1. Chapter 4 Alcohols and Alkyl Halides
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1

2. Overview of Chapter
This chapter introduces: functional groups, nomenclature, chemical reactions and their mechanisms. Focus will be on two reactions that yield alkyl halides:
(1) alcohol + hydrogen halide
ROH + HX  RX H2O
(2) alkane + halogen
RH X2  RX HX
Both are substitution reactions. 2

3. 4.1 Functional Groups 3

4. Families of Organic Compounds and their Functional Groups
A functional group is a structural unit in a molecule responsible for its characteristic behavior under a particular set of reaction conditions.
See front flyleaf and page 139 of the text.
Alcohol R-OH
Alkyl halide R-X (X = F, Cl, Br, I)
Amine primary amine: R-NH2
secondary amine: R2NH
tertiary amine: R3N 12

5. Families of Organic Compounds and their Functional Groups
Epoxide
Ether R-O-R'
Nitrile R-C≡N
Nitroalkane R-NO2
Sulfide R-S-R'
Thiol R-SH 12

6. Many Classes of Organic Compounds Contain a Carbonyl Group (C=O)H R R' O C
Ketone
Carbonyl group
Aldehyde O C O C O C R
OR'
Ester R R OH R
NH2
Acyl group
Carboxylic acid
Amide 15

7. 4.2 IUPAC Nomenclature of Alkyl Halides3

8. IUPAC Nomenclature There are several kinds of IUPAC nomenclature.
The two that are most widely used are: functional class nomenclature (parent) substitutive nomenclature (attachment)
Both types can be applied to alcohols and alkyl halides. 4

9. Functional Class Nomenclature of Alkyl Halides
Name the alkyl group and the halogen as separate words (alkyl + halide).
CH3F
CH3CH2CH2CH2CH2Cl
Methyl fluoride
Pentyl chloride
Halide is attached to the #1 C when naming as a halide.
CH3CH2CHCH2CH2CH3 Br H I
1-Ethylbutyl bromide
Cyclohexyl iodide 4

10. Substitutive Nomenclature of Alkyl Halides
When naming as halo-substituted alkanes, number the longest chain containing the halogen in the direction that gives the lowest number to the substituted carbon.
CH3CH2CH2CH2CH2F
1-Fluoropentane
Select the longest alkane chain when naming as an alkane.
CH3CH2CHCH2CH3 I
CH3CHCH2CH2CH3 Br
3-Iodopentane
2-Bromopentane 4

11. Substitutive Nomenclature of Alkyl Halides
CH3 Cl
Halogen and alkyl groups are of equal rank when it comes to numbering the chain.
Number the chain in the direction that gives the lowest number to the group (halogen or alkyl) that appears first. Cl
CH3 5

12. Substitutive Nomenclature of Alkyl Halides
CH3 Cl
5-Chloro-2-methylheptane
List substituents alphabetically. Cl
CH3
2-Chloro-5-methylheptane 5

13. 4.3 IUPAC Nomenclature of Alcohols

14. Functional Class Nomenclature of Alcohols
Name the alkyl group and add "alcohol" as a separate word.
CH3CH2OH
CH3CCH2CH2CH3 OH
CH3
Ethyl alcohol
CH3CHCH2CH2CH2CH3 OH
1,1-Dimethylbutyl alcohol
1-Methylpentyl alcohol 4

15. Substitutive Nomenclature of Alcohols
Name as "alkanols." Replace -e ending of alkane name with -ol.
Number chain in direction that gives lowest number to the carbon that bears the —OH group.
CH3CH2OH
CH3CCH2CH2CH3 OH
CH3
Ethanol
CH3CHCH2CH2CH2CH3 OH
2-Methyl-2-pentanol
or 2-Methylpentan-2-ol
2-Hexanol or Hexan-2-ol 4

16. Substitutive Nomenclature of Alcohols
CH3 OH
Hydroxyl groups outrank alkyl groups when it comes to numbering the chain.
Number the chain in the direction that gives the lowest number to the carbon that bears the OH group OH
CH3 5

17. Substitutive Nomenclature of Alcohols
CH3 OH
6-Methyl-3-heptanol OH
CH3
5-Methyl-2-heptanol 5

18. 4.4 Classes of Alcohols and Alkyl Halides9

19. Classification
Alcohols and alkyl halides are classified as primary secondary tertiary according to their "degree of substitution."
Degree of substitution is determined by counting the number of carbon atoms directly attached to the carbon that bears the halogen or hydroxyl group. 10

