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2 | 1 Chapter 2: Alkanes and Cycloalkanes; Conformational and Geometric Isomerism.

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Presentation on theme: "2 | 1 Chapter 2: Alkanes and Cycloalkanes; Conformational and Geometric Isomerism."— Presentation transcript:

1 2 | 1 Chapter 2: Alkanes and Cycloalkanes; Conformational and Geometric Isomerism

2 Alkanes AlkanesAlkanes are saturated hydrocarbons, containing only carbon–carbon single bonds. CycloalkanesCycloalkanes contain rings. Unsaturated hydrocarbons Aromatic hydrocarbonsUnsaturated hydrocarbons contain carbon–carbon double or triple bonds. Aromatic hydrocarbons are cyclic compounds structurally related to benzene. 2 | 2

3 2 | 3 Alkane bonds

4 2 | 4 Three-dimensional models of ethane, propane, and butane.

5 2 | 5 Names and Formulas of the First Ten Unbranched Alkanes

6 2 | 6 11undecane 12dodecane 13tridecane 14tetradecane 15pentadecane 16hexadecane 17heptadecane 18octadecane 19nonadecane 20icosane

7 2 | 7 All alkanes fit the general molecular formula C n H 2n+2 Unbranched alkanes are called normal alkanes, or n-alkanes. -CH 2 - group is called a methylene group. Alkanes

8 IUPAC Rules for naming Alkane 2 | 8 The root name of an alkane is that of the longest continuous chain of carbon atoms. Substituents are groups attached to the main chain of a molecule. Saturated substituents containing only C and H are called alkyl groups. The one-carbon alkyl group derived from methane is called a methyl group.

9 2 | 9

10 2 | 10

11 2 | 11

12 Alkyl and Halogen Substituents 2 | 12 The two-carbon alkyl group is the ethyl group. The propyl group and the isopropyl group are three-carbon groups attached to the main chain by the first and second carbons, respectively.

13 2 | 13 R is the general symbol for an alkyl group. The formula R-H herefore represents any alkane, The formula R-Cl stands for any alkyl chloride (methyl chloride, ethyl chloride, and so on). ine o Halogen substituents are named by changing the -ine ending of the element to -o.

14 2 | 14 2,2,4-trimethylpentane

15 2 | 15 Examples of Use of the IUPAC Rules

16 NOT  Rule (Cont’d) 2 | 16

17 5.When two substituents are present on the same carbon, use that number twice  Rule (Cont’d) 2 | 17

18 6.For identical substituents, use prefixes di-, tri-, tetra- and so on  Rule (Cont’d) NOT 2 | 18

19 7.When two chains of equal length compete for selection as parent chain, choose the chain with the greater number of substituents  Rule (Cont’d) NOT 2 | 19

20 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  Rule (Cont’d) NOT 2 | 20

21  Example 1 ● Find the longest chain as parent 2 | 21

22  Example 1 (Cont’d) ● Substituents: two methyl groups  dimethyl ● Use the lowest numbering for substituents 2 | 22

23  Example 1 (Cont’d) ● Complete name 2 | 23

24  Example 2 2 | 24

25  Example 2 (Cont’d) ● Find the longest chain as parent 2 | 25

26  Example 2 (Cont’d) ● Find the longest chain as parent ⇒ Nonane as parent 2 | 26

27  Example 2 (Cont’d) ● Use the lowest numbering for substituents 2 | 27

28  Example 2 (Cont’d) ● Substituents  3,7-dimethyl  4-ethyl 2 | 28

29  Example 2 (Cont’d) ● Substituents in alphabetical order  Ethyl before dimethyl (recall Rule 4 – disregard “di”) ● Complete name 2 | 29

30 3C.How to Name Branched Alkyl Groups  For alkanes with more than two carbon atoms, more than one derived alkyl group is possible  Three-carbon groups 2 | 30

31  Four-carbon groups 2 | 31

32  A neopentyl group 2 | 32

33  Example 1 2 | 33

34  Example 1 (Cont’d) ● Find the longest chain as parent 6-carbon chain 7-carbon chain 8-carbon chain 9-carbon chain 2 | 34

35 ⇒ Nonane as parent  Example 1 (Cont’d) ● Find the longest chain as parent 2 | 35

36  Example 1 (Cont’d) ● Use the lowest numbering for substituents 5,64,5 (lower numbering) ⇒ Use 4,5 2 | 36

37  Example 1 (Cont’d) ● Substituents  Isopropyl  tert-butyl ⇒ 4-isopropyl and 5-tert-butyl 2 | 37

38  Example 1 (Cont’d) ● Alphabetical order of substituents  tert-butyl before isopropyl ● Complete name 2 | 38

39  Example 2 2 | 39

40  Example 2 (Cont’d) ● Find the longest chain as parent 8-carbon chain 9-carbon chain 10-carbon chain ⇒ Decane as parent 2 | 40

