1. Che 163 Introductory Organic Chemistry ALKANES
Summer Quarter 2010

2. What is Organic chemistry?

3. What is Organic chemistry?
The study of carbon and its compounds.

4. What is Organic chemistry?
The study of carbon and its compounds.
First we will concentrate on compounds just containing carbon and hydrogen, these compounds are called hydrocarbons.

5. Hydrocarbon Classification
What is Organic chemistry?
The study of carbon and its compounds.
First we will concentrate on compounds just containing carbon and hydrogen, these compounds are called hydrocarbons.
Hydrocarbon Classification
Hydrocarbons
Alkanes
Cycloalkanes
Alkenes
Cycloalkenes
Alkynes

6. Alkanes (saturated) hydrocarbons, or aliphatic hydrocarbons)
General formula of CnH2n+2
Examples
a. CH4 b. C2H6 c. C3H?

7. Alkanes
General formula of CnH2n+2
Examples
a. CH4 b. C2H6 c. C3H8 d. C4H?

8. General formula of CnH2n+2 Examples CH4 b. C2H6 c. C3H8 d. C4H10
Alkanes
General formula of CnH2n+2
Examples
CH4 b. C2H6 c. C3H8 d. C4H10
Draw Lewis Structures
CH4
C2H6
C3H8

9. General formula of CnH2n+2 Examples CH4 b. C2H6 c. C3H8 d. C4H10
Alkanes
General formula of CnH2n+2
Examples
CH4 b. C2H6 c. C3H8 d. C4H10
Draw Lewis Structures
CH4
C2H6
C3H8
D. Polarity? Polar or nonpolar?

10. General formula of CnH2n+2 Examples CH4 b. C2H6 c. C3H8 d. C4H10
Alkanes
General formula of CnH2n+2
Examples
CH4 b. C2H6 c. C3H8 d. C4H10
Draw Lewis Structures
CH4
C2H6
C3H8
D. Polarity? Polar or nonpolar?
Nonpolar

11. E. Draw three dimensional structures, bond angles and hybridization.
Alkanes (Continued)
E. Draw three dimensional structures, bond angles and hybridization.
CH4
C2H6
C3H8
F. There are two different structures for C4H 10
Structure 1
Structure 2

12. 1. Primary (1◦) Carbon connected to one carbon atoms.
Types of carbon
1. Primary (1◦) Carbon connected to one carbon atoms.
2. Secondary (2◦) Carbon connected to two carbon atoms.
3. Tertiary (3◦) Carbon connected to three carbon atoms.
4. How many primary, secondary, and tertiary carbons in the
two different structures of C4H10
=>
Primary = ?
Secondary =
Tertiary =
Primary =
Secondary =
Tertiary =

13. 1. Primary (1◦) Carbon connected to one carbon atoms.
Types of carbon
1. Primary (1◦) Carbon connected to one carbon atoms.
2. Secondary (2◦) Carbon connected to two carbon atoms.
3. Tertiary (3◦) Carbon connected to three carbon atoms.
4. How many primary, secondary, and tertiary carbons in the
two different structures of C4H10
=>
Primary = 2
Secondary = ?
Tertiary =
Primary =
Secondary =
Tertiary =

14. 1. Primary (1◦) Carbon connected to one carbon atoms.
Types of carbon
1. Primary (1◦) Carbon connected to one carbon atoms.
2. Secondary (2◦) Carbon connected to two carbon atoms.
3. Tertirary (3◦) Carbon connected to three carbon atoms.
4. How many primary, secondary, and tertiary carbons in the
two different structures of C4H10
=>
Primary = 2
Secondary = 2
Tertiary = ?
Primary =
Secondary =
Tertiary =

15. 1. Primary (1◦) Carbon connected to one carbon atoms.
Types of carbon
1. Primary (1◦) Carbon connected to one carbon atoms.
2. Secondary (2◦) Carbon connected to two carbon atoms.
3. Tertiary (3◦) Carbon connected to three carbon atoms.
4. How many primary, secondary, and tertiary carbons in the
two different structures of C4H10
=>
Primary = 2
Secondary = 2
Tertiary = 3
Primary = ?
Secondary =
Tertiary =

