1. Structure and Stereochemistry of Alkanes
Organic Chemistry, 8th Edition L. G. Wade, Jr.
Chapter 3 Lecture
Structure and Stereochemistry of Alkanes
Rizalia Klausmeyer
Baylor University
Waco, TX
© 2013 Pearson Education, Inc.

2. Hydrocarbons are molecules that are made of carbon
and hydrogen ONLY.
Table: 03-T01
Chapter 3

3. Alkanes General formula: CnH2n+2
Found in everything from natural gas to petroleum.
The smaller alkanes have very low boiling points (b. p.); therefore, they are gases.
CH C2H C3H8
b. p oC oC oC
Chapter 3

4. Table: 03-T02.jpg
Chapter 3

5. Butane: C4H10 C H C H C H C H C H n-butane isobutane 3 2 3 3 3
Constitutional isomers are compounds with the same
molecular formula but the carbons are connected differently.
Chapter 3

6. Pentanes: C5H12 C H 3 2
n-pentane
isopentane
neopentane
Chapter 3

7. substituent parent Parent Alkanes Name is prefix + ane.
Prefix designates the number of carbon atoms.
Alkyl groups: substituents (branches) on parent alkane.
substituent
parent

8. IUPAC or Systematic Names
International Union of Pure and Applied Chemistry.
Standard method used internationally to name compounds.
Uses the longest chain of carbons as the main chain.
Common names kept: methane, ethane, propane, and butane.
Chapter 3

9. IUPAC Rules
Rule 1: Find the longest continuous chain of carbon atoms, and use the name of this chain as the base name of the compound.
Rule 2: Number the longest chain, beginning with the end of the chain nearest a substituent.
Rule 3: Name the groups attached to the longest chain as alkyl groups. Give the location of each alkyl group by the number of the main-chain carbon atom to which it is attached.
Write the alkyl groups in alphabetical order regardless of their position on the chain.
Chapter 3

10. Sub-rules for IUPAC nomenclature
If there are two or more longest chains of equal length:
Choose the one having the largest number of substituents.
Choose the one having the simples substituents.
If both ends of the root chain have equidistant substituents.
Begin numbering at the end nearest a third substituent, if one is present.
Begin numbering at the end nearest the first cited group (alphabetical order).
Chapter 3

11. Rule 1: The Main Chain Find the longest chain of consecutive carbons.
The longest chain is six carbons: hexane
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Figure: un.jpg
Chapter 3

12. Main Chain
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Figure: 03_01-04UN.jpg
Title:
Nomenclature: Same Length Chains
Caption:
When looking for the longest continuous chain, look to find all the different chains of that length. Often, the longest chain with the most substituents is not obvious.
Notes:
When there are two carbon chains of the same length, the one that has the most substituents must be chosen to name the compound.
When there are two longest chains of equal length, use the chain with the greatest number of substituents.
Chapter 3

13. Hint When looking for the longest continuous chain (to give the
base name), look to find all the
different chains of that length.
Often, the longest chain with
the most substituents is not
obvious.
Chapter 3

14. Rule 2: Numbering the Main Chain
Number the longest chain beginning at the end of the chain nearest a substituent.
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Figure: un.jpg
Chapter 3

15. Rule 3: Naming Alkyl Groups
Alkane name
Alkyl name
CH4
methane
–CH3
methyl group
CH3CH3
ethane
–CH2CH3
ethyl group
CH3CH2CH3
propane
–CH2CH2CH3
propyl group
CH3CH2CH2CH3
butane
–CH2CH2CH2CH3
butyl group
CH3CH2CH2CH2CH3
pentane
–CH2CH2CH2CH2CH3
pentyl group
Chapter 3

16. Common Alkyl Groups

17. Common Nonalkyl Groups
Seager SL, Slabaugh MR, Chemistry for Today: General, Organic and Biochemistry, 7th Edition, 2011