20. Classification H CH3CH2CH2CH2CH2F OH primary alkyl halide
secondary alcohol
CH3CCH2CH2CH3 OH
CH3
CH3CHCH2CH2CH3 Br
secondary alkyl halide
tertiary alcohol 4

21. 4.5 Bonding in Alcohols and Alkyl Halides9

22. alcohols and alkyl halides are polar
Dipole Moments
alcohols and alkyl halides are polar H C O + H C Cl H + + – – H H
 = 1.7 D
 = 1.9 D 12

23. alcohols and alkyl halides are polar
Dipole Moments
alcohols and alkyl halides are polar
 = 1.7 D
 = 1.9 D 12

24. Dipole-Dipole Attractive Forces
  –
  –
  –
  –
 – + 15

25. Dipole-Dipole Attractive Forces
  –
  –
  –
  –
 – + 15

26. 4.6 Physical Properties of Alcohols and Alkyl Halides: Intermolecular Forces
Boiling point
Solubility in water
Density 9

27. Effect of Structure on Boiling Point
CH3CH2CH3
CH3CH2F
CH3CH2OH
Molecular weight
Boiling point, °C
Dipole moment, D 17

28. Effect of Structure on Boiling Point
CH3CH2CH3
Molecular weight 44
Boiling point, °C -42
Dipole moment, D 0
Intermolecular forces are weak.
Only intermolecular
forces are induced
dipole-induced dipole
attractions. 17

29. Effect of Structure on Boiling Point
CH3CH2F
Molecular weight
Boiling point, °C
Dipole moment, D
A polar molecule; therefore dipole-dipole and dipole-induced dipole forces contribute to intermolecular attractions. 48
-32
1.9 17

30. Effect of Structure on Boiling Point
CH3CH2OH
Molecular weight
Boiling point, °C
Dipole moment, D
Highest boiling point; strongest intermolecular attractive forces.
Hydrogen bonding is stronger than other
dipole-dipole attractions. 46
+78
1.7 17

31. Figure 4.4 Hydrogen bonding in ethanol6

32. Boiling point increases with increasing number of halogens
Compound Boiling Point
CH3Cl -24°C
CH2Cl2 40°C
CHCl3 61°C
CCl4 77°C
Even though CCl4 is the only compound in this list without a dipole moment, it has the highest boiling point.
Induced dipole-induced dipole forces are greatest in CCl4 because it has the greatest number of Cl atoms. Cl is more polarizable than H. 6

33. But trend is not followed when halogen is fluorine
Compound Boiling Point
CH3CH2F °C
CH3CHF °C
CH3CF °C
CF3CF °C
Fluorine is not very polarizable and induced dipole- induced dipole forces decrease with increasing fluorine substitution. 6

34. Solubility in water Alkyl halides are insoluble in water.
Methanol, ethanol, isopropyl alcohol are completely miscible with water.
The solubility of an alcohol in water decreases with increasing number of carbons (compound becomes more hydrocarbon-like). 6

35. Figure 4.5 Hydrogen Bonding Between Ethanol and Water6

36. Density Alkyl fluorides and alkyl chlorides are less dense than water.
Alkyl bromides and alkyl iodides are more dense than water.
All liquid alcohols have densities of about 0.8 g/mL. 6

37. 4.7 Preparation of Alkyl Halides from Alcohols and Hydrogen Halides
ROH HX  RX H2O 9

38. Reaction of Alcohols with Hydrogen Halides
ROH HX  RX HOH
Hydrogen halide reactivity: HF HCl HBr HI
least reactive
most reactive 2

39. Reaction of Alcohols with Hydrogen Halides
ROH HX  RX HOH
Alcohol reactivity: CH3OH RCH2OH R2CHOH R3COH Methanol Primary Secondary Tertiary
least reactive
most reactive 2

40. Preparation of Alkyl Halides
25°C
(CH3)3COH + HCl
(CH3)3CCl + H2O
78-88%
73%
80-100°C OH +
HBr Br
H2O
CH3(CH2)5CH2OH + HBr
CH3(CH2)5CH2Br + H2O
87-90%
120°C 4

41. Preparation of Alkyl Halides
A mixture of sodium bromide and sulfuric acid may be used in place of HBr.
CH3CH2CH2CH2OH
CH3CH2CH2CH2Br
NaBr H2SO4
70-83%
heat 5

42. 4.8 Mechanism of the Reaction of Alcohols with Hydrogen Halides: Hammond’s Postulate9

