ch13 - Chapter 13 Conjugated Unsaturated Systems...

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Chapter 13 Conjugated Unsaturated Systems
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Chapter 13 2 Introduction Conjugated unsaturated systems have a p orbital on a carbon adjacent to a double bond The p orbital can come from another double or triple bond The p orbital may be the empty p orbital of a carbocation or a p orbital with a single electron in it (a radical) Conjugation affords special stability to the molecule Conjugated molecules can be detected using UV spectroscopy Allylic Substitution and the Allylic Radical Reaction of propene with bromine varies depending on reaction conditions At low temperature the halogen adds across the double bond At high temperature or at very low concentration of halogen an allylic substitution occurs
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Chapter 13 3 Allylic Chlorination (High Temperature) Allylic chlorination can be performed at high temperature in the gas phase The reaction is a free radical chain reaction In the initiation step a low concentration of chlorine radical is produced In the first propagation step an allyl radical is formed In the second propagation step the allyl radical reacts with molecular chlorine
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Chapter 13 4 Allylic radicals form readily because they are more stable than ordinary primary, secondary, tertiary, or vinyl radicals This trend is reflected in their respective C-H bond dissociation energies The relative stability of some carbon radicals is as follows:
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Chapter 13 5 Allylic Bromination with N -Bromosuccinimide Propene undergoes allylic bromination with N -bromosuccinimide (NBS) in the presence of light or peroxides NBS provides a continuous low concentration of bromine for the radical reaction A low bromine concentration favors allylic substitution over alkene addition The radical reaction is initiated by a small amount of bromine radical formed by exposure of NBS to light or peroxides
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Chapter 13 6 The propagation steps for allylic bromination with NBS are: A bromine radical reacts with propene to produce an allylic radical and HBr HBr reacts with NBS to produce a bromine molecule A molecule of bromine reacts with a propene radical to regenerate a bromine radical
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Chapter 13 7 The Stability of the Allyl Radical Both molecular orbital theory and resonance theory can explain the stability of allyl radicals Molecular Orbital Description of the Allyl Radical When an allylic hydrogen is abstracted to form an allyl radical, the developing p orbital on the sp 2 carbon overlaps with the p orbitals of the alkene The new p orbital is conjugated with the double bond p orbitals The radical electron and the π electrons of the alkene are delocalized over the entire conjugated system Delocalization of charge and electron density leads to increased stability
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Chapter 13 8 The three p orbitals of the allylic system combine to form three molecular orbitals The bonding molecular orbital contains two spin-paired electrons and this orbital
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ch13 - Chapter 13 Conjugated Unsaturated Systems...

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