1. Alkanes and cycloalkanes: Conformations of Molecules (分子的构象）
Chapter 4
Alkanes and cycloalkanes: Conformations of Molecules (分子的构象）

2. 4.1 Introduction of alkanes and cycloalkanes
General formula CnH2n+2 for alkanes
C3H CH3CH2CH3 Propane
General formula CnH2n for cycloalkanes (环烷烃）
CnH2n Cyclopropane

3. Sources of alkanes 4.1A Petroleum 4.1B Petroleum refining
Table Typical fractions obtained by distillation of petroleum
Mixtures of alkanes, fortunately, are perfectly suitable for uses as fuels, solvents, and lubricants, the primary uses of petroleum.

4. 4.1 C Cracking (裂解）
When a mixture of alkanes from the gas oil (C12 and higher) fraction is heated at very high temperatures (--500 oC) in the presence of a variety of catalysts, the, the molecules break apart and rearrange to smaller, more highly branched alkanes containing 5-10 carbon atoms. This process is called catalytic cracking (催化裂解）. Cracking can also be done in the absence of a catalyst---called thermal cracking (热裂解）

5. 2,2,4-trimethylpentane (isooctane 异辛烷)
Isooctane burns very smoothly without knocking, but heptane
Burns to produce much knocking.
A mixture of 87% isooctane and 13% heptane would be rated as
87-octane gasoline

6. 4.2 Shapes of alkanes
The tetrahedral carbon atoms their chains are zigzagged (锯齿形）and not at all straight chains （直链）
Unbranched chain alkanes (直链烷烃）
Branched chain alkanes (支链烷烃）

7. Primary, secondary and tertiary carbon atoms and hydrogen (伯，仲， 叔碳原子和氢原子）

8. How to write constitutional isomers ? (怎样写构形异构体 ?）

9. Problem 4.2 Write the following constitutional isomers.

10. 4.3 IUPAC nomenclature IUPAC nomenclature of alkanes
IUPAC nomenclature of alkyl halides
IUPAC nomenclature of alcohols

11. 4.3.1 IUPAC nomenclature of alkanes
What is it IUPAC?
The formal system of nomenclature used today is one proposed by the International Union of Pure and Applied Chemistry (IUPAC). This system was first developed in 1892.
Each different compound should have a different name and make a systematic set of rules

12. 4.3.2 Nomenclature of the unbranched alkanes

13. 4.3A Nomenclature of unbranched alkyl groups
If we remove one hydrogen atom from an alkane, we obtain what is called an alkyl group. These alkyl groups have names that end in –yl.

15. 4.3B Nomenclature of branched-Chain alkanes 支链烷烃的命名
Rules:
1. Locate the longest continuous chain of carbon atoms; this chain determines the parent name for the alkane （选母体 或 选主链）

16. 2. Number the longest chain beginning with the end of the chain nearer the substituent.(编号）

17. 3. Use the numbers obtained by application of rule 2 to designate the location of substituent group. (取代基定位）
The parent name is placed last.
The substituent group name is placed first.
Numbers are separated from words by a hyphen.

19. 4. When two or more substituents are present, give each substituent a number corresponding to its location on the longest-chain

20. 5. When two substituents are present on the same carbon atom, use that number twice

21. 6. When two or more substituents are identical, indicate this by the use of the prefixes di-, tri-, tetra-, and so on

22. 7.When two chains of equal length compete for selection as the parent chain, choose the chain with the greater number of substituents.

23. 8. When branching first occurs at an equal distance from either end of the longest chain, choose the name that gives the lower number at the first point of difference.

