Alkanes: Nomenclature, Isomers, and Conformations

1. Chapter 2Alkanes

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

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

9. 9

10. Formulas Molecular Formula

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

18. Methyl group • Attached to C3 3-methyl

19. 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

20. Problems • Name the following molecules

21. 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

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

23. 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

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

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

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

27. 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

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

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

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

31. 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

32. Problems • Name the following compounds 32

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

35. 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

36. Newman Projections 36

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

38. Energy vs Dihedral Angle 38

39. Butane 39

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

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

42. Energy vs Dihedral Angle 42

44. 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

45. 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

46. 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

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

48. 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

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

50. 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