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: $C<em>nH</em>{2n+2}$, where $n$ represents the number of carbon atoms.
  • Examples:
    • If $n=6$, then $C<em>6H</em>{(2 imes 6) + 2} = C<em>6H</em>{14}$ (hexane).
    • If $n=1$, then $C<em>1H</em>{(2 imes 1) + 2} = CH_4$ (methane).
    • If $n=8$, then $C<em>8H</em>{(2 imes 8) + 2} = C<em>8H</em>{18}$ (octane).
• Smallest Alkanes:
  • Methane ($CH_4$): The smallest organic molecule, found in the atmosphere, ground, oceans, and other planets (Mars, Jupiter).
  • Ethane ($C<em>2H</em>6$).
  • Propane ($C<em>3H</em>8$).
• 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