18. Applying the Naming Rules
4-ethyl
2-methyl
Name the groups attached to the longest chain as alkyl groups.
Give the location of each alkyl group by the number of the main-chain carbon atom to which it is attached.
Write the alkyl groups in alphabetical order regardless of their position on the chain.
4-ethyl-2-methylhexane
Chapter 3

19. Multiple Groups
When two or more of the same substituents are present, use the prefixes di-, tri-, tetra-, etc. to avoid having to name the alkyl group twice.
Three methyl groups at positions 2, 5, and 7.
2,5,7-trimethyldecane
Chapter 3

20. Solved Problem 3-1
Give the structures of 4-isopropyloctane and 5-t-butyldecane.
Solution: 4-Isopropyloctane has a chain of eight carbons, with an isopropyl group on the fourth carbon. 5-t-Butyldecane has a chain of ten carbons, with a t-butyl group on the fifth.
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Chapter 3

21. Complex Substituents
Complex alkyl groups are named by using the longest carbon chain.
Carbon number 1 of the alkyl group is the carbon attached to the main chain.
a (1-ethyl-2-methylpropyl) group
a (1,1,3-trimethylbutyl) group
When naming involves complex substituents, the first letter in the complex substituent, regardless of type, is used for alphabetizing
Chapter 3

22. Solved Problem 3-2
Give a systematic (IUPAC) name for the following compound.
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Figure: sp3.2
Chapter 3

23. Solved Problem 3-2: Solution
The longest carbon chain contains eight carbon atoms, so this compound is named as an octane. Numbering from left to right gives the first branch on C2; numbering from right to left gives the first branch on C3, so we number from left to right.
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Figure: sp3.2
4-isopropyl-2,2,3,6-tetramethyloctane
Chapter 3

24. Hint
When substituents are alphabetized, iso- is used as part of the alkyl group name, but the hyphenated prefixes are not.
Thus isobutyl is alphabetized with i, but n-butyl, tert-butyl, and sec-butyl are alphabetized with b.
Chapter 3

26. 2,3-dimethyl-1,3-butadiene
hexane
2,2,4-trimethylpentane
5-sec-butylnonane
2,3-dimethyl-1,3-butadiene
2-chloro-2,4-dimethylpentane
5-sec-butyl-4-methylnonane
2,2-dimethylbutane
3-ethyl-3-methylpentane
1-chloro-3-isopropylcyclopentane
1,2,4-trimethylcyclohexane
2,2,4,6,6-pentamethylheptane
3,5,dibromo-4-ethylheptane

27. Complex Substituents Chapter 3 File Name: AAAKPKO0
Figure: un.jpg
Chapter 3

28. Complex Substituents 3-ethyl-4,6-dimethyl-5-(1-methylbutyl)nonane
7-ethyl-2,4,6-trimethyl-5-(1-methylbutyl)nonane
3-ethyl-7-methyl-5-(2-methylbutyl)nonane
Chapter 3

29. Boiling Points of Alkanes
As the number of carbons in an alkane increases, the boiling point increases due to the larger surface area and the increased van der Waals attractions.
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Figure: 03_03.jpg
Title:
Boiling Points of Branched and Unbranched Alkanes
Caption:
Alkane boiling points. Comparison of the boiling points of the unbranched alkanes (blue) with those of some branched alkanes (red). Because of their smaller surface areas, branched alkanes have lower boiling points than unbranched alkanes.
Notes:
The only intermolecular force of nonpolar molecules are London dispersion forces which result from induced dipole attractions. Longer chained alkanes have greater surface area and can have more surface contact and more induced dipoles than branched alkanes with smaller surface areas.
Chapter 3

