1. William H. Brown Christopher S. Foote Brent L. Iverson
Organic Chemistry
William H. Brown
Christopher S. Foote
Brent L. Iverson

2. Alkanes
and
Cycloalkanes
Chapter 2

3. Structure Hydrocarbon: a compound composed only of carbon and hydrogen
Saturated hydrocarbon: a hydrocarbon containing only single bonds
Alkane: a saturated hydrocarbon whose carbons are arranged in an open chain
Aliphatic hydrocarbon: another name for an alkane

4. Hydrocarbons

5. Structure Shape tetrahedral about carbon
all bond angles are approximately 109.5°

6. Drawing Alkanes Line-angle formulas
an abbreviated way to draw structural formulas
each vertex and line ending represents a carbon

7. Constitutional Isomerism
Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms
example: C4H10

8. Constitutional Isomerism
do these formulas represent constitutional isomers?
find the longest carbon chain
number each chain from the end nearest the first branch
compare chain lengths as well the identity and location of branches

9. Constitutional Isomerism
World population
is about
6,000,000,000

10. Nomenclature - IUPAC Suffix -ane specifies an alkane
Prefix tells the number of carbon atoms

11. Nomenclature - IUPAC Parent name: the longest carbon chain
Substituent: a group bonded to the parent chain
alkyl group: a substituent derived by removal of a hydrogen from an alkane; given the symbol R-

12. Nomenclature - IUPAC
1.The name of a saturated hydrocarbon with an unbranched chain consists of a prefix and suffix
2. The parent chain is the longest chain of carbon atoms
3. Each substituent is given a name and a number
4. If there is one substituent, number the chain from the end that gives it the lower number

13. Nomenclature - IUPAC
5. If there are two or more identical substituents, number the chain from the end that gives the lower number to the substituent encountered first
indicate the number of times the substituent appears by a prefix di-, tri-, tetra-, etc.
use commas to separate position numbers

14. Nomenclature - IUPAC
6. If there are two or more different substituents,
list them in alphabetical order
number from the end of the chain that gives the substituent encountered first the lower number

15. Nomenclature - IUPAC
7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization
alphabetize the names of substituents first and then insert these prefixes

16. Nomenclature - IUPAC
Alkyl groups

17. Nomenclature - Common
The number of carbons in the alkane determines the name
all alkanes with four carbons are butanes, those with five carbons are pentanes, etc.
iso- indicates the chain terminates in -CH(CH3)2; neo- that it terminates in -C(CH3)3

18. Classification of C & H
Primary (1°) C: a carbon bonded to one other carbon
1° H: a hydrogen bonded to a 1° carbon
Secondary (2°) C: a carbon bonded to two other carbons
2° H: a hydrogen bonded to a 2° carbon
Tertiary (3°) C: a carbon bonded to three other carbons
3° H: a hydrogen bonded to a 3° carbon
Quaternary (4°) C: a carbon bonded to four other carbons

19. Cycloalkanes General formula CnH2n Structure and nomenclature
five- and six-membered rings are the most common
Structure and nomenclature
to name, prefix the name of the corresponding open-chain alkane with cyclo-, and name each substituent on the ring
if only one substituent, no need to give it a number
if two substituents, number from the substituent of lower alphabetical order
if three or more substituents, number to give them the lowest set of numbers and then list substituents in alphabetical order

20. Cycloalkanes Line-angle drawings each line represents a C-C bond
each vertex and line ending represents a C

21. Cycloalkanes
Example: name these cycloalkanes

22. Bicycloalkanes
Bicycloalkane: an alkane that contains two rings that share two carbons
Bicyclo[4.4.0]decane
(Decalin)
Bicyclo[4.3.0]nonane
(Hydrindane)
Bicyclo[2.2.1]heptane
(Norbornane)

23. Bicycloalkanes Nomenclature
parent is the alkane of the same number of carbons as are in the rings
number from a bridgehead, along longest bridge back to the bridgehead, then along the next longest bridge, etc.
show the lengths of bridges in brackets, from longest to shortest