43. A mechanism describes how reactants are converted to products.
About mechanisms
A mechanism describes how reactants are converted to products.
Mechanisms are often written as a series of chemical equations showing the elementary steps.
An elementary step is a reaction that proceeds by way of a single transition state.
Mechanisms can be shown likely to be correct, but cannot be proven correct. 7

44. About mechanisms For the reaction: (CH3)3COH + HCl (CH3)3CCl + H2O
tert-Butyl alcohol
tert-Butyl chloride
For the reaction:
(CH3)3CCl + H2O
the generally accepted mechanism involves three elementary steps.
Step 1 is a Brønsted acid-base reaction. 7

45. Step 1: Proton Transfer
Two molecules react in this elementary step; therefore it is bimolecular. .. H Cl : +
fast, bimolecular O
(CH3)3C –
tert-Butyloxonium ion
(an intermediate)
Like proton transfer from a strong acid to water, proton transfer from a strong acid to an alcohol is normally very fast. 9

46. Potential Energy Diagram for Step 1
Reaction coordinate H Cl  
(CH3)3CO
(CH3)3COH + H—Cl
+ Cl – +
(CH3)3CO H 22

47. Hammond's Postulate
If two succeeding states (such as a transition state and an unstable intermediate) are similar in energy, they are similar in structure.
Hammond's postulate permits us to infer the structure of something we can't study (transition state) from something we can study (reactive intermediate). 23

48. Step 2: Carbocation Formation+
(CH3)3C O H :
slow, unimolecular
tert-Butyl cation
(an intermediate)
Dissociation of the alkyloxonium ion involves bond-breaking, without any bond-making to compensate for it. It has a high activation energy and is slow.
A single molecule reacts in this step; therefore, it is unimolecular. 9

49. Potential Energy Diagram for Step 2
(CH3)3C O H
Potential energy
Reaction coordinate +
(CH3)3CO H
(CH3)3C H2O 22

50. Transition State for Carbocation Formation
Note the “carbocation character” in the transition state: 
(CH3)3C O H
Whatever stabilizes carbocations will stabilize the transition state leading to them. 23

51. Carbocation C R +
The key intermediate in reaction of secondary and tertiary alcohols with hydrogen halides is a carbocation.
A carbocation is a cation in which carbon has 6 valence electrons and a positive charge. 7

52. Figure 4.8 Structure of tert-Butyl Cation
CH3
H3C +
Positively charged carbon is sp2 hybridized.
All four carbons lie in same plane.
Unhybridized p orbital is perpendicular to plane of four carbons. 10

53. Step 3: Carbocation Capture
Bond formation between the positively charged carbocation and the negatively charged chloride ion is fast. .. Cl : – +
(CH3)3C +
fast, bimolecular
Two species are involved in this step. Therefore, this step is bimolecular. .. ..
(CH3)3C Cl :
tert-Butyl chloride 9

54. Step 3: Carbocation Capture+
The carbocation is the Lewis acid (an electrophile); the chloride ion is the Lewis base (a nucleophile). C
CH3
H3C + C
H3C
CH3 Cl Cl – + 9

55. Potential Energy Diagram for Step 3
(CH3)3C Cl -
Potential energy
Reaction coordinate
(CH3)3C + Cl– +
(CH3)3C—Cl 22

56. 4.9 Potential Energy Diagrams for Multistep Reactions: The SN1 Mechanism

57. Potential Energy Diagram - Overall
The potential energy diagram for a multistep mechanism is simply a collection of the potential energy diagrams for the individual steps.
Consider the three-step mechanism for the reaction of tert-butyl alcohol with HCl.
(CH3)3COH + HCl
(CH3)3CCl + H2O
25°C 9

58. carbocation formation
carbocation capture RX
proton transfer
ROH
ROH2 + 21

59. carbocation formation
proton transfer
ROH
ROH2 +
carbocation formation R+
carbocation capture RX ‡
(CH3)3C O H Cl + – ‡ 21

60. carbocation formation
proton transfer
ROH
ROH2 +
carbocation formation R+
carbocation capture RX ‡ ‡
(CH3)3C + O H 21

61. carbocation formation
proton transfer
ROH
ROH2 +
carbocation formation R+
carbocation capture RX ‡ ‡
(CH3)3C + Cl – 21

62. The mechanism just described is an example of an SN1 process.
Mechanistic Notation
The mechanism just described is an example of an SN1 process.
SN1 stands for substitution-nucleophilic- unimolecular.
The molecularity of the rate-determining step defines the molecularity of the overall reaction. 9