24. 4.3C Nomenclature of branched alkyl groups

25. Four-carbon groups

26. The following examples show how the names of these groups are employed

27. Problem 4.1 Give IUPAC names for the following compounds
3-Ethyl-2,2,3,4-tert-methylpentane

28. Problem 4.2 Write structureal formulas for the nine isomers of C7H16

30. 4.3E Nomenclature of alkyl halides

32. 4.3 F Nomenclature of alcohols

33. Several examples

34. Alcohols containing two hydroxyl groups are commonyl called glycols

35. 4.4 Nomenclature of cycloalkanes
4.4A Monocyclic compounds (单环化合物）

36. Naming substituted cycloalkanes

38. Problem 4.5 Give names for the following substituted cycloalkanes

39. Problem 4.5 Answers 1-tert-butyl-3-methylcyclohexane
1,3-dimethylcyclobutane
1-butylcyclohexane
1-Chloro-2,4-dimethylcyclohexane
2-Chlorocyclopentanol
3-(1,1-Dimethylethyl)cyclohexanol or 3-tert-butylcyclohexanol

40. 4.4B Bicyclic compounds
We name compounds containing two fused or bridged rings as bicycloalkanes and we use the name of the alkane corresponding to the total number of carbon atoms in the rings as the parent name

42. If substituents are present, we number the bridged ring system beginning at one bridgehead first
(1,7-二甲基二环[2.2.1]庚烷） （1-甲基-2-乙基二环[1.1.0]丁烷）

43. If substituents are present, we number from large ring to small ring

44. 3-Ethyl-8-methylbicyclo[4.3.0]nonane
3-乙基-8-甲基-二环[4.3.0]壬烷

45. Problem 4.6 Give names for each of the following bicyclic alkanes:

46. Answers A Bicyclo[2.2.0]hexane B Bicyclo[4.4.0]decane
C bicyclo[2.2.2]octane
D methylbicyclo[3.2.0]heptane
E methylbicyclo[4.2.1]nonane

47. (F) write the structure of a bicyclic compound that is an isomer of bicyclo[2.2.1]heptane and give its name
Bicyclo[2.2.1]heptane Bicyclo[3.2.0]heptane
3. Bicyclo[3.1.1]heptane Bicyclo[4.1.0]heptane

48. 4.5 Physical properties of alkanes and cycloalkanes
Boiling points
Melting points
Density
Solubility

49. 4.6 Sigma bonds and bond rotation
Groups bonded only by a sigma bond can undergo rotation about that bond with respect to each other. The temporary molecular shapes that result from rotations of groups about single bonds are called conformation of the molecule.
An analysis of the energy changes that a molecule undergoes as
groups rotate about single bonds is called a conformational analysis.

50. 4.6.1 Let us consider the ethane molecule as an example

51. Ethane 乙烷 ：
书写：

52. 4.6.3 锯架（sawhorse formula)透视式及书写

53. Newman Projection formula 纽曼投影式

54. Newman Projection formula 纽曼投影式

55. 纽曼投影式(Newman projection)及书写

56. 模型
锯架式
纽曼式
楔型式 模型
锯架式
纽曼式
楔型式

57. Conformational analysis

58. 纽曼投影式(Newman projection)及书写

59. Discussion
In ethane the difference in energy between the staggered and eclipsed conformation is 2.8 kcal mol-1 (12 KJ mol-1). This small barrier to rotation is called the torsion barrier of the single bond (单键的纽转障碍）. Unless the temperature is extremely low (-250 oC) many ethane molecules will have enough energy to surmount this barrier.

60. 4.7 Conformational analysis of butane
If we consider rotation about the C2-C3 bond of butane,
torsional strain plays a part, too.

61. We should look at the important conformations of butane(四种极限构象式)
Anti 对位交叉式（反叠式）
An Eclipsed部分重叠式（反错式）
gauche邻位交叉式（顺错式）
An eclipsed全重叠式（顺叠式）

63. Discussions
The eclipsed conformations represent energy maxima in the potential energy diagram ( Torsional strain and Van der waals repulsion)
While the barriers to rotation in a butane molecule are larger than those of an ethane molecule
The stable order of conformations are;

64. 4.8 The relative stabilities of cycloalkanes: Ring strain (环张力）
4.8A Heats of combustion (燃烧热）
The heat of combustion of a compound is the enthalpy change for the complete oxidation of the compound.
For a hydrocarbon complete oxidation means converting it to carbon dioxide and water.
CH4 + 2O2 = CO2 + 2H2O kcal mol -1

65. Consider, as an example, the combustion of butane and isobutane

66. Why?
Since butane liberates more heat on combustion than isobutane, it must contain relatively more potential energy. Isobutane, therefore, must be more stable