24. IUPAC - General prefix-infix-suffix
prefix tells the number of carbon atoms in the parent
infix tells the nature of the carbon-carbon bonds
suffix tells the class of compound
Nature of Carbon-Carbon
Bonds in the Parent Chain
Suffix
Class
Infix -e
hydrocarbon
-an-
all single bonds
-ol
alcohol
-en-
one or more double bonds
-al
aldehyde
-yn-
one or more triple bonds
-amine
amine
-one
ketone
-oic acid
carboxylic acid

25. IUPAC - General prop-en-e = propene eth-an-ol = ethanol
but-an-one = butanone
but-an-al = butanal
pent-an-oic acid = pentanoic acid
cyclohex-an-ol = cyclohexanol
eth-yn-e = ethyne
eth-an-amine = ethanamine

26. Conformations
Conformation: any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond
Newman projection: a way to view a molecule by looking along a carbon-carbon single bond

27. Conformations
Staggered conformation: a conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as far apart as possible from the atoms or groups on an adjacent carbon

28. Conformations
Eclipsed conformation: a conformation about a carbon-carbon single bond in which the atoms or groups of atoms on one carbon are as close as possible to the atoms or groups of atoms on an adjacent carbon

29. Conformations Torsional strain also called eclipsed interaction strain
strain that arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation
the torsional strain between eclipsed and staggered ethane is approximately 12.6 kJ (3.0 kcal)/mol

30. Conformations
Dihedral angle (Q): the angle created by two intersecting planes

31. Conformations
Ethane as a function of dihedral angle

32. Conformations The origin of torsional strain in ethane
originally thought to be caused by repulsion between eclipsed hydrogen nuclei
alternatively, caused by repulsion between electron clouds of eclipsed C-H bonds
theoretical molecular orbital calculations suggest that the energy difference is not caused by destabilization of the eclipsed conformation but rather by stabilization of the staggered conformation
this stabilization arises from the small donor-acceptor interaction between a C-H bonding MO of one carbon and the C-H antibonding MO on an adjacent carbon; this stabilization is lost when a staggered conformation is converted to an eclipsed conformation

33. Conformations anti conformation
a conformation about a single bond in which the groups lie at a dihedral angle of 180°

34. Conformations Steric strain (nonbonded interaction strain):
the strain that arises when atoms separated by four or more bonds are forced closer to each other than their atomic (contact) radii will allow
Angle strain:
strain that arises when a bond angle is either compressed or expanded compared to its optimal value
The total of all types of strain can be calculated by molecular mechanics programs
such calculations can determine the lowest energy arrangement of atoms in a given conformation, a process called energy minimization

35. Conformations
conformations of butane as a function of dihedral angle

36. Anti Butane Energy-minimized anti conformation
the C-C-C bond angle is 111.9° and all H-C-H bond angles are between and 107.9°
the calculated strain is 9.2 kJ (2.2 kcal)/mol

37. Eclipsed Butane
calculated energy difference between (a) the non-energy-minimized and (b) the energy-minimized eclipsed conformations is 5.6 kJ (0.86 kcal)/mol

38. Cyclopropane
angle strain: the C-C-C bond angles are compressed from 109.5° to 60°
torsional strain: there are 6 sets of eclipsed hydrogen interactions
strain energy is about 116 kJ (27.7 kcal)/mol

39. Cyclobutane
puckering from planar cyclobutane reduces torsional strain but increases angle strain
the conformation of minimum energy is a puckered “butterfly” conformation
strain energy is about 110 kJ (26.3 kcal)/mol

40. Cyclopentane
puckering from planar cyclopentane reduces torsional strain, but increases angle stain
the conformation of minimum energy is a puckered “envelope” conformation
strain energy is about 42 kJ (6.5 kcal)/mol

41. Cyclohexane
Chair conformation: the most stable puckered conformation of a cyclohexane ring
all bond C-C-C bond angles are 110.9°
all bonds on adjacent carbons are staggered