63. Mechanistic Notation
The molecularity of the rate-determining step defines the molecularity of the overall reaction.
(CH3)3C + O H
The rate-determining step is a unimolecular dissociation of alkyloxonium ion. 9

64. 4.10 Structure, Bonding and Stability of Carbocations9

65. Carbocations C R +
Most carbocations are too unstable to be isolated, but occur as reactive intermediates in a number of reactions.
When R is an alkyl group, the carbocation is stabilized compared to R = H. 7

66. Carbocations C H + H3C + H C H Methyl cation least stable
Ethyl cation (a primary carbocation) more stable than CH3+ 7

67. Carbocations H3C + CH3 H3C + CH3 C C H CH3
Isopropyl cation (a secondary carbocation) more stable than CH3CH2+
tert-Butyl cation (a tertiary carbocation) more stable than (CH3)2CH+ 7

68. Figure 4.14 Stabilization of Carbocations via the Inductive Effect
Positively charged carbon pulls electrons in  bonds closer to itself. + 18

69. Figure 4.14 Stabilization of Carbocations via the Inductive Effect
Positive charge is "dispersed ", i.e., shared by carbon and the three atoms attached to it.    18

70. Figure 4.14 Stabilization of Carbocations via the Inductive Effect
Electrons in C—C bonds are more polarizable than those in C—H bonds; therefore, alkyl groups stabilize carbocations better than H.  
Electronic effects transmitted through bonds are called "inductive effects." 18

71. Figure 4.15 Stabilization of Carbocations via Hyperconjugation
Electrons in this  bond can be shared by positively charged carbon because the  orbital can overlap with the empty 2p orbital of positively charged carbon 18

72. Figure 4.15 Stabilization of Carbocations via Hyperconjugation
Notice that an occupied orbital of this type is available when sp3 hybridized carbon is attached to C+, but is not available when H is attached to C+. Therefore, alkyl groups stabilize carbocations better than H does.  18

73. 4.11 Effect of Alcohol Structure on Reaction Rate9

74. R O H +
• • O H
• • +
Slow step is: R +
The more stable the carbocation, the faster it is formed. 3o > 2o > 1o > Me for stability and rate.
Tertiary carbocations are more stable than secondary, which are more stable than primary, which are more stable than methyl.
Tertiary alcohols react faster than secondary, which react faster than primary, which react faster than methanol. 22

75. High activation energy for formation of methyl cation.
Eact
CH3 d+ O H
• •
CH3 + O H
• •
CH3 O H +
• • 22

76. Smaller activation energy for formation of primary carbocation.
RCH2 d+ O H
• •
Eact
RCH2 + O H
• •
RCH2 O H +
• • 22

77. Activation energy for formation of secondary carbocation is less than that for formation of primary carbocation.
R2CH d+ O H
• •
R2CH + O H
• •
Eact
R2CH O H +
• • 22

78. Activation energy for formation of tertiary carbocation is less than that for formation of secondary carbocation.
Eact
R3C d+ O H
• •
R3C + O H
• •
R3C O H +
• • 22

79. 4.12 Reaction of Methyl and Primary Alcohols with Hydrogen Halides: The SN2 Mechanism9

80. Preparation of Alkyl Halides
Primary carbocations are too high in energy to allow SN1 mechanism. Yet, primary alcohols are converted to alkyl halides.
Primary alcohols react by a mechanism called SN2 (substitution-nucleophilic-bimolecular).
120°C
CH3(CH2)5CH2OH + HBr
CH3(CH2)5CH2Br + H2O
87-90% 4

81. The SN2 Mechanism
This is a two-step mechanism for conversion of alcohols to alkyl halides:
(1) proton transfer to alcohol to form alkyloxonium ion
(2) bimolecular displacement of water from alkyloxonium ion by halide
120°C
CH3(CH2)5CH2OH + HBr
CH3(CH2)5CH2Br + H2O
87-90% 27

82. Mechanism Step 1: Proton transfer from HBr to 1-heptanol O
CH3(CH2)5CH2 .. .. : + H Br : .. H + H O : .. Br –
fast, bimolecular
Heptyloxonium ion
CH3(CH2)5CH2 9

83. Mechanism Step 2: Reaction of alkyloxonium ion with bromide ion. + H O
CH3(CH2)5CH2 Br : .. – +
1-Bromoheptane
slow, bimolecular .. Br :
CH3(CH2)5CH2 + O H 9