30. Melting Points of Alkanes
Melting points increase as the carbon chain increases.
Alkanes with an even number of carbons have higher melting points than those with an odd number of carbons.
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Figure: 03_04.jpg
Title:
Melting Points of Alkanes
Caption:
Alkane melting points. The melting point curve for n-alkanes with even numbers of carbon atoms is slightly higher than that for alkanes with odd numbers of carbons.
Notes:
In solids, the packing of the molecules into a three dimensional structure affects the melting point. When molecules can pack in neat order avoiding empty pockets the melting point will be higher than when the packing is not ordered. Alkanes with an even number of carbons pack better than those with an odd number of carbons.
Chapter 3

31. Alkane Sources
Alkanes are obtained from petroleum and petroleum by-products.
Fractional distillation will separate the crude oil into mixtures of alkanes with a range of boiling points.
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Figure: UN
Chapter 3

32. Catalytic Cracking and Hydrocracking
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Figure: un
Chapter 3

33. Ethane Representations
Two sp3 hybrid carbons.
Rotation about the C—C sigma bond.
Conformations are different arrangements of atoms caused by rotation about a single bond.
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Figure: 03_04-10un.jpg
Title:
Ethane
Caption:
Ethane, the two-carbon alkane, is composed of two methyl groups with overlapping sp3 hybrid orbitals forming a sigma bond between them.
Notes:
Ethane has two sp3 carbons. The C-C bond distance is 1.54Å and there is free rotation along this bond.
Chapter 3

34. Conformations of Ethane
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Figure: 03_04-11un.jpg
Title:
Conformations of Ethane
Caption:
The different arrangement formed by rotations about a single bond are called conformations, and a specific is called conformer. Pure conformers cannot be isolated in most cases, because the molecules are constantly rotating through all the possible conformations.
Notes:
Some conformations can be more stable than others.
Pure conformers cannot be isolated in most cases, because the molecules are constantly rotating through all the possible conformations.
Chapter 3

35. Newman Projections
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Figure: 03_05.jpg
Title:
Newman Projections
Caption:
The Newman projection looks straight down the carbon-carbon bond.
Notes:
The Newman projection is the best way to judge the stability of the different conformations of a molecule.
The Newman projection is the best way to judge the stability of the different conformations of a molecule.
Chapter 3

36. Ethane Conformations
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Figure: 03_07.jpg
Title:
Conformational Analysis of Ethane
Caption:
The torsional energy of ethane is lowest in the staggered conformation. The eclipsed conformation is about 3.0 kcal/mol (12.6 kJ/mol) higher in energy. At room temperature, this barrier is easily overcome, and the molecules rotate constantly.
Notes:
The staggered conformations are lower in energy than the eclipsed conformation because the staggering allows the electron clouds of the C-H bonds to be as far apart as possible. The energy difference is only 3 kcals/mol which can be easily overcome at room temperature.
The torsional energy of ethane is lowest in the staggered conformation. The eclipsed conformation is about 3.0 kcal/mol (12.6 kJ/mol) higher in energy. At room temperature, this barrier is easily overcome, and the molecules rotate constantly.
Chapter 3

37. Propane Conformations
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Figure: 03_08.jpg
Title:
The Newman Projection of Propane
Caption:
Propane is shown here as a perspective drawing and as a Newman projection looking down one of the carbon-carbon bonds.
Notes:
Propane is shown here as a perspective drawing and as a Newman projection looking down the C1—C2 bond.
Chapter 3

38. Propane Conformations
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Figure: 03_09.jpg
Title:
Conformational Analysis of Propane
Caption:
Torsional energy of propane. When a bond of propane rotates, the torsional energy varies much like it does in ethane, but with 0.3 kcal/mol (1.2 kJ/mol) of additional torsional energy in the eclipsed conformation.
Notes:
Much like ethane the staggered conformations of propane is lower in energy than the eclipsed conformations. Since the methyl group occupies more space than a hydrogen, the torsional strain will be 0.3 kcal/mol higher for propane than for ethane.
The staggered conformations of propane is lower in energy than the eclipsed conformations. Since the methyl group occupies more space than a hydrogen, the torsional strain will be 0.3 kcal/mol higher for propane than for ethane.
Chapter 3