16. 1. Primary (1◦) Carbon connected to one carbon atoms.
Types of carbon
1. Primary (1◦) Carbon connected to one carbon atoms.
2. Secondary (2◦) Carbon connected to two carbon atoms.
3. Tertiary (3◦) Carbon connected to three carbon atoms.
4. How many primary, secondary, and tertiary carbons in the
two different structures of C4H10
Primary = 2
Secondary = 2
Tertiary = 3
Primary = 3
Secondary = ?
Tertiary =

17. 1. Primary (1◦) Carbon connected to one carbon atoms.
Types of carbon
1. Primary (1◦) Carbon connected to one carbon atoms.
2. Secondary (2◦) Carbon connected to two carbon atoms.
3. Tertiary (3◦) Carbon connected to three carbon atoms.
4. How many primary, secondary, and tertiary carbons in the
two different structures of C4H10
Primary = 2
Secondary = 2
Tertiary = 3
Primary = 3
Secondary = 0
Tertiary = ?

18. 1. Primary (1◦) Carbon connected to one carbon atoms.
Types of carbon
1. Primary (1◦) Carbon connected to one carbon atoms.
2. Secondary (2◦) Carbon connected to two carbon atoms.
3. Tertiary (3◦) Carbon connected to three carbon atoms.
4. How many primary, secondary, and tertiary carbons in
the two different structures of C4H10
Primary = 3
Secondary = 0
Tertiary = 1
Primary = 2
Secondary = 2
Tertiary = 3

19. Constitutional Isomers (Structural Isomers) are different compounds of the same formula. The different structures from the previous slide for the formula C4H10 is an example of Constitutional isomers.
How many isomers are there of an alkane containing five carbons (C5H10)?

20. NOMENCLATURE Common system
Works best for low molecular weight hydrocarbons
Steps to give a hydrocarbon a common name:
Count the total number of carbon atoms in the molecule.
Use the Latin root from the following slide that corresponds to the number of carbon atoms followed by the suffix “ane”.
Unbranced hydrocarbons use the prefix normal, or n-,
Branched hydrocarbons use specific prefixes, as shown on a subsequent slide

21. Latin Hydrocarbon Roots Latin Hydrocarbon Roots
Examples
Latin Hydrocarbon Roots
Number of Carbons
Latin
Root 1
meth 2
eth 3
prop 4
but 5
pent 6
hex 7
hept 8
oct 9
non 10
dec 11
undec
Latin Hydrocarbon Roots
Number of Carbons
Latin
Root 12
dodec 13
tridec 14
tetradec 15
pentadec 16
hexadec 17
heptadec 18
octadec 19
nonadec 20
eicos 21
unicos 22
doicos H
n-butane H
isobutane H H
H C H H
H C C C H H
H C H H H
neopentane

22. 2. Systematic System of Nomenclature (IUPAC)
Find the longest continuous chain of carbon atoms.
Use a Latin root corresponding to the number of carbons in the
longest chain of carbons.
Follow the root with the suffix of “ane” for alkanes
Carbon atoms not included in the chain are named as
substituents preceding the root name with Latin root followed
by “yl” suffix.
Number the carbons, starting closest to the first branch.
Name the substituents attached to the chain, using the carbon
number as the locator in alphabetical order.
Use di-, tri-, etc., for multiples of same substituent.
If there are two possible chains with the same number of
carbons, use the chain with the most substituents.

23. Substituent Names (Alkyl groups)

24. Systematic Nomenclature continued.
Which one?

25. Systematic Nomenclature continued.
Which one?
The one with the most number of substituents

26. Systematic Nomenclature continued.
Which one?
The one with the least number of substituents
The top structure has four substituents and the bottom has three
Substituents.

27. Systematic Nomenclature continued.
Which one?
The one with the least number of substituents
The top structure has four substituents and the bottom has three
Substituents.
Name = ?

28. Systematic Nomenclature continued.
Which one?
The one with the least number of substituents
The top structure has four substituents and the bottom has three
Substituents.
Name = ? heptane

29. Systematic Nomenclature continued.
Which one?
The one with the least number of substituents
The top structure has four substituents and the bottom has three
Substituents.
Name = 3,3,5-trimethyl-4-propylheptane

30. Another Example:
Name = 3-ethyl-2,6-dimethylheptane

31. Another Example: Name = 2,6-dimethyl-3-ethylheptane
Notice substituents are in alphabetical order; di, tri, etc. do not participate in the alphabetical order

32. Line Structures A quicker way to write sturctures
(Condensed Structure)
methyl
ethyl
(A line structure of the above condensed structure)
methyl

33. 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. 3 1 1 2
1-methyl-3-(1,2-dimethylpropyl)cyclohexane

34. Alkane Physical Properties
Solubility: hydrophobic (not water soluble)
Density: less than 1 g/mL (floats on water)
Boiling points increase with increasing carbons (little less for branched chains) due to dispersion forces being larger.
Melting points increase with increasing carbons (less for odd-number of carbons).

