1. Chapter 3 Structure and Stereochemistry of Alkanes
Organic Chemistry, 5th Edition L. G. Wade, Jr.
Chapter 3 Structure and Stereochemistry of Alkanes

2. ALKANE FORMULAS Four carbon/or hydogen atoms bonded to
each carbon atom
All C-C single bonds
NOTE: always an even number of
hydrogen atoms in a hydrocarbon
Ratio: CnH2n+2
Alkane homologs : each member in a
alkane series different from the next
member by a CH2 group
Chapter 3

3. CH3CH2CH2CH2CH2CH3
CH3CH2CH2CH2CH3
C6H14
C5H12
CH2
Chapter 3

4. Lower Membered Alkanes trivial (common) names
Condensed
Line-angle
CH4
METHANE
ETHANE
PROPANE
Chapter 3

5. BUTANES
n-BUTANE
iso-BUTANE
Note: 1 H here
Chapter 3

6. Pentanes
=>
Chapter 3

7. Writing Isomers -Heptanes
1. Start with longest straight chain possible
2. Shorten chain by one carbon and add the "extra" at all possible positions, starting with left-handed side. 2-Methylhexane
Obviously you do not add the extra carbon to end carbon!!!
3-Methylhexane
2-Methylhexane
Chapter 3
3-Methylhexane
2-Methylhexane

8. 3. Shorten chain by one more carbon
3. Shorten chain by one more carbon. This gives a 5-carbon chain (pentane). Two carbons are to be added as two methyl groups or a single two-carbon group (ethyl group).
THIS is 3-methylhexane!!
Chapter 3

9. 2,2,3-trimethylbutane 3,3-dimethylpentane
Shorten chain by yet one more carbon. This gives a 4-carbon chain
(butane). Three carbons are to be added as three methyl group.
NOTE: Can’t derive another butane chain by adding an ethyl group.
2,2,3-trimethylbutane
3,3-dimethylpentane
Chapter 3

10. 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

11. 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

12. Longest Chain
The number of carbons in the longest chain determines the base name: ethane, hexane. (Listed in Table 3.2, page 81.)
If there are two possible chains with the same number of carbons, use the chain with the most substituents.
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Chapter 3

13. 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
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Chapter 3

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

15. Propyl Groups H H n-propyl isopropyl A primary carbon
A secondary carbon
=>
Chapter 3

16. Butyl Groups H H n-butyl sec-butyl A primary carbon A secondary carbon
=>
Chapter 3

17. Isobutyl Groups H H isobutyl tert-butyl A tertiary carbon =>
A primary carbon
Chapter 3

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

19. 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

20. 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

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

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

23. 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

24. 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

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

26. 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

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

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

29. Conformational Analysis
Torsional strain: resistance to rotation.
For ethane, only 3.0 kcal/mol
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Chapter 3

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

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

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

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

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

35. Conformational Analysis
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Chapter 3

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

37. 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

38. 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

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

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

41. Heats of Combustion Alkane + O2  CO2 + H2O
166.6
164.0
158.7
158.6
=>
158.3
Long-chain
157.4
157.4
Chapter 3

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

43. Cyclopropane (2) Torsional strain because of eclipsed hydrogens =>
Chapter 3

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

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

46. 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

47. Chair Conformer
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Chapter 3

48. Boat Conformer
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Chapter 3

49. Conformational Energy
Chapter 3
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50. Axial and Equatorial Positions
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Chapter 3

51. Monosubstituted Cyclohexanes
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Chapter 3

52. 1,3-Diaxial Interactions
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Chapter 3

53. Disubstituted Cyclohexanes
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Chapter 3

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

55. 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.
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Chapter 3

56. 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

57. 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

58. End of Chapter 3
Chapter 3