Ch03_Alkanes___their_stereochemistry

Chapter 3: Alkanes and Their Properties

3.1 Introduction to Alkanes

• The bristlecone pine, the oldest living organism on Earth, highlights the significance of complex organic compounds like alkanes.

• Alkanes are simple, saturated hydrocarbons that consist of only carbon and hydrogen.

3.1.1 Why Study Alkanes?

• Alkanes are unreactive yet essential for introducing critical concepts in organic chemistry.

• There are over 195 million known organic compounds, classified into various families based on structure and behavior.

• Alkanes represent the simplest family of organic compounds, serving as a foundation for understanding more complex groups.

3.2 Alkanes and Their Isomers

• Definition: Alkanes are saturated hydrocarbons with the general formula CnH2n+2.

  • Saturated means they contain only single C-H and C-C bonds.

  • Example: Methane (CH4), Ethane (C2H6), Propane (C3H8).

• As the number of carbon atoms increases, more structural isomers can form:

  • C4H10: Butane (straight-chain) vs. Isobutane (branched).

  • The number of potential alkane isomers increases significantly with carbon count:

    • C6H14: 5 isomers

    • C10H22: 75 isomers

    • C15H32: 4347 isomers

3.2.1 Isomer Types

• Isomers: Compounds with the same formula but different structures.

  • Constitutional isomers differ in connectivity (e.g., Butane vs. Isobutane).

  • More complex forms of isomerism can exist beyond constitutional isomerism.

3.3 Alkyl Groups

• Derived by removing a hydrogen atom from an alkane, creating alkyl groups:

  • Example: Methane (CH4) to Methyl (–CH3) and Ethane (C2H6) to Ethyl (–CH2CH3).

• Alkyl groups are key in forming larger organic compounds and are denoted by replacing the -ane suffix with -yl.

• Straight-chain and branched alkyl groups contribute to the diversity of organic compounds.

3.4 Naming Alkanes

• The IUPAC nomenclature defines how compounds are systematically named:

  1. Identify the longest carbon chain (parent chain).

  2. Number the chain to give substituted groups the lowest possible numbers.

  3. Identify and number substituents.

  4. Write the name as a single word using prefixes, hyphens, and commas appropriately.

• Example of naming: The longest chain identifies the primary alkane; substituents are named and numbered.

3.5 Properties of Alkanes

• Alkanes are chemically inert and typically only react under specific conditions (combustion).

• Physical Properties: Alkanes exhibit regular patterns of boiling and melting points influenced by molecular size and branching:

  • Larger alkanes generally have higher boiling/melting points due to increased dispersion forces.

  • Increased branching leads to lower boiling points due to reduced surface area.

3.6 Conformations of Ethane

• Conformations refer to the different spatial arrangements of atoms that result from rotation around C-C bonds.

• Key concepts include:

  • Staggered conformation: more stable, minimizes torsional strain.

  • Eclipsed conformation: less stable, subject to torsional strain.

• Energy barriers influence the accessibility of conformations in molecules.

3.7 Conformations of Other Alkanes

• Similar patterns apply to larger alkanes, where the conformational situation becomes more complex.

• Propane exhibits a torsional barrier higher than ethane due to additional interactions.

• For butane, the energy landscape includes anti and gauche conformations, reflecting varying stability.

3.8 Application: Gasoline Chemistry

• Alkanes are crucial in fuels like gasoline, derived from petroleum through fractional distillation and catalytic processes.

• Understanding the chemistry of alkanes aids in refining processes and improving fuel efficiency.

Key Terms

• Alkane: Saturated hydrocarbon with single bonds.

• Isomer: Compounds with the same formula but different structures.

• Alkyl Group: Fragment of an alkane remaining after an H atom is removed.

• Conformation: Different spatial arrangements of a molecule resulting from bond rotations.