39. Butane Conformations
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Figure: 03_10.jpg
Title:
Newman Projections of Butane
Caption:
Butane conformations. Rotations about the center bond in butane give different molecular shapes. Three of these conformations are given specific names.
Notes:
For butane there will be two different staggered conformations: gauche and anti. The gauche conformation has a dihedral angle of 60° between the methyl groups while the anti conformation has a dihedral angle of 180° between the methyl groups. There distinct eclipsed conformation when the dihedral angle between the methyl groups is 0°, this conformation is referred to as totally eclipsed.
Butane has two different staggered conformations: gauche (60° between the methyl groups) and anti (180° between the methyl groups).
The eclipsed conformation where the dihedral angle between the methyl groups is 0° is referred to as totally eclipsed.
Chapter 3

40. Conformational Analysis of Butane
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Figure: 03_11.jpg
Title:
Conformational Analysis of Butane
Caption:
Torsional energy of butane. The anti conformation is lowest in energy, and the totally eclipsed conformation is highest in energy.
Notes:
The eclipsed conformations are higher in energy than the staggered conformations of butane, especially the totally eclipsed conformation. Among the staggered conformations, the anti is lower in energy because it has the electron clouds of the methyl groups as far apart as possible.
Chapter 3

41. Steric Strain in Butane
The totally eclipsed conformation is higher in energy because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion.
This kind of interference between two bulky groups is called steric strain or steric hindrance.
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Figure: 03_11-01UN.jpg
Title:
Totally Eclipsed Conformation of Butane
Caption:
The totally eclipsed conformation is about 1.4 kcal (5.9 kJ) higher in energy than the other eclipsed conformations, because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion. This kind of interference between two bulky groups is called steric strain or steric hindrance.
Notes:
The other eclipsed conformations are lower in energy than the totally eclipsed conformation but are still more unstable than the staggered conformations.
Chapter 3

42. Cycloalkanes contain rings of carbon atoms.
Cycloalkanes: CnH2n
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Figure: 03_12.jpg
Title:
Cycloalkanes
Caption:
Structures of some cycloalkanes.
Notes:
The molecular formula of alkanes is CnH2n, two hydrogen less than an open chain alkane. Their physical properties resemble those of alkanes.
Cycloalkanes contain rings of carbon atoms.
Chapter 3

43. Physical Properties of Alkanes
Nonpolar.
Relatively inert.
Boiling point and melting point depend on the molecular weight.
Table: 03-T04
Chapter 3

44. Cycloalkane Nomenclature
Cycloalkane is the main chain: alkyl groups attached to the cycloalkane will be named as alkyl groups.
If only one alkyl group is present, then no number is necessary.
ethylcyclopentane
Chapter 3

45. Cycloalkane Nomenclature
If there are two or more substituents, number the main chain to give all substituents the lowest possible number. 1 1 3 3
1,3-dimethylcyclohexane
3-ethyl-1,1-dimethylcyclohexane
Chapter 3

46. Students accidentally draw cyclic
Hint
Students accidentally draw cyclic
structures when acyclic structures are intended, and vice versa. Always verify whether the name contains the prefix cyclo-.
Chapter 3

47. Cycloalkanes as Substituents
The cycloalkane becomes a substituent when the acyclic portion of the molecule contains fewer carbons than the cyclic part or when there is a more important functional group in the molecule.
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Figure: un
Chapter 3

48. Geometric Isomers Same side: cis- Opposite side: trans-1
Same side: cis-
cis-1,2-dimethylcyclohexane 2 2
Opposite side: trans- 1
trans-1-ethyl-2-methylcyclohexane
Chapter 3

