Alkanes: Preparation, Reactions and Applications

• Alkanes, the simplest and most fundamental class of hydrocarbons, are characterized by their single covalent bonds between carbon atoms.
• This section delves into the methods of preparation, key chemical reactions, and the diverse applications of alkanes.

General Methods of Preparation

1. Catalytic Hydrogenation:

   • Alkenes and alkynes are converted to alkanes by the addition of hydrogen in the presence of a catalyst such as platinum (Pt), palladium (Pd), or nickel (Ni).
   • Example: Ethene (CH2=CH2) is hydrogenated to ethane (CH3-CH3) using Pt, Pd, or Ni as a catalyst.

2. Wurtz Reaction:

   • This involves the reaction of alkyl halides with sodium metal in dry ether to produce alkanes.
   • Example: Ethyl bromide (CH3CH2Br) reacts with sodium (Na) to form butane (CH3CH2-CH2CH3) and sodium bromide (NaBr).

3. Reduction of Carboxylic Acids:

   • Carboxylic acids are reduced to alkanes using strong reducing agents like lithium aluminum hydride (LiAlH4) or borane (BH3).
   • Example: Ethanoic acid (CH3COOH) is reduced to ethane (CH3-CH3) using LiAlH4.

4. Decarboxylation:

   • Carboxylic acids are decarboxylated to alkanes by heating or using a base, releasing carbon dioxide (CO2).
   • Example: Ethanoic acid (CH3COOH) undergoes decarboxylation to produce methane (CH4) and CO2.

Chemical Reactions of Alkanes

• Alkanes are saturated hydrocarbons, meaning they consist of carbon and hydrogen atoms bonded together by single covalent bonds.
• Due to their relatively inert nature, alkanes exhibit limited reactivity compared to other classes of organic compounds.
• However, they can undergo a few important reactions:

1. Combustion

• • Alkanes readily undergo combustion reactions in the presence of oxygen to produce carbon dioxide and water.
  • The general equation for the complete combustion of an alkane, such as methane (CH₄), is as follows:
    • Alkane + Oxygen → Carbon Dioxide + Water
  • For example, the combustion of methane is:
    • CH₄ + 2O₂ → CO₂ + 2H₂O

2. Halogenation

• • Alkanes can undergo halogenation, where halogens (such as chlorine or bromine) replace hydrogen atoms in the alkane molecule. This reaction requires ultraviolet (UV) light or heat to initiate.
  • The reaction proceeds through a free radical mechanism.
  • For example, the halogenation of methane with chlorine gas (Cl₂) proceeds as follows:
    • CH₄ + Cl₂ (UV light/heat) → CH₃Cl + HCl
  • This reaction can continue, leading to the formation of products such as:
    • Dichloromethane (CH₂Cl₂)
    • Trichloromethane (chloroform, CHCl₃)
    • Tetrachloromethane (carbon tetrachloride, CCl₄)

3. Cracking

• • Alkanes can undergo thermal decomposition, known as cracking, to produce smaller hydrocarbons.
  • This process is often used in the petroleum industry to obtain shorter-chain hydrocarbons with higher commercial value.
  • For example, the cracking of octane (C₈H₁₈) may produce ethene (C₂H₄) and propene (C₃H₆):
    • C₈H₁₈ → C₂H₄ + C₃H₆ + other products

Applications of Alkanes

• Alkanes have widespread applications across various industries due to their availability and chemical properties:
  1. Fuels: Key components of natural gas, gasoline, diesel, and kerosene, essential for heating, transportation, and power generation.
  2. Petrochemical Industry: Used as raw materials for producing plastics, solvents, and lubricants, vital in manufacturing and industrial processes.
  3. Pharmaceuticals: Serve as solvents or starting materials in the synthesis of drugs.
  4. Cosmetics: Used as solvents, emollients, and thickeners in lotions, creams, and makeup.
  5. Food Industry: Utilized as carriers for flavors and in food-grade waxes to protect and preserve produce.

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