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Organic Chemistry, Fourth Edition

Janice Gorzynski Smith
University of Hawai’i

Chapter 15
Lecture Outline
Prepared by Layne A
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Permission required for reproduction or display
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• A radical is a reactive intermediate with a single unpaired
electron, formed by homolysis of a covalent bond
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• Half-headed arrows are used to show the movement of
electrons in radical processes
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• A carbon radical is sp2 hybridized and trigonal planar, like
carbocations
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3

Bond Dissociation Energies
• Bond dissociation energies for the cleavage of C—H bonds
are used to measure stability
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• Cleaving a stronger bond requires more energy
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4

2

Stability of Radicals
Figure 15
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• Some radical reactions are carried out in the presence of a
radical initiator
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• Heating a peroxide readily causes homolysis of the weak O–O
bond, forming two RO• radicals
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6

3

Reaction of a Radical X• with a C–H Bond
• A radical X•, once formed, rapidly reacts with whatever is
available, usually a stable  or  bond
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7

Reaction of a Radical X• with a C=C Bond
• A radical X• can also add to the  bond of a carbon–carbon
double bond
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8

4

Inhibition of Radicals by Molecular Oxygen
• Occasionally, two radicals react to form a sigma bond
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• Reaction with oxygen causes the reaction to slow down or
stop, as X–O–O• radicals are not as reactive as halogen
radicals
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9

Radical Halogenation of Alkanes
• In the presence of heat or light, alkanes react with halogens to
form alkyl halides by a radical substitution reaction
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• Reaction with F2 is too violent, and reaction with I2 is too slow
to be useful
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• When a single hydrogen atom on a carbon has been replaced
by a halogen atom, monohalogenation has taken place
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• Monohalogenation can be achieved experimentally by adding
halogen X2 to an excess of alkane
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2
Complete halogenation of
CH4 using excess Cl2

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6

Halogenation of Alkanes—Mechanism
• Three facts about halogenation suggest that the mechanism
involves radical, not ionic, intermediates:

13

Common Steps of Radical Reactions
• Radical halogenation has three distinct steps:

• This type of mechanism that involves two or more repeating
steps is called a chain mechanism
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15

Energy Changes in Radical Propagation
Figure 15
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4

17

Product Mixture in Radical Chlorination
• Chlorination of CH3CH2CH3
CH3CH2CH2Cl and (CH3)2CHCl
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18

9

Radical Halogenation of Alkanes
• Since the observed ratio between CH3CH2CH2Cl and
(CH3)2CHCl is 1:1, the 2° C–H bonds must be more reactive
than the 1° C–H bonds
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19

Chlorination vs Bromination
• Although alkanes undergo radical substitutions with both Cl2
and Br2, chlorination and bromination exhibit two important
differences
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Chlorination is faster than bromination
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Chlorination is unselective, yielding a mixture of
products, but bromination is more selective, often
yielding one major product
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• Calculating Ho using bond dissociation energies reveals that
abstraction of a 1° or 2° hydrogen by Br• is endothermic
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21

Energy Diagram for Endothermic
Reaction—Bromination
Figure 15
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• The more stable radical is formed faster, and often a single
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radical halogenation product predominates
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• Since chlorination has an exothermic rate-determining step,
the transition state to form both radicals resembles the same
starting material, CH3CH2CH3
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Figure 15
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12

Predicting Stereochemistry of Reactions

25

Stereochemistry from Achiral Starting Material
• Halogenation of an achiral starting material such as
CH3CH2CH2CH3 forms two constitutional isomers by replacement
of either a 1° or 2° hydrogen
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• 2-Chlorobutane has a new stereogenic center, and so an equal
amount of two enantiomers must form—a racemic mixture
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• Cl2 then reacts with it from either side to form an equal amount
of two enantiomers
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• Radical halogenation reactions at a stereogenic center occur
with racemization
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• Since no bond is broken to the stereogenic center at C2, its
configuration is retained during the reaction
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• A pair of diastereomers is formed
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• Current research suggests that chlorofluorocarbons (CFCs),
used extensively as refrigerants and propellants, are responsible
for destroying ozone in the upper atmosphere
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31

Alternatives to CFCs
• The overall result is that O3 is consumed as a reactant and O2
is formed
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• New alternatives to CFCs are hydrochlorofluorocarbons
(HCFCs) and hydrofluorocarbons (HFCs) such as CH2FCF3
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16

Radical Halogenation at an Allylic Carbon
• An allylic carbon is a carbon adjacent to a double bond
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• The bond dissociation energy for this process is even less
than that for a 3° C–H bond (91 kcal/mol)
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33

Stability of Allyl Radicals
• The allyl radical is more stable than other radicals because
the  bond and the unpaired electron are delocalized
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• Declocalizing electron density lowers the energy of the
hybrid, thus stabilizing the allyl radical
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• NBS contains a weak N–Br bond that is homolytically cleaved
with light to generate a bromine radical, initiating an allylic
halogenation reaction
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18

Formation of Bromine from NBS
• The HBr formed in Step [2] reacts with NBS to form a low
concentration of Br2
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37

Radical vs Ionic Bromination
• An alkene with allylic C–H bonds undergoes two different
reactions depending on the reaction conditions
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• The Br2 produced from NBS, present in very low
concentrations, must first react with the double bond to form
the bridged bromonium ion
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• If concentrations of both intermediates—the bromonium ion
and Br− are low (as is the case here), the overall rate of
addition is very slow, and the products of the very fast and
facile radical chain reaction predominate
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• A mixture results because the reaction proceeds by way of a
resonance-stabilized radical
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Figure 15
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• Naturally occurring antioxidants such as vitamin E prevent
radical reactions that can cause cell damage
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• Vitamin E and BHT are radical inhibitors, which terminate
radical chain mechanisms by reacting with the radical
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• Radicals (R•) abstract a hydrogen atom from the OH group of an
antioxidant, forming a new resonance-stabilized radical
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• Because oxidative damage to lipids in cells is thought to play a
role in the aging process, many antiaging formulations contain
antioxidants
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• Radicals react with alkenes via radical chain mechanisms
consisting of:
• Initiation, propagation and termination steps

• HBr adds to alkenes to form alkyl bromides in the presence of
heat, light, or peroxides
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• The addition of HBr to alkenes in the presence of heat, light,
or peroxides proceeds via a radical mechanism
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Path [A] forms the less stable 1° radical
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Path [B] forms the more stable 2° radical
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47

Radical vs Ionic Addition of HBr
• Depending on the reaction conditions, a different species
initially reacts with the p bond accounting for the difference in
regioselectivity
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• Ionic addition involves initial attack by a proton
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• Both propagation steps for HBr addition are exothermic, so
propagation is exothermic (energetically favorable) overall
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Figure 15
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• They include biologically important compounds such as
proteins and carbohydrates, as well as synthetic plastics such
as polyethylene, polyvinyl chloride (PVC) and polystyrene
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of

joining

together

of

• For example, joining ethylene monomers together forms the
polymer polyethylene, a plastic used in milk containers and
plastic bags
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• The identity of Z affects the physical properties of the
resulting polymer
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52

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53

Radical Polymerization
• In radical polymerization, the more substituted radical always
adds to the less substituted end of the monomer, a process
called head-to-tail polymerization

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