49. Stabilities of Cycloalkanes
Five- and six-membered rings are the most common in nature.
Carbons of cycloalkanes are sp3 hybridized and thus require an angle of 109.5º.
When a cycloalkane carbon has an angle other than 109.5º, there will not be optimum overlap and the compound will have angle strain.
Angle strain is sometimes called Baeyer strain in honor of Adolf von Baeyer, who first explained this phenomenon.
Torsional strain arises when all the bonds are eclipsed.
Chapter 3

50. Angle Strain in Cyclopropane
The bond angles are compressed to 60° from the usual 109.5° bond angle of sp3 hybridized carbon atoms.
This severe angle strain leads to nonlinear overlap of the sp3 orbitals and “bent bonds.”
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Figure: 03_15.jpg
Title:
Angle Strain in Cyclopropane
Caption:
Angle strain in cyclopropane. The bond angles are compressed to 60° from the usual 109.5° bond angle of sp3 hybridized carbon atoms. This severe angle strain leads to nonlinear overlap of the sp3 orbitals and “bent bonds.”
Notes:
The angle compression of cyclopropane is 49.5°. The high reactivity of cyclopropanes is due to the non-linear overlap of the sp3 orbitals.
Chapter 3

51. Torsional Strain in Cyclopropane
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Figure: 03_16.jpg
Title:
Conformations of Cyclopropane
Caption:
Torsional strain in cyclopropane. All the carbon-carbon bonds are eclipsed, generating torsional strain that contributes to the total ring strain.
Notes:
The angle strain and the torsional strain in cyclopropane make this ring size extremely reactive.
All the C—C bonds are eclipsed, generating torsional strain that contributes to the total ring strain.
Chapter 3

52. Cyclobutane: C4H8
The ring strain of a planar cyclobutane results from two factors: angle strain from the compressing of the bond angles to 90° rather than the tetrahedral angle of 109.5° and torsional strain from eclipsing of the bonds.
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Figure: 03_14.jpg
Title:
Ring Strain and Torsional Strain of Cyclobutane
Caption:
The ring strain of a planar cyclobutane results from two factors: Angle strain from the compressing of the bond angles to 90° rather than the tetrahedral angle of 109.5°, and torsional strain from eclipsing of the bonds.
Notes:
The angle compression for butane is 19.5°. Angle strain and torsional strain account for the high reactivity of 4-membered rings.
Chapter 3

53. Nonplanar Cyclobutane
Cyclic compound with four carbons or more adopt nonplanar conformations to relieve ring strain.
Cyclobutane adopts the folded conformation (“envelope”) to decrease the torsional strain caused by eclipsing hydrogens.
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Figure: 03_17.jpg
Title:
Conformations of Cyclobutane
Caption:
The conformation of cyclobutane is slightly folded. Folding gives partial relief from the eclipsing of bonds, as shown in the Newman projection. Compare this actual structure with the hypothetical planar structure in Figure 3-14.
Notes:
Cyclic compound with 4 carbons or more adopt non-planar conformations to relieve ring strain. Cyclobutane adopts the folded conformation to decrease the torsional strain caused by eclipsing hydrogens.
Chapter 3

54. Cyclopentane: C5H10
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Figure: 03_18.jpg
Title:
Conformations of Cyclopentane
Caption:
The conformation of cyclopentane is slightly folded, like the shape of an envelope. This puckered conformation reduces the eclipsing of adjacent CH2 groups.
Notes:
To relieve ring strain, cyclopentane adopts the envelope conformation.
The conformation of cyclopentane is slightly folded, like the shape of an envelope. This puckered conformation reduces the eclipsing of adjacent methylene (CH2) groups.
Chapter 3

55. Chair Conformation of Cyclohexane
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Figure: 03_19.jpg
Title:
Conformations of Cyclohexane
Caption:
The chair conformation of cyclohexane has one methylene group puckered upward and another puckered downward. Viewed from the Newman projection, the chair conformation has no eclipsing of the carbon-carbon bonds. The bond angles are 109.5°.
Notes:
Cyclohexane can adopt four non-planar conformations: chair, boat, twist boat, and half-chair. The most stable conformation is the chair because it has all the C-H bonds staggered.
Chapter 3