84. Energy Diagram CH2 OH2 Br CH3(CH2)4CH2 ‡ proton transfer ROH ROH2 + RX+ Br –
CH3(CH2)4CH2 ‡
Energy Diagram
proton transfer
ROH
ROH2 + RX ‡ 31

85. 4.13 More on Activation Energy9

86. Arrhenius equation
A quantitative relationship between the activation energy, the rate constant, and the temperature:
A is the preexponential, or frequency factor (related to the collision frequency and geometry), R=8.314 x 10-3 kJ/Kmol, T is in K.
The true activation barrier is known as DG‡ and takes into account the enthalpy and entropy of activation. 33

87. 4.14 Other Methods for Converting Alcohols to Alkyl Halides9

88. ROH + conc. HCl + ZnCl2  RCl + HOH
Lucas Test
The Lucas test distinguishes 1o, 2o and 3o alcohols. The Lucas reagent is conc. HCl + ZnCl2.
ROH + conc. HCl + ZnCl2  RCl + HOH
3o alcohols react instantaneously,
2o alcohols react in about 5 minutes and
1o alcohols require 20 minutes or more. 33

89. Reagents for ROH to RX
These are better preparative methods to make alkyl chlorides and bromides. Thionyl chloride SOCl2 + ROH  RCl + HCl + SO2 Phosphorus tribromide PBr3 + 3ROH  3RBr + H3PO3 33

90. Examples SOCl2 CH3CH(CH2)5CH3 CH3CH(CH2)5CH3 K2CO3 Cl OH (81%)
(Pyridine often used instead of K2CO3)
PBr3
(CH3)2CHCH2OH
(CH3)2CHCH2Br
(55-60%) 34

91. 4.15 Halogenation of Alkanes
RH + X2  RX + HX 9

92. exothermic for Cl2 and Br2
Energetics
RH + X2  RX + HX
explosive for F2
exothermic for Cl2 and Br2
endothermic for I2 2

93. 4.16 Chlorination of Methane9

94. Chlorination of Methane
carried out at high temperature (400 °C).
CH Cl2  CH3Cl HCl
CH3Cl + Cl2  CH2Cl HCl
CH2Cl Cl2  CHCl HCl
CHCl Cl2  CCl HCl
A multiplicity of products is possible. 6

95. 4.17 Structure and Stability of Free Radicals9

96. Alkyl Radicals R . C
Most free radicals in which carbon bears the unpaired electron are too unstable to be isolated.
Alkyl radicals are classified as primary, secondary, or tertiary in the same way that carbocations are. 7

97. Figure 4.17 Structure of Methyl Radical
Methyl radical is planar, which suggests that carbon is sp2 hybridized and that the unpaired electron is in a p orbital. 10

98. 3o > 2o > 1o > Methyl
Alkyl Radicals R . R C R
The order of stability of free radicals is the same as for carbocations.
3o > 2o > 1o > Methyl 7

99. Alkyl Radical Stability
H3C H .
Ethyl radical (primary) C H .
Methyl radical
less stable than C
H3C
CH3 H .
Isopropyl radical (secondary)
less stable than
less stable than C
H3C
CH3 .
tert-Butyl radical (tertiary) 7

100. Alkyl Radicals
The order of stability of free radicals can be determined by measuring bond strengths.
By "bond strength" we mean the energy required to break a covalent bond.
A chemical bond can be broken in two different ways—heterolytically or homolytically. 7

101. In a heterolytic cleavage, one atom retains both electrons.
Homolytic
In a homolytic bond cleavage, the two electrons in the bond are divided equally between the two atoms. One electron goes with one atom, the second with the other atom.
In a heterolytic cleavage, one atom retains both electrons.
Heterolytic – + 9

102. Homolytic
The species formed by a homolytic bond cleavage of a neutral molecule are free radicals. Therefore, measure enthalpy cost of homolytic bond cleavage to gain information about stability of free radicals.
The more stable the free-radical products, the weaker the bond, and the lower the bond- dissociation energy. 9

103. Measures of Free Radical Stability
Bond-dissociation enthalpy measurements tell us that isopropyl radical is 13 kJ/mol more stable than n-propyl.
CH3CH2CH2 + H .
CH3CHCH3 + H .
410
397
CH3CH2CH3 12

104. Measures of Free Radical Stability
Bond-dissociation enthalpy measurements tell us that tert-butyl radical is 30 kJ/mol more stable than isobutyl. .
(CH3)2CHCH2 + H
(CH3)3C + H . .
410
380
(CH3)3CH 12

