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4: Structure and Stereochemistry of Alkanes

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    Learning Objectives

    After reading this chapter and completing ALL the exercises, a student can be able to

    • distinguish between the different hydrocarbon functional groups - refer to section 4.1
    • explain & predict the physical properties of alkanes including relative bp and solubility in a mixture - refer to section 4.2
    • interpret and draw the rotation about a carbon-carbon single bond using Newman projections and sawhorse structures - refer to section 4.3 - 4.5
    • correlate energies of conformations with rotational energy diagrams and predict the most stable conformations for butane, higher alkanes, cyclohexane, mono-substituted cyclohexanes, and disubstituted cyclohexanes - refer to sections 4.3, 4.3, 4.4, 4.5, and 4.7, 4.8, and 4.10 respectively
    • explain the partial rotation of carbon-carbon single bonds in rings - refer to section 4.6
    • explain ring strain and its relationship to cycloalkane stability - refer to section 4.6
    • draw cyclohexane conformations (chair & boat) - refer to section 4.7
    • draw mono-substituted cyclohexane conformers (chair only) - refer to section 4.8
    • identify & draw the geometric (cis/trans) isomers of cycloalkanes - refer to section 4.9
    • draw di-substituted cyclohexane conformers (chair only) - refer to section 4.10
    • recognize and draw the three ways to join two rings - refer to section 4.11
    • describe the uses and sources of alkanes - refer to section 4.12
    • recognize and distinguish between the two major reactions of alkanes (combustion and halogenation) - refer to section 4.13

    • 4.1: Hydrocarbon Functional Groups
      Hydrocarbons are organic compounds that consist entirely of carbon and hydrogen atoms.  Hydrocarbons can form different functional groups based upon their carbon bonding patterns as alkanes, alkenes, alkynes, or arenes.
    • 4.2: Physical Properties of Alkanes
      Alkanes are not very reactive and have little biological activity; alkanes are colorless, odorless non-polar compounds.
    • 4.3: Structure and Conformations of Alkanes
      The carbon-carbon single bonds of alkanes rotate freely.  Conformers are the same molecule shown with different sigma bond rotations.  Newman projections are one way to communicate bond rotation.
    • 4.4: Conformations of Butane
      The conformations of butane are studied to introduce the language and energetic considerations of single bond rotation when alkyl group interactions can occur.
    • 4.5: Conformations of Higher Alkanes
      Pentane and higher alkanes have conformational preferences similar to ethane and butane. Each dihedral angle tries to adopt a staggered conformation and each internal C-C bond attempts to take on an anti conformation to minimize the potential energy of the molecule.
    • 4.6: Cycloalkanes and Ring Strain
      For cyclic alkanes, only partial rotation of carbon-carbon single bonds can occur.  The actual shape of the carbon ring distorts from the traditional geometric shapes to reduce steric hindrance and ring strain to lower the overall  potential energy of the molecule.
    • 4.7: Cyclohexane Conformations
      Cyclohexane rings are notably stable.  Understanding the conformations of cyclohexane and their relative energies is helpful when studying the chemistry of simple carbohydrates (monosaccharides).
    • 4.8: Conformations of Monosubstituted Cyclohexanes
      For monosubstituted cyclohexanes, the axial or equatorial orientation of the substituent influences the overall potential energy of the conformation.
    • 4.9: Cis-trans Isomerism in Cycloalkanes
      Stereoisomerism is possible for cycloalkanes with two different substituent groups (not counting other ring atoms).  Cis and trans isomers are unique compounds.
    • 4.10: Conformations of Disubstituted Cyclohexanes
      Because six-membered rings are so common among natural and synthetic compounds and  its conformational features are rather well understood, we shall focus on the six-membered cyclohexane ring to study the energetic relationship of conformation and overall potential energy.
    • 4.11: Joined Rings
      Two or more rings can be fused together into a bicyclic or spirocyclic system.  The steroid fused ring system is a notable example.
    • 4.12: Uses and Sources of Alkanes
      The primary sources for alkanes are oil and natural gas.  Alkanes are important raw materials for the chemical industry and are used as fuels for motors.
    • 4.13: Reactions of Alkanes - a Brief Overview
      Alkanes (the most basic of all organic compounds) undergo very few reactions. The two reactions of more importaces is combustion and halogenation, (i.e., substitution of a single hydrogen on the alkane for a single halogen) to form a haloalkane. The halogen reaction is very important in organic chemistry because it opens a  gateway to further chemical reactions.
    • 4.14: Additional Exercises
      This section has additional exercises for the key learning objectives of this chapter.
    • 4.15: Solutions to Additional Exercises
      This section has the solutions to the additional exercises from the previous section.

    4: Structure and Stereochemistry of Alkanes is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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