56. Chair Conformation
The chair is the most stable conformational isomer of cyclohexane.
The chair has no eclipsing interactions.
Bond angles in the chair conformation are 109.5°.
Chapter 3

57. Boat Conformation of Cyclohexane
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Figure: 03_20.jpg
Title:
Boat Conformation of Cyclohexane
Caption:
In the symmetrical boat conformation of cyclohexane, eclipsing of bonds results in torsional strain. In the actual molecule, the boat conformation is skewed to give the twist boat, a conformation with less eclipsing of bonds and less interference between the two flagpole hydrogens.
Notes:
In the boat conformation all bonds are staggered except for the “flagpole” hydrogens. There is steric hindrance between these hydrogens so the molecule twists a little producing the twist boat conformation which is 1.4 kcal (6 kJ) lower in energy than the boat.
Chapter 3

58. Boat and Twisted Boat Conformation
Eclipsing bonds result in torsional strain.
The twist boat conformation has fewer eclipsing bond interactions and less interference between the flagpole hydrogens.
Chapter 3

59. Conformational Energy Diagram of Cyclohexane
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Figure: 03_21.jpg
Title:
Conformational Energy Diagram of Cyclohexane
Caption:
Conformational energy of cyclohexane. The chair conformation is most stable, followed by the twist boat. To convert between these two conformations, the molecule must pass through the unstable half-chair conformation.
Notes:
Interconversion between chair conformations require that cyclohexane go through its higher energy conformations.
Chapter 3

60. Axial and Equatorial Positions
Axial bonds (red) are directed vertically parallel to the axis of the ring.
Equatorial bonds (green) are directed outward toward the equator of the molecule.
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Figure: 03_22.jpg
Title:
Chair Conformation of Cyclohexane
Caption:
The axial bonds are directed vertically, parallel to the axis of the ring. The equatorial bonds are directed outward, toward the equator of the ring. As they are numbered here, the odd-numbered carbons have their upward bonds axial and their downward bonds equatorial. The even-numbered carbons have their downward bonds axial and their upward bonds equatorial.
Notes:
All the C-H bonds are staggered in the chair conformation. Axial hydrogens are pointed straight up or down, parallel to the axis of the ring. Equatorial hydrogens, like their name suggests, are pointed out of the ring along the “equator” of the molecule.
Chapter 3

61. Chair–Chair Interconversion
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Figure: 03_23.jpg
Title:
Chair-Chair Interconversion
Caption:
Chair-chair interconversion of methylcyclohexane. The methyl group is axial in one conformation, and equatorial in the other.
Notes:
The most important result in chair conversion is that any substituent
that is axial in the original conformation becomes equatorial in the
new conformation.
Chapter 3

62. Axial Methyl in Methylcyclohexane
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Figure: 03_24.jpg
Title:
Newman Projection of Methylcyclohexane: Methyl Axial
Caption:
(a) When the methyl substituent is in an axial position on C1, it is gauche to C3. (b) The axial methyl group on C1 is also gauche to C5 of the ring.
Notes:
In the Newman projection it is easier to see the steric interaction between the methyl substituent and the hydrogens and carbons of the ring.
Chapter 3

63. Equatorial Methyl Group
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Figure: 03_25.jpg
Title:
Newman Projection of Methylcyclohexane: Methyl Equatorial
Caption:
Looking down the C1-C2 bond of the equatorial conformation, we find that the methyl group is anti to C3.
Notes:
An equatorial methyl group will be anti to the C3. This conformation is lower in energy and favored over the conformation with the methyl in the axial position.
Chapter 3

64. 1,3-Diaxial Interaction
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Figure: 03_26.jpg
Title:
1,3-Diaxial Interaction
Caption:
The axial substituent interferes with the axial hydrogens on C3 and C5. This interference is called a 1,3-diaxial interaction.
Notes:
The axial substituent interferes with the axial hydrogens on C3 and C5. This interference is called a 1,3-diaxial interaction.
Chapter 3

