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

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

3. Alkane bonds

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

5. Names and Formulas of the First Ten Unbranched Alkanes
Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Names and Formulas of the First Ten Unbranched Alkanes

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

7. Alkanes All alkanes fit the general molecular formula CnH2n+2
Unbranched alkanes are called normal alkanes,
or n-alkanes.
-CH2- group is called a methylene group.

8. IUPAC Rules for naming Alkane
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.

12. Alkyl and Halogen Substituents
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. 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).
Halogen substituents are named by changing the -ine ending of the element to -o.

14. 2,2,4-trimethylpentane

15. Examples of Use of the IUPAC Rules

16. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Rule (Cont’d)
NOT
NOT

17. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Rule (Cont’d)
When two substituents are present on the same carbon, use that number twice

18. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Rule (Cont’d)
For identical substituents, use prefixes di-, tri-, tetra- and so on
NOT
NOT

19. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Rule (Cont’d)
When two chains of equal length compete for selection as parent chain, choose the chain with the greater number of substituents
NOT

20. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Rule (Cont’d)
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
NOT

21. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 1
Find the longest chain as parent

22. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 1 (Cont’d)
Use the lowest numbering for substituents
Substituents: two methyl groups
dimethyl

23. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 1 (Cont’d)
Complete name

24. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2

25. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2 (Cont’d)
Find the longest chain as parent

26. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2 (Cont’d)
Find the longest chain as parent
⇒ Nonane as parent

27. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2 (Cont’d)
Use the lowest numbering for substituents

28. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2 (Cont’d)
Substituents
3,7-dimethyl
4-ethyl

29. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2 (Cont’d)
Substituents in alphabetical order
Ethyl before dimethyl (recall Rule 4 – disregard “di”)
Complete name

30. 3C. How to Name Branched Alkyl Groups
Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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

31. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Four-carbon groups

32. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
A neopentyl group

33. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 1

34. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 1 (Cont’d)
Find the longest chain as parent
6-carbon
chain
7-carbon
chain
8-carbon
chain
9-carbon
chain

35. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 1 (Cont’d)
Find the longest chain as parent
⇒ Nonane as parent

36. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 1 (Cont’d)
Use the lowest numbering for substituents
5,6
4,5
(lower numbering)
⇒ Use 4,5

37. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 1 (Cont’d)
Substituents
Isopropyl
tert-butyl
⇒ 4-isopropyl and 5-tert-butyl

38. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 1 (Cont’d)
Alphabetical order of substituents
tert-butyl before isopropyl
Complete name

39. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2

40. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2 (Cont’d)
Find the longest chain as parent
8-carbon
chain
9-carbon
chain
⇒ Decane as parent
10-carbon
chain

41. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2 (Cont’d)

42. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2 (Cont’d)
Use the lowest numbering for substituents
5,6
5,6
⇒ Determined using the next Rules

43. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2 (Cont’d)
Substituents
sec-butyl
Neopentyl
But is it
5-sec-butyl and 6-neopentyl or
5-neopentyl and 6-sec-butyl ?

44. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2 (Cont’d)
Since sec-butyl takes precedence over neopentyl
5-sec-butyl and 6-neopentyl
Complete name

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

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. Boiling points of the normal alkanes In Isomers

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

49. Pentane: ball-and-stick model

50. 2,2-Dimethylpropane space filling model

51. Pentane: space filling model

52. 2,2-Dimethylpropane dash-wedge model

53. Pentane: dash-wedge model

54. 3E. How to Name Alkyl Halides
Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
3E. How to Name Alkyl Halides
Rules
Halogens are treated as substituents (as prefix)
F: fluoro Br: bromo
Cl: chloro I: iodo
Similar rules as alkyl substituents

55. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Examples

56. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
3F. How to Name Alcohols
IUPAC substitutive nomenclature: a name may have as many as four features
Locants, prefixes, parent compound, and suffixes

57. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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

58. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Examples

59. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 4

60. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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

61. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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) 6
⇒ Use 2

62. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 4 (Cont’d)
Parent and suffix
2-Heptanol
Substituents
Propyl
Complete name
3-Propyl-2-heptanol

63. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
How to Name Cycloalkanes
4A. How to Name Monocyclic Cycloalkanes
Cycloalkanes with only one ring
Attach the prefix cyclo-

64. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Substituted cycloalkanes

65. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 1
Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid

66. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2
(lowest numbers of substituents are 1,2,4 not 1,3,4)

67. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 3
(the carbon bearing the OH should have the lowest numbering, even though 1,2,4 is lower than 1,3,4)

68. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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

69. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
4B. How to Name Bicyclic Cycloalkanes
Bicycloalkanes
Alkanes containing two fused or bridged rings
Total # of carbons = 7
Bicycloheptane
Bridgehead

70. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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

71. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Other examples

72. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
How to Name Alkenes & Cycloalkenes
Rule
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

73. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Rule
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

74. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Examples

75. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Rule
Indicate the locations of the substituent groups by the numbers of the carbon atoms to which they are attached
Examples

76. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Examples (Cont’d)

77. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Rule
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

78. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example

79. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Rule
Name compounds containing a C=C and an alcohol group as alkenols (or cycloalkenols) and give the alcohol carbon the lower number
Examples

80. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Examples (Cont’d)

81. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Rule
Vinyl group & allyl group

82. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Rule
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

83. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example

84. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example (Cont’d)

85. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example (Cont’d)
Complete name

86. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
How to Name Alkynes
Alkynes are named in much the same way as alkenes, but ending name with –yne instead of –ene
Examples

87. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Examples (Cont’d)

88. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
OH group has priority over C≡C

89. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Physical Properties of Alkanes & Cycloalkanes
Boiling points & melting points

90. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
C6H14 Isomer
Boiling Point (oC)
68.7
63.3
60.3 58
49.7

91. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Physical Constants of Cycloalkanes
# of C Atoms
Name
bp (oC)
mp (oC)
Density
Refractive Index 3
Cyclopropane
-33
-126.6 - 4
Cyclobutane 13
-90
1.4260 5
Cyclopentane 49
-94
0.751
1.4064 6
Cyclohexane 81
6.5
0.779
1.4266 7
Cycloheptane
118.5
-12
0.811
1.4449 8
Cyclooctane
149
13.5
0.834

92. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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

93. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
8A. Newman Projections & How to Draw Them

94. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
8B. How to Do a Conformational Analysis
f1 = 60o
f2 = 180o

95. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
f = 0o

96. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid

97. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Conformational Analysis of Butane

99. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid

100. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid

101. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid

102. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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

103. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
10A. Cyclopropane
sp3 hybridized carbon
(normal tetrahedral bond angle is 109.5o) q
Internal bond angle (q) ~60o (~49.5o deviated from the ideal tetrahedral angle)

104. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid

105. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
10B. Cyclobutane q
Internal bond angle (q) ~88o (~21o deviated from the normal 109.5o tetrahedral angle)

106. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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 90o instead of 88o), but torsional strain would be considerably larger because all eight C–H bonds would be eclipsed

107. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
10C. Cyclopentane
If cyclopentane were planar, q ~108o, very close to the normal tetrahedral angle of 109.5o
However, planarity would introduce considerable torsional strain (i.e. 10 C–H bonds eclipsed)
Therefore cyclopentane has a slightly bent conformation

108. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Conformations of Cyclohexane: The Chair & the Boat

109. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
The boat conformer of cyclohexane is less stable (higher energy) than the chair form due to
Eclipsed conformation
1,4-flagpole interactions

110. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
The twist boat conformation has a lower energy than the pure boat conformation, but is not as stable as the chair conformation

111. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Energy diagram

112. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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

113. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
12A. How to Draw Chair Conformational Structures
When you draw chair conformational structures, try to make the corresponding bonds parallel in your drawings

114. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Axial hydrogen atoms in chair form
The axial bonds are all either up or down, in a vertical orientation

115. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Equatorial hydrogen atoms in chair form
The equatorial bonds are all angled slightly

116. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
12B. A Conformational Analysis of Methylcyclohexane
Substituted cyclohexane
Two different chair forms

117. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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

118. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
At 25oC G
% of Equatorial
% of Axial F 60 40
CH3 95 5
iPr 97 3
tBu
> 99.99
< 0.01

119. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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

120. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Disubstituted Cycloalkanes: Cis-Trans Isomerism

121. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
13A. Cis-Trans Isomerism and Conformation Structures of Cyclohexanes
Trans-1,4-Disubstituted Cyclohexanes

122. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Upper bond
Lower bond
Upper-lower bonds means the groups are trans

123. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Cis-1,4-Disubstituted Cyclohexanes

124. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Cis-1-tert-Butyl-4-methylcyclohexane

125. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Trans-1,3-Disubstituted Cyclohexanes

126. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Trans-1-tert-Butyl-3-methylcyclohexane

127. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Cis-1,3-Disubstituted Cyclohexanes

128. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Trans-1,2-Disubstituted Cyclohexanes

129. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Cis-1,2-Disubstituted Cyclohexane

130. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Bicyclic & Polycyclic Alkanes

131. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
C60 (Buckminsterfullerene)

132. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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

133. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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)

134. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Synthesis of Alkanes and Cycloalkanes
16A. Hydrogenation of Alkenes & Alkynes

135. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Examples
Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid

136. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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)

137. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Index of hydrogen deficiency (Cont’d)
Saturated acyclic alkanes: CnH2n+2
Each double bond or ring: 2 hydrogens less
Each double bond or ring provides one unit of hydrogen deficiency

138. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
e.g.
Hexane: C6H14
C6H12
C6H14
– C6H12
Index of hydrogen
deficiency (IHD) = H2
= one pair of H2
= 1

139. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Examples
IHD = 2
IHD = 3
IHD = 2
IHD = 4

140. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
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

141. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 1: IHD of C4H6Cl2
Count Cl as H
C4H6Cl2 ⇒ C4H8
A C4 acyclic alkane:
C4H2(4)+2 = C4H10
C4H10
– C4H8 H2
IHD of C4H6Cl2 =
one pair of H2 = 1
Possible structures

142. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 2: IHD of C5H8O
Ignore oxygen
C5H8O ⇒ C5H8
A C5 acyclic alkane:
C5H2(5)+2 = C5H12
C5H12
– C5H8 H4
IHD of C4H6Cl2 =
two pairs of H2 = 2
Possible structures

143. Organic Chemistry-1 CHEM 231, Prepared by. Dr. Khalid Ahmad Shadid
Example 3: IHD of C5H7N
Subtract 1 H for each N
C5H7N ⇒ C5H6
A C5 acyclic alkane:
C5H2(5)+2 = C5H12
C5H12
– C5H6 H6
IHD of C4H6Cl2 =
three pair of H2 = 3
Possible structures

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.