41  Example 2 (Cont’d) 2 | 41

42  Example 2 (Cont’d) ● Use the lowest numbering for substituents 5,6 ⇒ Determined using the next Rules 5,6 2 | 42

43  Example 2 (Cont’d) ● Substituents  sec-butyl  Neopentyl But is it ● 5-sec-butyl and 6-neopentyl or ● 5-neopentyl and 6-sec-butyl ? 2 | 43

44  Example 2 (Cont’d) ● Since sec-butyl takes precedence over neopentyl  5-sec-butyl and 6-neopentyl ● Complete name 2 | 44

45 Physical Properties of Alkanes and Nonbonding Intermolecular Interactions 2 | 45 Hydrogen Bonding: (a) polar water molecule: ball-and- stick model Hydrogen Bonding: (b) hydrogen bonding between water molecules

46 2 | 46 Molecules with partially positive and partially negative ends Van der Waals attractions. Hydrogen bonding and van der Waals attractions are nonbonding intermolecular interactions.

47 2 | 47 Boiling points of the normal alkanes In Isomers

48 2 | 48 2,2-Dimethylpropane ball-and-stick model

49 2 | 49 Pentane: ball-and-stick model

50 2 | 50 2,2-Dimethylpropane space filling model

51 2 | 51 Pentane: space filling model

52 2 | 52 2,2-Dimethylpropane dash-wedge model

53 2 | 53 Pentane: dash-wedge model

54 3E.How to Name Alkyl Halides  Rules ● Halogens are treated as substituents (as prefix) F: fluoroBr: bromo Cl: chloroI: iodo ● Similar rules as alkyl substituents 2 | 54

55  Examples 2 | 55

56 3F.How to Name Alcohols  IUPAC substitutive nomenclature: a name may have as many as four features ● Locants, prefixes, parent compound, and suffixes 2 | 56

57  Rules ● Select the longest continuous carbon chain to which the hydroxyl is directly attached. Change the name of the alkane corresponding to this chain by dropping the final –e and adding the suffix –ol ● Number the longest continuous carbon chain so as to give the carbon atom bearing the hydroxyl group the lower number. Indicate the position of the hydroxyl group by using this number as a locant 2 | 57

58  Examples 2 | 58

59  Example 4 2 | 59

60  Example 4 (Cont’d) ● Find the longest chain as parent Longest chain but does not contain the OH group 7-carbon chain containing the OH group ⇒ Heptane as parent 2 | 60

61  Example 4 (Cont’d) ● Use the lowest numbering for the carbon bearing the OH group 2 (lowest number of the carbon bearing the OH group) ⇒ Use | 61

62  Example 4 (Cont’d) ● Parent and suffix  2-Heptanol ● Substituents  Propyl ● Complete name  3-Propyl-2-heptanol 2 | 62

63 4.How to Name Cycloalkanes 4A.How to Name Monocyclic Cycloalkanes  Cycloalkanes with only one ring ● Attach the prefix cyclo- 2 | 63

64  Substituted cycloalkanes 2 | 64

65  Example 1 2 | 65

66  Example 2 (lowest numbers of substituents are 1,2,4 not 1,3,4) 2 | 66

67  Example 3 (the carbon bearing the OH should have the lowest numbering, even though 1,2,4 is lower than 1,3,4) 2 | 67

68  Cycloalkylalkanes ● When a single ring system is attached to a single chain with a greater number of carbon atoms ● When more than one ring system is attached to a single chain 2 | 68

69 4B.How to Name Bicyclic Cycloalkanes  Bicycloalkanes ● Alkanes containing two fused or bridged rings  Total # of carbons = 7 ● Bicycloheptane  Bridgehead 2 | 69

70  Example (Cont’d)  Between the two bridgeheads ● Two-carbon bridge on the left ● Two-carbon bridge on the right ● One-carbon bridge in the middle  Complete name ● Bicyclo[2.2.1]heptane 2 | 70

71  Other examples 2 | 71

72 5.How to Name Alkenes & Cycloalkenes  Rule 1.Select the longest chain that contains C=C as the parent name and change the name ending of the alkane of identical length from –ane to –ene 2 | 72

73  Rule 2.Number the chain so as to include both carbon atoms of C=C, and begin numbering at the end of the chain nearer C=C. Assign the location of C=C by using the number of the first atom of C=C as the prefix. The locant for the alkene suffix may precede the parent name or be placed immediately before the suffix 2 | 73

74 ● Examples 2 | 74

75  Rule 3.Indicate the locations of the substituent groups by the numbers of the carbon atoms to which they are attached ● Examples 2 | 75

