alkanes break into smaller alkanes and alkenes.
• Example: Pentane ($C_5H_{12}$) -> Smaller alkane ($C_2H_6$) + Alkene ($C_3H_6$)

Conclusion

• Summary of alkanes and their reactions.
• Future lectures will cover Alkenes and Alkynes.