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Alkanes: Nomenclature, Isomers, and Conformations

Learn about the nomenclature, isomers, and conformations of alkanes in this informative chapter. Understand how to name and identify different compounds, distinguish between constitutional isomers, and analyze the different conformations of alkanes. Explore the physical properties, boiling points, and classifications of carbon substitution in alkanes.

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Alkanes: Nomenclature, Isomers, and Conformations

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  1. Chapter 2Alkanes

  2. Hydrocarbons Compounds that contain only carbon and hydrogen Two classes: Aliphatic and aromatic 2

  3. Unbranched Alkanes Referred to as normal or n-alkanes Possess a linear carbon chain 5

  4. 9

  5. Formulas Molecular Formula

  6. Problems • How many hydrogens does n-Octadecane, an alkane containing 18 carbons, have? • Give the molecular, structural, condensed, and skeletal formulas for n-Octadecane • Estimate the boiling point and density of n-Octadecane

  7. Isomers C4H10 Compounds with the same molecular formula, but different structural formula Constitutional Isomers/Structural Isomers: compounds with the same molecular formula but different atom connectivity 12

  8. Organic Nomenclature Standardized by International Union of Pure and Applied Chemistry (IUPAC) The current system is called substitutive nomenclature Rules for alkane nomenclature extend to most other compound classes Apply the following rules: 13

  9. Substitutive Nomenclature of Alkanes 1. Unbranched alkanes are named according to number of carbons 2. If branched, find the longest continuous carbon chain and identify this as the principle/parent chain 14

  10. Substitutive Nomenclature of Alkanes 3. If two chains are equal in length, select the one with the most substituents 4. Number the principle chain, giving the lower number to the first branching point 15

  11. Substitutive Nomenclature of Alkanes 5. Identify the name of each branch and to which carbon on the parent chain it is attached • Branching groups are called substituents • Those derived from alkanes are alkyl groups 16

  12. Methyl group • Attached to C3 3-methyl

  13. Construct the name • Location of branch (3) • Name of branch (methyl) • Note: a hyphen goes between the location and branch • Name of parent chain (hexane) 19

  14. Problems • Name the following molecules

  15. 7. When more than one of the same substituent is present: • Indicate which carbon each substituent is on • Use Greek prefixes (di-, tri-, tetra-) to indicate how many of each substituent you have 21

  16. Substitutive Nomenclature of Alkanes 8. For multiple substituents, select the numbering scheme that gives the smaller number at first point of difference 22

  17. 9. Cite substituents in alphabetical order regardless of location Di-, tri-, tetra-, and hyphenated prefixes tert- and sec- are ignored Iso-, neo-, cyclo- are not ignored 23

  18. Problems • Name the following molecules: • Draw 2-bromo-3-chloro-4-fluoro-2,3,4-trimethylheptane

  19. 10. If the numbering of different groups is not resolved, the first-cited group gets the lowest number 25

  20. Highly Condensed Structures Highly condensed structures are commonly used 26

  21. Classification of Carbon Substitution Primary (1°) carbon: A carbon bonded to 1 other carbon Secondary (2°) carbon: A carbon bonded to 2 other carbons Tertiary (3°) carbon: A carbon bonded to 3 other carbons Quaternary (4°) carbon: A carbon bonded to 4 other carbons 27

  22. Classification of Carbon Substitution Similarly, hydrogens may also be classified as primary, secondary, tertiary, or quaternary 28

  23. Problems • Locate the primary, secondary, tertiary, and quaternary carbons in the following molecule

  24. Cycloalkanes Alkanes with closed loops or rings Add the prefix cyclo Note that cyclohexane has 2 less hydrogens than hexane 30

  25. Nomenclature of Cyloalkanes The same nomenclature rules are followed Do not forget the cyclo part of the name If the noncyclic carbon chain contains more carbons than the ring, it is named as the parent chain 31

  26. Problems • Name the following compounds 32

  27. Problems • Draw the following compounds • 1-chloro-4-ethylcyclohexane • 2-bromo-1,1-dimethylcyclobutane • 1,1,2,2-tetramethylcyclopropane

  28. Conformations of Alkanes • Conformational isomers. Rotation about a single bond leads to a series of conformers • A Newman projection is a visual tool to inspect conformers as viewed down a bond 35

  29. Newman Projections 36

  30. Staggered and Eclipsed Conformers Two energetic extremes are found for ethane Other dihedral angles are possible 37

  31. Energy vs Dihedral Angle 38

  32. Butane 39

  33. Conformations of Butane Additional conformers are possible for butane 40

  34. Problem • Draw the Newman projections for the different eclipsed conformers of butane • Are there any conformers that are energetically equivalent?

  35. Energy vs Dihedral Angle 42

  36. Space-Filling Models of Butane Conformers 2.3 Conformations of Alkanes van der Waals repulsion creates a torsional strain encouraging rotation towards a more stable conformer The most stable conformer dominates 44

  37. Conformational Analysis 2.3 Conformations of Alkanes Staggered conformers are preferred van der Waals repulsion influences conformer populations Rotation about single bonds is rapid except at very low temperatures 45

  38. Boiling Points • Boiling point: Temperature at which vapor pressure of substance = atmospheric pressure • B.P. of unbranched alkanes increases by 20 – 30 °C per carbon • Homologous series: differs by CH2 groups 2.6 Physical Properties of Alkanes

  39. Intermolecular Interactions for Alkanes • Electron clouds can be temporarily distorted 2.6 Physical Properties of Alkanes

  40. Intermolecular Interactions for Alkanes • Induced dipole • van der Waals attraction (or a dispersion interaction) • Greater intermolecular forces = higher b.p. 2.6 Physical Properties of Alkanes

  41. Molecular Shape and Boiling Point • Greater branching = lower b.p. • Molecules that are spherical have less surface area 2.6 Physical Properties of Alkanes

  42. Melting Points • Melting point: Temperature at which a substance transforms from solid to liquid • A narrow m.p. is an indicator of purity • Branching interferes with crystal packing leading to lower m.p. values • Symmetric molecules tend to have unusually high m.p’s 2.6 Physical Properties of Alkanes

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