65. tert-Butylcyclohexane
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Figure: 03_26-14un.jpg
Title:
Conformations with Extremely Bulky Groups
Caption:
Some groups are so bulky that they are extremely hindered in axial positions. Cyclohexanes with tertiary-butyl substituents show that an axial t-butyl group is severely hindered. Regardless of the other groups present, the most stable conformation has a t-butyl group in an equatorial position. The following figure shows the severe steric interactions in a chair conformation with a t-butyl group axial.
Notes:
Alkyl substituents on cyclohexane rings will tend to be equatorial to avoid 1,3-diaxial interactions. Groups like tert-butyl are so bulky that it will force the chair conformation where it is in the equatorial position, regardless of other groups present.
Substituents are less crowded in the equatorial positions.
Chapter 3

66. Cis-1,3-dimethylcyclohexane
Cis-1,3-dimethylcyclohexane can have both methyl groups in axial positions or both in equatorial positions.
The conformation with both methyl groups being equatorial is more stable.
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Figure: 03_26-02un.jpg
Title:
Chair Conformations of cis-1,3-Dimethylcyclohexane
Caption:
Two chair conformations are possible for cis-1,3-dimethylcyclohexane. The unfavorable conformation has both methyl groups in axial positions, with a 1,3-diaxial interaction between them. The more stable conformation has both methyl groups in equatorial positions.
Notes:
Alkyl substituents on cyclohexane rings will tend to be equatorial to avoid 1,3-diaxial interactions. cis-1,3-dimethylcyclohexane can have both methyl groups on axial positions but the conformation with both methyls in equatorial positions is favored.
Chapter 3

67. Trans-1,3-dimethylcyclohexane
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Figure: 03_26-03un.jpg
Title:
Chair Conformations of trans-1,3-Dimethylcyclohexane
Caption:
Either of the chair conformations of trans-1,3-dimethylcyclohexane has one methyl group in an axial position and one in an equatorial position. These conformations have equal energies, and they are present in equal amounts.
Notes:
Alkyl substituents on cyclohexane rings will tend to be equatorial to avoid 1,3-diaxial interactions. trans-1,3-dimethylcyclohexane has one methyl group axial and the other equatorial. Chair interconversion would still produce an axial and an equatorial methyl. In this case both chairs have the same energy, and they are present in equal amounts.
Both conformations have one axial and one equatorial methyl group so they have the same energy.
Chapter 3

68. Cis-1,4-ditertbutylcyclohexane
The most stable conformation of cis-1,4-di-tert-butylcyclo hexane is the twist boat. Both chair conformations require one of the bulky t-butyl groups to occupy an axial position.
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Figure: 03_26-15un.jpg
Title:
Conformation of 1,4-di-t-butylcyclohexane
Caption:
The most stable conformation of cis-1,4-di-t-butylcyclohexane is a twist boat. Either of the chair conformations requires one of the bulky t-butyl groups to occupy an axial position.
Notes:
Since tert-butyl groups are most stable in the equatorial positions, when two t-butyl groups are present they will force the cyclohexane to interconvert to the twist boat conformation.
Chapter 3

69. Solved Problem 3-3
(a) Draw both chair conformations of cis-1, dimethylcyclohexane, and determine which conformer is more stable.
(b) Repeat for the trans isomer.
(c) Predict which isomer (cis or trans) is more stable.
Copyright © 2006 Pearson Prentice Hall, Inc.
Chapter 3

70. Solved Problem 3-3: Solution (a)
(a) There are two possible chair conformations for the cis isomer, and these two conformations interconvert at room temperature. Each of these conformations places one methyl group axial and one equatorial, giving them the same energy.
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Figure in page 117, 8th Edition – solution A
Chapter 3

