1. Chapter 3 Structure and Stereochemistry of Alkanes
Organic Chemistry, 6th Edition L. G. Wade, Jr.
Chapter 3 Structure and Stereochemistry of Alkanes
ã 2006, Prentice Hall

2. Classification Review
Chapter 3

3. Alkane Formulas All C-C single bonds Saturated with hydrogens
Ratio: CnH2n+2
Alkane homologs: CH3(CH2)nCH3
Same ratio for branched alkanes
Chapter 3

4. Common Names Isobutane, “isomer of butane”
Isopentane, isohexane, etc., methyl branch on next-to-last carbon in chain.
Neopentane, most highly branched
Five possible isomers of hexane, 18 isomers of octane and 75 for decane!
Chapter 3

5. Alkane Examples
Chapter 3

6. IUPAC Names Find the longest continuous carbon chain.
Number the carbons, starting closest to the first branch.
Name the groups attached to the chain, using the carbon number as the locator.
Alphabetize substituents.
Use di-, tri-, etc., for multiples of same substituent.
Chapter 3

7. Longest Chain
The number of carbons in the longest chain determines the base name: ethane, hexane. (Listed in Table 3.2, page 82.)
If there are two possible chains with the same number of carbons, use the chain with the most substituents.
Chapter 3

8. Number the Carbons
Start at the end closest to the first attached group.
If two substituents are equidistant, look for the next closest group. 1 2 3 4 5 6 7
Chapter 3

9. Name Alkyl Groups CH3-, methyl CH3CH2-, ethyl CH3CH2CH2-, n-propyl
CH3CH2CH2CH2-, n-butyl
Chapter 3

10. Propyl Groups H H n-propyl isopropyl A primary carbon
A secondary carbon
Chapter 3

11. Butyl Groups H H n-butyl sec-butyl A primary carbon A secondary carbon
Chapter 3

12. Isobutyl Groups H H isobutyl tert-butyl A tertiary carbon
A primary carbon
Chapter 3

13. Alphabetize Alphabetize substituents by name.
Ignore di-, tri-, etc. for alphabetizing.
3-ethyl-2,6-dimethylheptane
Chapter 3

14. Complex Substituents
If the branch has a branch, number the carbons from the point of attachment.
Name the branch off the branch using a locator number.
Parentheses are used around the complex branch name. 1 2 3
1-methyl-3-(1,2-dimethylpropyl)cyclohexane
Chapter 3

15. Physical Properties Solubility: hydrophobic Density: less than 1 g/mL
Boiling points increase with increasing carbons (little less for branched chains).
Melting points increase with increasing carbons (less for odd- number of carbons).
Chapter 3

16. Boiling Points of Alkanes
Branched alkanes have less surface area contact,
so weaker intermolecular forces.
Chapter 3

17. Melting Points of Alkanes
Branched alkanes pack more efficiently into
a crystalline structure, so have higher m.p.
Chapter 3

18. Branched Alkanes Lower b.p. with increased branching
Higher m.p. with increased branching
Examples: H C 3 2
bp 60°C
mp -154°C
bp 58°C
mp -135°C
bp 50°C
mp -98°C
Chapter 3

19. Major Uses of Alkanes C1-C2: gases (natural gas)
C3-C4: liquified petroleum (LPG)
C5-C8: gasoline
C9-C16: diesel, kerosene, jet fuel
C17-up: lubricating oils, heating oil
Origin: petroleum refining
Chapter 3

20. Reactions of Alkanes Combustion
Cracking and hydrocracking (industrial)
Halogenation
Chapter 3

21. Conformers of Alkanes
Structures resulting from the free rotation of a C-C single bond
May differ in energy. The lowest-energy conformer is most prevalent.
Molecules constantly rotate through all the possible conformations.
Chapter 3

22. Ethane Conformers Staggered conformer has lowest energy.
Dihedral angle = 60 degrees
model H
Newman
projection
sawhorse
Chapter 3

