1. Chapter 2 Alkanes

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
Compounds with the same molecular formula, but different structural formula
Constitutional Isomers/Structural Isomers: compounds with the same molecular formula but different atom connectivity
C4H10 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. Alkanes with closed loops or rings Add the prefix cyclo
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 Angle38

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 Angle42

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

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

46. B.P. of unbranched alkanes increases by 20 – 30 °C per carbon
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. A narrow m.p. is an indicator of purity
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

51. M.P. increases with number of carbons
Melting Points
M.P. increases with number of carbons
2.6 Physical Properties of Alkanes

52. Other Physical Properties
Dipole moment: A measure of polarity
Solubility: Important for determining which solvents can be used (e.g., for reactions)
Density: Determines whether a compound will be the upper of lower layer if mixed with an immiscible liquid (e.g., alkane + water)
2.6 Physical Properties of Alkanes

53. Alkanes are the least reactive organic compounds
Combustion
Alkanes are the least reactive organic compounds
Alkanes react rapidly with O2 in combustion
A commercially important reaction with increasingly significant global impact
Analytically useful for determining molecular formula
2.7 Combustion

54. The Petroleum Feedstock
Most alkanes come from petroleum
Composed mostly of alkanes and aromatic hydrocarbons
Purified via fractional
distillation
2.8 Occurrence and Use of Alkanes

55. Industrial Fractionation of Petroleum

57. A functional group is a characteristically bonded group of atoms
Functional Groups
A functional group is a characteristically bonded group of atoms
Each functional group exhibits its own particular chemical reactivity
Alkanes may be viewed as the blank template upon which functional groups are placed
2.9 Functional Groups, Compound Classes, and the “R” Notation

59. Compound Classes
Compounds with the same functional group comprise a compound class
Some compounds may contain more than one functional group

60. The “R” and “Ph” Notation
A convenient way to represent a generic structure or portion of a molecule
R Notation: R represents all alkyl groups
A Benzene ring may be called a “phenyl group” and can be represented by Ph-

61. The “Ar” Notation
Use Ar- for more highly substituted rings