42. Cyclohexane
In a chair conformation, six H are equatorial and six are axial

43. Cyclohexane
For cyclohexane, there are two equivalent chair conformations
all C-H bonds equatorial in one chair are axial in the alternative chair, and vice versa

44. Cyclohexane
Boat conformation: a puckered conformation of a cyclohexane ring in which carbons 1 and 4 are bent toward each other
there are four sets of eclipsed C-H interactions and one flagpole interaction
a boat conformation is less stable than a chair conformation by 27 kJ (6.5 kcal)/mol

45. Cyclohexane Twist-boat conformation
approximately 41.8 kJ (5.5 kcal)/mol less stable than a chair conformation
approximately 6.3 kJ (1.5 kcal)/mol more stable than a boat conformation

46. Cyclohexane

47. Methylcyclohexane
Equatorial and axial methyl conformations

48. G0 axial ---> equatorial
given the difference in strain energy between axial and equatorial conformations, it is possible to calculate the ratio of conformations using the following relationship

49. Cis,Trans Isomerism Stereoisomers: compounds that have
the same molecular formula
the same connectivity
a different orientation of their atoms in space
Cis,trans isomers
stereoisomers that are the result of the presence of either a ring (this chapter) or a carbon-carbon double bond (Chapter 5)

50. Isomers
relationships among isomers

51. Cis,Trans Isomers
1,2-Dimethylcyclopentane

52. Cis,Trans Isomerism
1,4-Dimethylcyclohexane

53. Cis,Trans Isomerism trans-1,4-Dimethylcyclohexane
the diequatorial-methyl chair conformation is more stable by approximately 2 x (7.28) = kJ/mol

54. Cis,Trans Isomerism
cis-1,4-Dimethylcyclohexane

55. Cis,Trans Isomerism
The decalins

56. Steroids
The steroid nucleus
Cholestanol

57. Bicycloalkanes
Norbornane drawn from three different perspectives

58. Bicycloalkanes
Adamantane

59. Physical Properties Intermolecular forces of attraction (example)
ion-ion (Na+ and Cl- in NaCl)
ion-dipole (Na+ and Cl- solvated in aqueous solution)
dipole-dipole and hydrogen bonding
dispersion forces (very weak electrostatic attraction between temporary dipoles)

60. Physical Properties
Low-molecular-weight alkanes (methane....butane) are gases at room temperature
Higher molecular-weight alkanes (pentane, decane, gasoline, kerosene) are liquids at room temperature
High-molecular-weight alkanes (paraffin wax) are semisolids or solids at room temperature

61. Physical Properties
Constitutional isomers have different physical properties

62. Oxidation of Alkanes
Oxidation is the basis for their use as energy sources for heat and power
heat of combustion: heat released when one mole of a substance in its standard state is oxidized to carbon dioxide and water

63. Heat of Combustion
Heat of combustion for constitutional isomers

64. Heats of Combustion For constitutional isomers [kJ (kcal)/mol]
( )
( )
(1304.6)
(1303.0) 8 C O 2 + 9 H 2 O

65. Heat of Combustion
strain in cycloalkane rings as determined by heats of combustion

66. Sources of Alkanes Natural gas Petroleum Coal 90-95% methane
gases (bp below 20°C)
naphthas, including gasoline (bp °C)
kerosene (bp °C)
fuel oil (bp °C)
lubricating oils (bp above 350°C)
asphalt (residue after distillation)
Coal

67. Gasoline
Octane rating: the percent 2,2,4-trimethylpentane (isooctane) in a mixture of isooctane and heptane that has equivalent antiknock properties

68. Synthesis Gas
A mixture of carbon monoxide and hydrogen in varying proportions which depend on the means by which it is produced

69. Synthesis Gas
Synthesis gas is a feedstock for the industrial production of methanol and acetic acid
it is likely that industrial routes to other organic chemicals from coal via methanol will also be developed

70. Alkanes and Cycloalkanes
End Chapter 2