23. Ethane Conformers (2) Eclipsed conformer has highest energy
Dihedral angle = 0 degrees
Chapter 3

24. Conformational Analysis
Torsional strain: resistance to rotation.
For ethane, only 12.6 kJ/mol
Chapter 3

25. Propane Conformers Note slight increase in torsional strain
due to the more bulky methyl group.
Chapter 3

26. Butane Conformers C2-C3 Highest energy has methyl groups eclipsed.
Steric hindrance
Dihedral angle = 0 degrees
totally eclipsed
Chapter 3

27. Butane Conformers (2) Lowest energy has methyl groups anti.
Dihedral angle = 180 degrees
anti
Chapter 3

28. Butane Conformers (3) Methyl groups eclipsed with hydrogens
Higher energy than staggered conformer
Dihedral angle = 120 degrees
eclipsed
Chapter 3

29. Butane Conformers (4) Gauche, staggered conformer
Methyls closer than in anti conformer
Dihedral angle = 60 degrees
gauche
Chapter 3

30. Conformational Analysis
Chapter 3

31. Higher Alkanes Anti conformation is lowest in energy.
“Straight chain” actually is zigzag.
Chapter 3

32. Cycloalkanes Rings of carbon atoms (-CH2- groups) Formula: CnH2n
Nonpolar, insoluble in water
Compact shape
Melting and boiling points similar to branched alkanes with same number of carbons
Chapter 3

33. Naming Cycloalkanes Cycloalkane usually base compound
Number carbons in ring if >1 substituent.
First in alphabet gets lowest number.
May be cycloalkyl attachment to chain.
Chapter 3

34. Cis-Trans Isomerism Cis: like groups on same side of ring
Trans: like groups on opposite sides of ring
Chapter 3

35. Cycloalkane Stability
5- and 6-membered rings most stable
Bond angle closest to 109.5
Angle (Baeyer) strain
Measured by heats of combustion per -CH2 -
Chapter 3

36. Heats of Combustion/CH2 Alkane + O2  CO2 + H2O
697.1
686.1
664.0
663.6 kJ/mol
662.4
Long-chain
658.6 kJ
658.6
Chapter 3

37. Cyclopropane Large ring strain due to angle compression
Very reactive, weak bonds
Chapter 3

38. Cyclopropane (2) Torsional strain because of eclipsed hydrogens
Chapter 3

39. Cyclobutane Angle strain due to compression
Torsional strain partially relieved by ring-puckering
Chapter 3

40. Cyclopentane
If planar, angles would be 108, but all hydrogens would be eclipsed.
Puckered conformer reduces torsional strain.
Chapter 3

41. Cyclohexane Combustion data shows it’s unstrained.
Angles would be 120, if planar.
The chair conformer has 109.5 bond angles and all hydrogens are staggered.
No angle strain and no torsional strain.
Chapter 3

42. Chair Conformer
Chapter 3

43. Boat Conformer
Chapter 3

44. Conformational Energy
Chapter 3

45. Axial and Equatorial Positions
Chapter 3

46. Monosubstituted Cyclohexanes
Chapter 3

47. 1,3-Diaxial Interactions
Chapter 3

48. Disubstituted Cyclohexanes
Chapter 3

49. Cis-Trans Isomers
Bonds that are cis, alternate axial-equatorial around the ring.
One axial, one equatorial
Chapter 3

50. Bulky Groups
Groups like t-butyl cause a large energy difference between the axial and equatorial conformer.
Most stable conformer puts t-butyl equatorial regardless of other substituents.
Chapter 3

51. Bicyclic Alkanes Fused rings share two adjacent carbons.
Bridged rings share two nonadjacent C’s.
bicyclo[2.2.1]heptane
bicyclo[3.1.0]hexane
Chapter 3

52. Cis- and Trans-Decalin
Fused cyclohexane chair conformers
Bridgehead H’s cis, structure more flexible
Bridgehead H’s trans, no ring flip possible.
trans-decalin
cis-decalin
Chapter 3

53. Bicyclo[4.4.0]decane
Chapter 3

54. End of Chapter 3
Chapter 3