76 ● Examples (Cont’d) 2 | 76

77  Rule 4.Number substituted cycloalkenes in the way that gives the carbon atoms of C=C the 1 and 2 positions and that also gives the substituent groups the lower numbers at the first point of difference 2 | 77

78 ● Example 2 | 78

79  Rule 5.Name compounds containing a C=C and an alcohol group as alkenols (or cycloalkenols) and give the alcohol carbon the lower number ● Examples 2 | 79

80 ● Examples (Cont’d) 2 | 80

81  Rule 6.Vinyl group & allyl group 2 | 81

82  Rule 7.Cis vs. Trans ● Cis: two identical or substantial groups on the same side of C=C ● Trans: two identical or substantial groups on the opposite side of C=C 2 | 82

83  Example 2 | 83

84  Example (Cont’d) 2 | 84

85  Example (Cont’d) ● Complete name 2 | 85

86 6.How to Name Alkynes  Alkynes are named in much the same way as alkenes, but ending name with –yne instead of –ene  Examples 2 | 86

87  Examples (Cont’d) 2 | 87

88  OH group has priority over C≡C 2 | 88

89 7.Physical Properties of Alkanes & Cycloalkanes  Boiling points & melting points 2 | 89

90 C 6 H 14 IsomerBoiling Point ( o C)

91 Physical Constants of Cycloalkanes # of C AtomsNamebp ( o C)mp ( o C)Density Refractive Index 3Cyclopropane Cyclobutane Cyclopentane Cyclohexane Cycloheptane Cyclooctane

92 8.Sigma Bonds & Bond Rotation  Two groups bonded by a single bond can undergo rotation about that bond with respect to each other ● Conformations – temporary molecular shapes that result from a rotation about a single bond ● Conformer – each possible structure of conformation ● Conformational analysis – analysis of energy changes that occur as a molecule undergoes rotations about single bonds 2 | 92

93 8A.Newman Projections & How to Draw Them 2 | 93

94  1 = 60 o  2 = 180 o 8B.How to Do a Conformational Analysis 2 | 94

95  = 0 o 2 | 95

96 2 | 96

97 9.Conformational Analysis of Butane 2 | 97

98 2 | 98

99 2 | 99

100 2 | 100

101 0o0o 180 o 60 o 2 | 101

102 10.The Relative Stabilities of Cycloalkanes: Ring Strain  Cycloalkanes do not have the same relative stability due to ring strain  Ring strain comprises: ● Angle strain – result of deviation from ideal bond angles caused by inherent structural constraints ● Torsional strain – result of dispersion forces that cannot be relieved due to restricted conformational mobility 2 | 102

103 10A.Cyclopropane sp 3 hybridized carbon (normal tetrahedral bond angle is o )  Internal bond angle (  ) ~60 o (~49.5 o deviated from the ideal tetrahedral angle)  2 | 103

104 2 | 104

105 10B.Cyclobutane  Internal bond angle (  ) ~88 o (~21 o deviated from the normal o tetrahedral angle)  2 | 105

106  Cyclobutane ring is not planar but is slightly folded.  If cyclobutane ring were planar, the angle strain would be somewhat less (the internal angles would be 90 o instead of 88 o ), but torsional strain would be considerably larger because all eight C–H bonds would be eclipsed 2 | 106

107 10C.Cyclopentane  If cyclopentane were planar,  ~108 o, very close to the normal tetrahedral angle of o  However, planarity would introduce considerable torsional strain (i.e. 10 C–H bonds eclipsed)  Therefore cyclopentane has a slightly bent conformation 2 | 107

108 11.Conformations of Cyclohexane: The Chair & the Boat 2 | 108

109  The boat conformer of cyclohexane is less stable (higher energy) than the chair form due to ● Eclipsed conformation ● 1,4-flagpole interactions 2 | 109

110  The twist boat conformation has a lower energy than the pure boat conformation, but is not as stable as the chair conformation 2 | 110

111  Energy diagram 2 | 111

112 12.Substituted Cyclohexanes: Axial & Equatorial Hydrogen Groups  The six-membered ring is the most common ring found among nature’s organic molecules  The chair conformation of a cyclohexane ring has two distinct orientations for the bonds that project from the ring: axial and equatorial 2 | 112

113 12A.How to Draw Chair Conformational Structures  When you draw chair conformational structures, try to make the corresponding bonds parallel in your drawings 2 | 113

114  Axial hydrogen atoms in chair form The axial bonds are all either up or down, in a vertical orientation 2 | 114

115  Equatorial hydrogen atoms in chair form The equatorial bonds are all angled slightly 2 | 115

116 12B.A Conformational Analysis of Methylcyclohexane  Substituted cyclohexane Two different chair forms 2 | 116

117  The chair conformation with axial G is less stable due to 1,3-diaxial interaction  The larger the G group, the more severe the 1,3-diaxial interaction and shifting of the equilibrium from the axial-G chair form to the equatorial-G chair form 2 | 117

