Chapter 2: Chemistry of Organic Compounds - Alkanes

Organic Chemistry: Fundamentals of Alkanes

Introduction to Organic Chemistry

• Definition of Organic Compound: A compound that contains elements of carbon.
• Organic Chemistry: The study of compounds with carbon.
• Historical Context: The terms "organic compound" and "inorganic compound" originated from an older classification system based on the source of compounds:
  • Inorganic Compounds: Believed to be obtained from minerals.
  • Organic Compounds: Believed to be obtained exclusively from living organisms (vegetable or animal sources).
• Wohler's Discovery (1828): Wohler synthesized urea, an organic compound, in a lab from inorganic compounds. This breakthrough led scientists to synthesize organic compounds in the lab, breaking the original classification rule.
• Modern Definition: Today, organic chemistry is the chemistry of carbon-based compounds.
• Key Elements: Most, and virtually all, organic compounds contain carbon and hydrogen. They may also contain oxygen, nitrogen, and phosphorus.
• Prevalence: There are over 10,000,000 known compounds containing carbon and hydrogen (and potentially oxygen or nitrogen). A basic understanding of organic compounds is crucial because they are ubiquitous (e.g., sugars, proteins, DNA, RNA, vitamins, material fibers, pesticides).

Hydrocarbons

• Definition: Hydrocarbons are organic compounds that contain only hydrogen and carbon atoms.
• Classification: Hydrocarbons are further classified into subcategories, one of the most important being alkanes.

Alkanes

• Definition: Alkanes are a class of hydrocarbons characterized by single carbon-carbon bonds.
• Saturated Hydrocarbons: Alkanes are also known as saturated hydrocarbons because each carbon atom in the compound has the maximum possible number of hydrogen atoms attached.
• Aliphatic Hydrocarbons: Another name for alkanes, derived from the Greek word "aliphar," meaning fat or oil, due to their oily/greasy nature.
• Molecular Formula for Linear Alkanes: CnH{2n+2}, where n represents the number of carbon atoms.
  • Examples:
    • If n=6, then C6H{(2 imes 6) + 2} = C6H{14} (hexane).
    • If n=1, then C1H{(2 imes 1) + 2} = CH_4 (methane).
    • If n=8, then C8H{(2 imes 8) + 2} = C8H{18} (octane).
• Smallest Alkanes:
  • Methane (CH_4): The smallest organic molecule, found in the atmosphere, ground, oceans, and other planets (Mars, Jupiter).
  • Ethane (C2H6).
  • Propane (C3H8).
• Natural Gas Composition: Natural gas typically contains about 75\% methane, 10\% ethane, and 5\% propane.
• Physical Properties - Boiling Point Trend: As the number of carbon atoms (n) in an alkane increases, its boiling point generally increases.
  • n=1-4 (methane, ethane, propane, butane): Gaseous at room temperature (e.g., methane bp =-161.5^ ext{o}C).
  • n=5-50: Liquid at room temperature.
  • n>50: Solid at room temperature (e.g., A C_{20} alkane has a boiling point of 343^ ext{o}C).
• Structure and Hybridization of Alkanes:
  • All carbon-carbon and carbon-hydrogen bonds in alkanes are single (sigma) bonds.
  • Hybridization: All carbon atoms in alkanes are sp^3 hybridized.
  • Bond Angle: The bond angle around each carbon atom is approximately 109^ ext{o} (specifically 109.5^ ext{o}).
  • Molecular Shape: Each carbon center has a tetrahedral shape (or pyramidal when considering bonds to hydrogen).
  • Conformation: Larger alkanes (e.g., pentane) exhibit a zigzag conformation due to the 109.5^ ext{o} bond angles, where carbon atoms alternate between being slightly