chapter 12 radicals reactions of alkanes
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Organic Chemistry 6 th Edition Paula Yurkanis Bruice. Chapter 12 Radicals Reactions of Alkanes. Petroleum is a complex mixture of alkanes and cycloalkanes that can be separated by distillation:. Alkanes are very unreactive compounds because. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 12
• Radicals• Reactions of Alkanes
Organic Chemistry 6th Edition
Paula Yurkanis Bruice
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Petroleum is a complex mixture of alkanes and cycloalkanes that can be separated by distillation:
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Alkanes are very unreactive compounds because they have only strong bonds and atoms with no partial charges
However, alkanes do react with Cl2 and Br2:
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Consider the relative stabilities of alkyl radicals:
Alkyl groups stabilize carbocations about 5–10 times better than they stabilize radicals:
Radicals: Resonance > HyperconjugationCarbocations: Hyperconjugation > Resonance
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Why does hyperconjugation stabilize carbocations more than radicals?
Carbocation:Both electrons
are in the bondingorbital
Radical:One of theelectrons is
in the antibondingorbital
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The Distribution of Products Depends on Probability and Reactivity
Probability is based on relative number of primary and secondary protons, 6:4
However, secondary hydrogens are more reactive than primary hydrogens
Therefore, probability and reactivity both contribute to product distribution
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To determine the the product distribution, consider both probability and reactivity:
• Probability: the number of hydrogens that can be abstracted that will lead to the formation of the particular product
• Reactivity: the relative rate at which a particular hydrogen is abstracted
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An example of calculating the distribution of radical chlorination products:
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The Reactivity–Selectivity PrincipleRadical bromination is more selective than radical chlorination:
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Reactivity–Selectivity PrincipleMonochlorination reactions:
Monobromination reactions:
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Why is the bromine radical more selective than the chlorine radical?
Hammond postulate: • Endothermic, a product-like (radical) transition state• Exothermic, a reactant-like (alkane) transition state
Radical stability important
Radical stability not important
Therefore, the bromine atom more readily distinguishes 3º, 2º, and 1º hydrogens
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The more reactive a species is, the less selective it will be:
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What about fluorination and iodination?
Alkanes undergo chlorination and bromination, but not iodination
Fluorination is too violent a reaction to be useful:
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Addition of Radicals to an Alkene
Peroxide is used to generate Br radical in trace quantities
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An alkyl peroxide is a radical initiator
The electrophile adds to the sp2 carbon that is bonded tothe greater number of hydrogens
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Radicals do not rearrange as readily as carbocations:
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Why is the peroxide effect observed only for the additionof HBr to alkene?
Because both of the propagation steps in the HBr addition reaction are exothermic.
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Stereochemistry of Radical Substitution Reactions
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Stereochemistry of Radical Addition Reactions
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Why are both enantiomers formed?
Consider the first propagation step:
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The more stable radicals form faster:
Radical Substitution of Benzylic and Allylic Hydrogens
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The percentage of substitution at the benzylic or allyliccarbon is greater for bromination. Note the reactivity–selectivity principle:
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Because of the presence of an alkene functional group, N-bromosuccinimide is used as a brominating agent for allylic compounds:
No Br2 addition to the alkene
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Mechanism of NBS Bromination
Azobisisobutylnitrileradical initiator
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Advantage: The low concentrations of Br2 and HBr present during NBS bromations result in no alkene addition reactions:
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Designing a Multistep Synthesis
Example 1
Synthesis:
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Example 2
Racemic
Racemic
Synthesis:
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Example 3
Retosynthetic analysis:
Synthesis:
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Radical Reactions in Biological SystemsCytochrome-mediated hydroxylation of an alkane:
Conversion of nicotine to a polar metabolite:
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The Heme Cofactor of a Cytochrome
The FeII is converted to the reactive FeV oxide
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Antihistamine Metabolism: Allegra Cytochrome
oxidation
Site of oxidation depends on
accessibility rather than radical stability
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Other Reactions of Radicals
Reaction of ethers to form highly explosive peroxides:
Caution: Ethers may have dangerous levels of peroxides!
Reactive Oxygen Species:
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Reactive Oxygen Species (ROS) Are Common in Biological Systems
Oxidation of an unsaturated fat:
Terminating the radical chain reaction with a phenolic compound:
Radical is resonance stabilized and not propagated
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Radical Chain Terminators: Antioxidants
Water-soluble
Fat-soluble
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Commonly Used Antioxidant PreservativesBHT is used in packaging:
Hydroquinone is used to preserve ethers:
A radical that cannot propagate:• Resonance stabilized
• Sterically hindered
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Naturally Occurring AntioxidantsRutin is a bioflavonoid
glycoside with antioxidant and anticancer properties:
Found in citrus fruit and berries
Bioflavonoids afford resonance-stabilized
radicals:
Stars show the resonance delocation
of the radical
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Green Tea AntioxidantsNote the abundance of phenolic hydroxyls:
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Green Tea Antioxidants
Formed from catechinsduring tea fermentation
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Radicals and Stratospheric Ozone
• Ozone is a major constituent of smog
• Ozone shields Earth from harmful radiation
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Chlorofluorocarbons remain very stable in the atmosphere until they reach the stratosphere:
The chlorine radicals are ozone-removing agents: