chapter 9 reactions of alkanes radicals
DESCRIPTION
Chapter 9 Reactions of Alkanes Radicals. Irene Lee Case Western Reserve University. Alkanes are very unreactive compounds because . However, alkanes do react with Cl 2 and Br 2. they have only strong s bonds and atoms with no partial charges. Reaction of Alkane with Cl 2 or Br 2. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 9
Reactions of Alkanes
Radicals
Irene LeeCase Western Reserve
University
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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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Reaction of Alkane with Cl2 or Br2
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Factors that determine product distribution …
The rate-determining step of the overall reaction is hydrogen abstraction
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Consider the relative stabilities of alkyl radicals,
The stable alkyl radical is formed faster, therefore 2-chlorobutane is formed faster
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pg 341 middle
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In determining the relative amounts of products obtained,both probability and reactivity should be considered
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 hydrogenis abstracted
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The Reactivity–Selectivity PrincipleA bromine radical is less reactive and more selective than a chlorine radical
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Why are the relative rates of radical formation so different between the bromine versus the chlorine radical?
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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 notiodination Fluorination is too violent of a reaction to be useful
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Consider the relative stabilities of radicals
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The more stable radicals form faster
Radical Substitution of Benzylic Hydrogens
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Because of the reactivity of allylic hydrogens,
a milder brominating reagent can be used
A very important and useful reaction
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Stereochemistry of Radical Substitution Reactions
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Why are both enantiomers formed?
Consider the first propagation step
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What happens if the reactant already has an asymmetriccarbon and the radical substitution reaction creates a second asymmetric carbon?
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Radical Reactions in Biological Systems
Liver enzymes & cytochrome P450
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Radical Inhibitors/ Antioxidants
BHA & BHT: are synthetic chemicals “bad”?