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

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

37. Reactions of Alkanes II. Cracking reaction
I. Combustion reaction
II. Cracking reaction
III. Halogenation reaction (substitution reaction)

38. Which isomer of C5H12 has the most monochloro isomers?
Sample problem:
Which isomer of C5H12 has the most monochloro isomers?
Problem solving process:
Step 1 draw the isomers of C5H12
Step 2 react each isomer with chlorine
Step 3 count the products

39. Which isomer of C5H12 has the most monochloro isomers?
Sample problem:
Which isomer of C5H12 has the most monochloro isomers?
Problem solving process:
Step 1 draw the isomers of C5H10
Step 2 react each isomer with chlorine
Step 3 count the products

40. Which isomer of C5H12 has the most monochloro isomers?
Sample problem:
Which isomer of C5H12 has the most monochloro isomers?
Problem solving process:
Step 1 draw the isomers of C5H10
Step 2 react each isomer with chlorine
Step 3 count the products

41. Which isomer of C5H12 has the most monochloro isomers?
Sample problem:
Which isomer of C5H12 has the most monochloro isomers?
Problem solving process:
Step 1 draw the isomers of C5H10
Step 2 react each isomer with chlorine
Step 3 count the products

42. Which isomer of C5H12 has the most monochloro isomers?
Sample problem:
Which isomer of C5H12 has the most monochloro isomers?
Problem solving process:
Step 1 draw the isomers of C5H10
Step 2 react each isomer with chlorine
Step 3 count the products

43. Which isomer of C5H12 has the most monochloro isomers?
Sample problem:
Which isomer of C5H12 has the most monochloro isomers?
Problem solving process:
Step 1 draw the isomers of C5H10
Step 2 react each isomer with chlorine
Step 3 count the products
Winner!

44. 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.

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

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

47. Conformational Analysis
Torsional strain: resistance to rotation.
For ethane, only 12.6 kJ/mol
=>

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

49. Butane Conformers C2-C3 Highest energy has methyl groups eclipsed.
Steric hindrance
Dihedral angle = 0 degrees
=>
totally eclipsed (methyl groups)

50. Butane Conformers (2) Lowest energy has methyl groups anti.
Dihedral angle = 180 degrees
=>
Staggered-anti

51. Butane Conformers (3) Methyl groups eclipsed with hydrogens
Higher energy than staggered conformer
Dihedral angle = 120 degrees
=>
Eclipsed (hydrogen and methyl)

52. Butane Conformers (4) Gauche, staggered conformer
Methyls closer than in anti conformer
Dihedral angle = 60 degrees
=>
Staggered-gauche

53. Conformational Analysis

54. 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
Slightly unsaturated compared to alkanes

55. Naming Cycloalkanes Count the number of carbons in the cycle
If the bonds are single then use the suffix “ane”
First substituent in alphabet gets lowest number.
May be cycloalkyl attachment to chain.

56. Cis-Trans Isomerism (a type of stereoisomerism)
Cis: like groups on same side of ring
Trans: like groups on opposite sides of ring

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

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

59. Cyclopropane Large ring strain due to angle compression
Very reactive, weak bonds
=>

60. Cyclopropane (2)
Torsional strain because of eclipsed hydrogens

61. Cyclobutane Angle strain due to compression
Torsional strain partially relieved by ring puckering
=>

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

63. 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.

64. Chair Conformer

65. Boat Conformer

66. Conformational Energy

67. Axial and Equatorial Positions

68. Monosubstituted Cyclohexanes

69. 1,3-Diaxial Interactions

70. Disubstituted Cyclohexanes

71. Cis-Trans Isomers
Bonds that are cis, alternate axial-equatorial around the ring.
=>
One axial, one equatorial

72. Bulky Groups Groups like t-butyl cause a large energy
difference between the axial and equatoria
l conformer.
Most stable conformer puts t-butyl equatorial
regardless of other substituents.
=>

73. End of Chapter 2