118 At 25 o C G% of Equatorial% of Axial F6040 CH i Pr973 t Bu> 99.99< | 118

119 12C.1,3-Diaxial Interactions of a tert-Butyl Group  The chair conformation with axial tert-butyl group is less stable due to 1,3-diaxial interaction 1,3-diaxial interaction 2 | 119

120 13.Disubstituted Cycloalkanes: Cis-Trans Isomerism 2 | 120

121 13A.Cis-Trans Isomerism and Conformation Structures of Cyclohexanes  Trans-1,4-Disubstituted Cyclohexanes 2 | 121

122 Upper bond Lower bond  Upper-lower bonds means the groups are trans 2 | 122

123  Cis-1,4-Disubstituted Cyclohexanes 2 | 123

124  Cis-1-tert-Butyl-4-methylcyclohexane 2 | 124

125  Trans-1,3-Disubstituted Cyclohexanes 2 | 125

126  Trans-1-tert-Butyl-3-methylcyclohexane 2 | 126

127  Cis-1,3-Disubstituted Cyclohexanes 2 | 127

128  Trans-1,2-Disubstituted Cyclohexanes 2 | 128

129  Cis-1,2-Disubstituted Cyclohexane 2 | 129

130 14.Bicyclic & Polycyclic Alkanes 2 | 130

131 C 60 (Buckminsterfullerene) 2 | 131

132 15.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; they do not break unless alkanes are heated to very high temperatures 2 | 132

133  Because carbon and hydrogen atoms have nearly the same electronegativity, the carbon–hydrogen bonds of alkanes are only slightly polarized  This low reactivity of alkanes toward many reagents accounts for the fact that alkanes were originally called paraffins (parum affinis, Latin: little affinity) 2 | 133

134 16.Synthesis of Alkanes and Cycloalkanes 16A. Hydrogenation of Alkenes & Alkynes 2 | 134

135  Examples 2 | 135

136 17.How to Gain Structural Inform- ation from Molecular Formulas & Index of Hydrogen Deficiency  Index of hydrogen deficiency (IHD) ● The difference in the number of pairs of hydrogen atoms between the compound under study and an acyclic alkane having the same number of carbons ● Also known as “degree of unsaturation” or “double-bond equivalence” (DBE) 2 | 136

137  Index of hydrogen deficiency (Cont’d) ● Saturated acyclic alkanes: C n H 2n+2 ● Each double bond or ring: 2 hydrogens less ● Each double bond or ring provides one unit of hydrogen deficiency 2 | 137

138  e.g. Hexane: C 6 H 14 Index of hydrogen deficiency (IHD) = – C 6 H 12 C 6 H 14 H2H2 = one pair of H 2 = 1 C 6 H 12 2 | 138

139  Examples IHD = 2IHD = 3 IHD = 2IHD = 4 2 | 139

140 17A.Compounds Containing Halogen, Oxygen, or Nitrogen  For compounds containing ● Halogen – count halogen atoms as though they were hydrogen atoms ● Oxygen – ignore oxygen atoms and calculate IHD from the remainder of the formula ● Nitrogen – subtract one hydrogen for each nitrogen atom and ignore nitrogen atoms 2 | 140

141  Example 1:IHD of C 4 H 6 Cl 2 ● Count Cl as H  C 4 H 6 Cl 2 ⇒ C 4 H 8 ● A C 4 acyclic alkane: C 4 H 2(4)+2 = C 4 H 10 IHD of C 4 H 6 Cl 2 = – C 4 H 8 C 4 H 10 H2H2 one pair of H 2 = 1 ● Possible structures 2 | 141

142  Example 2:IHD of C 5 H 8 O ● Ignore oxygen C5H8O ⇒ C5H8C5H8O ⇒ C5H8 ● A C 5 acyclic alkane: C 5 H 2(5)+2 = C 5 H 12 IHD of C 4 H 6 Cl 2 = – C 5 H 8 C 5 H 12 H4H4 two pairs of H 2 = 2 ● Possible structures 2 | 142

143  Example 3:IHD of C 5 H 7 N ● Subtract 1 H for each N  C 5 H 7 N ⇒ C 5 H 6 ● A C 5 acyclic alkane: C 5 H 2(5)+2 = C 5 H 12 IHD of C 4 H 6 Cl 2 = – C 5 H 6 C 5 H 12 H6H6 three pair of H 2 = 3 ● Possible structures 2 | 143

144 2 | 144

145 2 | 145

146 Chlorination of hydrocarbons is a substitution reaction in which a chlorine atom is substituted for a hydrogen atom. Likewise in bromination reactions, a bromine atom is substituted for a hydrogen atom. 2 | 146

147 2 | 147


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