71. Solved Problem 3-3: Solution (b)
(b) There are two chair conformations of the trans isomer that interconvert at room temperature. Both methyl groups are axial in one, and both are equatorial in the other. The diequatorial conformation is more stable because neither methyl group occupies the more hindered axial position.
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Figure in page 117, 8th Edition – solution B
Chapter 3

72. Solved Problem 3-3: Solution (c)
(c) The trans isomer is more stable. The most stable conformation of the trans isomer is diequatorial and therefore about 7.6 kJ/mol (1.8 kcal/mol) lower in energy than either conformation of the cis isomer, each having one methyl axial and one equatorial. Remember that cis and trans are distinct isomers and cannot interconvert.
Copyright © 2006 Pearson Prentice Hall, Inc.
Chapter 3

73. Solved Problem 3-4
Draw the most stable conformation of trans-1-ethyl-3-methylcyclohexane.
Solution: First, we draw the two conformations. The ethyl group is bulkier than the methyl group, so the conformation with the ethyl group equatorial is more stable.
File Name: AAAKPNH0
Figure in page 118, 8th Edition – solution to problem 3-4
Chapter 3

74. Bicyclic Systems
Fused rings share two adjacent carbon atoms and the bond between them.
Bridged rings share two nonadjacent carbon atoms and one or more carbon atoms (the bridge) between them.
Spirocyclic compounds are rare; the two rings share only one carbon.
ile Name: AAAKPNR0
Figure: 03_26-16un.jpg
Title:
Bicyclic Compounds
Caption:
Two or more rings can be joined into bicyclic or polycyclic systems. There are three ways that two rings may be joined. Fused rings are most common, sharing two adjacent carbon atoms and the bond between them. Bridged rings are also common, sharing two nonadjacent carbon atoms (the bridgehead carbons) and one or more carbon atoms (the bridge) between them. Spirocyclic compounds, in which the two rings share only one carbon atom, are relatively rare.
Notes:
Three examples of bicyclic ring systems can be fused, bridged, or spirocyclic. Fused and bridged bicyclic rings are joined together by two carbons; in fused bicycles the two carbons are adjacent while in bridged bicycles the carbons are nonadjacent. Spirocycles are joined by only one carbon.
Chapter 3

75. Nomenclature of Bicyclic Systems
File Name: AAAKPNS0
Figure: 03_26-18un.jpg
Title:
Nomenclature of Bicyclic Compounds
Caption:
The name of a bicyclic compound is based on the name of the alkane having the same number of carbons as there are in the ring system. This name follows the prefix bicyclo and a set of brackets enclosing three numbers. The following examples contain eight carbon atoms and are named bicyclo[4.2.0]octane and bicyclo[3.2.1]octane, respectively.
Notes:
When naming bicyclic rings, the alkane name used will denote the total amount of carbons in the compound. The prefix bicyclo is used followed by three numbers in brackets. These three numbers represent the number of carbons that bridge (connect) the two shared carbons. In the case of spirocycles, the prefix spiro is used instead of bicycle and only two numbers are written.
Bicyclo [#.#.#]alkane
where the #s are the numbers of carbons on the bridges (in decreasing order) and the alkane name includes all the carbons in the compound.
Chapter 3

76. Decalin
File Name: AAAKPNX0
Figure: 03_27.jpg
Title:
cis- and trans-Decalin
Caption:
cis-Decalin has a ring fusion where the second ring is attached by two cis bonds. trans-Decalin is fused using two trans bonds. The six-membered rings in cis- and trans-decalin assume chair conformations.
Notes:
The correct IUPAC name for decalin is bicyclo[3.3.0]decane but it is commonly known as decalin. There are two possible geometric isomers for decalin: cis and trans.
Cis-decalin has a ring fusion where the second ring is attached by two cis bonds. Trans-decalin is fused using two trans bonds. Trans-decalin is more stable because the alkyl groups are equatorial.
Chapter 3