Why substitution of allylic hydrogens?
المؤلف:
University of Missouri System
المصدر:
Organic Chemistry ii
الجزء والصفحة:
.................
2-10-2020
1958
Why substitution of allylic hydrogens?
As the table below shows, the dissociation energy for the allylic C-H bond is lower than the dissociation energies for the C-H bonds at the vinylic and alkylic positions. This is because the radical formed when the allylic hydrogen is removed is resonance-stabilized. Hence, given that the halogen concentration is low, substitution at the allylic position is favored over competing reactions. However, when the halogen concentration is high, addition at the double bond is favored because a polar reaction outcompetes the radical chain reaction.
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