68. 4.8 B Heats of combustion of cycloalkanes

69. Table 4.8 Ring strain of cycloalkanes

70. Results discussion
Cyclohexane has the lowest heat of combustion per CH2 group. We can assume, therefore, that cyclohexane has no ring strain.
The combustion of cyclopropane evolves the greatest amount of heat per CH2 group. Therefore, molecules of cyclopropane must have the greatest ring strain.
The combustion of cyclobutane evolves the second largest amount of heat per CH2 group and, therefore, cyclobutane has the second largest amount of ring strain
Cyclopentane and cycloheptane have about the same modest amount of ring strain.

71. 4.9 The origin of ring strain in cyclopropane and cyclobutane: Angle strain and torsional strain
The normal tetrahedral bond angle of an sp3-hybridized atom is 109.5o.
How about angle of the cyclopropane and cyclobutane ?

72. Cyclopropane

73. Orbital overlap is less effective in cyclopropane
这种由于键角偏离正常键角而引起的
张力---角张力（angle strain）

74. A Newman projection formula as viewed along one carbon-carbon bond shows the eclipsed hydrogens

75. Cyclobutane also has considerable angle strain

76. The ‘Bent’ or ‘envelope’ form of cyclopentane
The internal angles of a regular pentagon are 108o, a value very close to the normal tetrahedral bond angles of 109.5o. Therefore, if cyclopentane molecules were planar, they would have very little angle strain

77. 4.10 Conformations of cyclohexane
1. Chair conformation 椅式构象
The carbon-carbon bond angles are all 109.5o
It is no angle strain
The chair conformation is free of torsional strain
The hydrogen atoms at opposite corners of the cyclohexane ring are maximally separated in chair conformations
There are two of conformations (chair and boat)
The chair form is most stable conformation

78. 2. The conformation of cyclohexane环己烷的构象
1、椅式构象（图）
Axial bond up直立键（a键）：
    6个（3上、3下）
Equatorial 平伏键（e键）：

79. 3. 特点（图）
相邻原子： 交叉式
非键合原子：
交叉式
因此，椅式构象稳定

80. 4. How to write the chair conformation?

81. 5. The inversion of chair conformation 构象翻转

82. 6. The boat conformation 船式构象
相邻原子： 重叠式
非键合原子：
重叠式
因此，船式构象不稳定

84. The inversion of conformations

85. Fig. 4.17 The relative energies of the various conformations of
Cyclohexane (more than 99% of the molecules are chair conformation)

86. 4.11 Substituted cyclohexanes: Axial (直立键 a-键）and equatorial (平伏键e-键）hydrogen atoms

87. The conformations analysis of a substituent cyclohexane

88. The conformations analysis of methylcyclohexane

89. The conformation of tert-butylcyclohexane

90. The conformations analysis of tert-butylcyclohexane

91. 4.12 Disubstituted cycloalkanes: Cis-trans isomerism
1. Cis-and trans-1,2-dimethylcyclopentanes
Cis-1,2-Dimethylcyclopentane trans-1,2-Dimethylcyclopentane
bp, 99.5 oC bp, 91.9oC

92. 2. 1,3-Dimethylcyclopentanes
Cis-1,3-Dimethylcyclohexane Trans-1,3-Dimethylcyclohexane
bp, oC bp, oC

93. 3. 1,3-Dichlorocyclopentanes
Cis-1,3-Dichlorocyclopentane trans-1,3-Dichlorocyclopentane

94. Problem Write structures for the cis and trans isomers of (a) 1,2-dimethylcyclopropane and (b) 1,2-dibromocyclobutane

95. 4. 1,2-, 1,3-, and 1,4-Dimethylcyclohexane
Cis-1,2-Dimethylcyclohexane trans-1,2-Dimethylcyclohexane

96. 1,3-Dimethylcyclohexane
Cis-1,3-Dimethylcyclohexane trans-1,3-Dimethylcyclohexane

97. 1,4-Dimethylcyclohexane
Cis-1,4-Dimethylcyclohexane trans-1,4-Dimethylcyclohexane

98. 4.12A Cis-trans isomerism and conformational structures
trans-1,4-Dimethylcyclohexane

99. Cis-1,4-Dimethylcyclohexane

100. Cis-1,4-Dimethylcyclohexane

101. Sample problem; Consider each of the following conformational structures and tell whether each is cis or trans
cis-1,2-Dichlorocyclohexane b. cis-1,3-Dichlorocyclohexane
c. trans-1,2-Dichlorocyclohexane

102. Problem (a) Write structural formulas for the two chair conformations of cis-1-tert-butyl-4-methylcyclohexane. (b) Are these two conformations equivalent? (d) Which would be the preferred conformation at equilibrium?