105. 4.18 Mechanism of Methane Chlorination9

106. Free Racical Halogenation (Cl2 or Br2)
All free-radical mechanisms involve three steps:
Initiation step - generates free radicals needed for the mechanism.
Propagation steps - Produces the reaction product(s).
Termination steps – decreases the number of free radicals to slow or stop the reaction. 16

107. Mechanism of Chlorination of Methane
Free-radical chain mechanism.
Initiation step: : .. Cl .. Cl : . : Cl .. . +
The initiation step "gets the reaction going" by producing free radicals—chlorine atoms from chlorine molecules in this case.
Initiation step is followed by propagation steps. Each propagation step consumes one free radical but generates another one. 16

108. Mechanism of Chlorination of Methane Almost all of the product is formed by repetitive cycles of the two propagation steps.
First propagation step:
Cl: .. .
H : Cl: ..
H3C .
H3C : H + +
Second propagation step: +
Cl: .. .
H3C : Cl 18

109. Mechanism of Chlorination of Methane
First propagation step:
Cl: .. .
H : Cl: ..
H3C .
H3C : H + +
Second propagation step: : .. Cl
H3C .. : Cl
Cl: .. .
H3C . + +
Net: : .. Cl
H3C .. : Cl
H : Cl: ..
H3C : H + + 18

110. Termination Steps
Stops a chain reaction by consuming free radicals. Hardly any product is formed by termination step because concentration of free radicals at any instant is extremely low.
Cl: .. .
H3C .. : Cl
H3C . + 20

111. 4.19 Halogenation of Higher Alkanes

112. Chlorination of Alkanes
can be used to prepare alkyl chlorides from alkanes in which all of the hydrogens are equivalent to one another.
420°C
CH3CH3 + Cl2
CH3CH2Cl + HCl
(78%) h
+ Cl2 Cl
+ HCl
(73%) 22

113. Chlorination of Alkanes
Major limitation: Chlorination gives every possible monochloride derived from original carbon skeleton.
Not much difference in reactivity of nonequivalent hydrogens in molecule. 23

114. Example
Chlorination of butane gives a mixture of 1-chlorobutane and 2-chlorobutane, primary vs secondary hydrogens.
(28%)
CH3CH2CH2CH2Cl
Cl2
CH3CH2CH2CH3 h
CH3CHCH2CH3
(72%) Cl 23

115. Percentage of product that results from substitution of a given hydrogen, assuming that every collision with chlorine atoms is productive.
10% 24

116. Percentage of product that actually results from replacement of a given hydrogen.
18%
18%
4.6%
4.6%
4.6%
4.6%
4.6%
4.6%
18%
18% 24

117. Relative Rates of Hydrogen Atom Abstraction
divide by 4.6
18%
4.6%
4.6
= 1
4.6 18
= 3.9
A secondary hydrogen is abstracted 3.9 times faster than a primary hydrogen by a chlorine atom. 26

118. Similarly, chlorination of 2-methylbutane gives a mixture of isobutyl chloride and tert-butyl chloride, primary vs tertiary.
CH3CCH2Cl
CH3 H
(63%)
(37%) h
Cl2 H
CH3CCH3
CH3 Cl
CH3CCH3
CH3 27

119. Percentage of product that actually results from replacement of a given hydrogen.
7.0%
37% 28

120. Relative Rates of Hydrogen Atom Abstraction
divide by 7
7.0 37
= 1
= 5.3 7 7
A tertiary hydrogen is abstracted 5.3 times faster than a primary hydrogen by a chlorine atom. 29

121. Selectivity of Free-radical Halogenation
R3CH > R2CH2 > RCH3 chlorination: bromination: Chlorination of an alkane gives a mixture of every possible isomer having the same skeleton as the starting alkane. Useful for synthesis only when all hydrogens in a molecule are equivalent. Bromination is highly regioselective for substitution of tertiary hydrogens. Major synthetic application is in synthesis of tertiary alkyl bromides. 31

122. Synthetic Application of Chlorination of an AlkaneCl
Cl2 h
(64%)
Chlorination is useful for synthesis only when all of the hydrogens in a molecule are equivalent. 32

123. Synthetic Application of Bromination of an Alkane
CH3CCH2CH2CH3
CH3 Br
CH3CCH2CH2CH3
CH3
Br2 h
(76%)
Bromination is highly selective for substitution of tertiary hydrogens.
Major synthetic application is in synthesis of tertiary alkyl bromides. 33

124. End of Chapter 4 Alcohols and Alkyl Halides9