103. Cis-1-tert-butyl-4-methylcyclohexane

104. Trans-1,3-Dimethylcyclohexane

105. The large group puts in the equatorial position as soon as possible

106. Cis-1-tert-butyl-3-methylcyclohexane

107. Problem (a) Write chair conformation for cis- and trans-1,2-dimethylcyclohexane. (b) For which isomer (cis or trans) are the two conformations equivalent? (c) For the isomer where the two conformations are not equivalent, which conformation is more stable? (d) Which conformation would be more highly populated at equilibrium?

108. (a) Conformations of cis isomer are equivalent (e,a) and (a,e)

109. (b) Conformations of trans isomer are not equivalent, (e, e) and (a, a)
( c ) The trans (e, e) conformation is more stable than the trans (a, a)
( d ) The trans (e, e) would be more highly populated at equilibrium

110. 4.13 Bicyclo and polycyclic alkanes

111. Decalin shows cis-trans conformations isomerism

112. Adamantane (金刚烷） is a tricyclic system that contains a three dimensional array of cyclohexane

114. 4.14 Chemical reactions of alkanes
Alkanes, as a class, are characterized by a general inertness to many chemical reagents.
Carbon-carbon and carbon-hydrogen bonds are quite strong.
Carbon and hydrogen atoms have nearly the same electronegativity, the carbon-hydrogen bonds of alkanes are only slightly polarized
It is of lower reactivity of alkanes toward many reagents.
When heated, alkanes also react with chlorine and bromine, and they react explosively with fluorine

115. 4.15 Synthesis of alkanes and cycloalkanes
4.15A Hydrogenation of alkenes (烯烃的氢化）

116. Specific examples

118. Problem Three different alkenes will react with hydrogen in the presence of a platinum or nickel catalyst to yield butane. What are their structures? Show the reactions.

120. 4.15B Reduction of alkyl halides (卤代烷烃的还原）
Most alkyl halides react with zinc (锌）and aqueous acid to produce an alkane. In this reaction zinc acts as a reducing agent （还原剂）and causes the halogen of the alkyl halide to be replaced by hydrogen.

121. Specific example

122. Problem In addition to isopentyl bromide, three other alkyl halides will yield isopentane when they are treated with zinc and aqueous acid. What are their structures? Show the reactions.

124. 4.15C Lithium Dialkylcuprates (二烷基铜锂）; The Corey-Posner, Whitesides-House synthesis
The overall synthesis provides, for example, a way for coupling the alkyl groups of two alkyl halides to produce an alkane:

127. 4.16 Planning organic synthesis
In Planning syntheses we are required to think backward, to work our way backwards from relatively complex molecules to simpler ones that will act as the precursor（前体）（ for our target molecule. We carry out what is called a retrosynthetic analysis (逆合成分析 或反向合成分析）。

130. Problem Outline a synthesis of each of the following alkanes from appropriate alkyl halides using the Corey-Posner ect.
(a) Propane (b) Butane © 2-Methylbutane (d) 2,7-Dimethyloctane (e) Ethylcyclohexane (f) Isopropylcyclopentane (g) 3-Methylcyclopentene

132. Problem 4.14 Outline methods showing how hexane could be prepared starting with:
(a) A bromopropane (b) A bromobutane
(C) A bromopentane (d) A bromohexane
(e) A hexane

133. Synthesis of hexane

134. 4.17 Some important terms and concepts
That is very important terms and concepts, you should remember all of them.

135. Homeworks
4.19, 4.30, 4.31, 4.32, 4.36
Problem 4.14