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CHEM 125A: FRESHMAN ORGANIC CHEMISTRY II - Lecture 17 - Professor J. Michael McBride

Alkynes; Conjugation in Allylic Intermediates and Dienes


Because of their unusual acidity very strong base makes it possible to isomerize an internal acetylene to the less stable terminal isomer. Many chemical reactions may be understood in terms of localized bonds, but the special stability of conjugated systems requires considering delocalized orbitals or "resonance." Equilibrium constants, rates, and regiochemistry in systems involving allylic cations, anions, transition states, and free radicals demonstrate that allylic conjugation is worth about 13 kcal/mole. Regioselection in addition of DCl to 1,3-pentadiene reveals rapid collapse of an allylic ion pair. Allylic substitution of bromine can be favored over Br2 addition by using NBS to control Br2 concentration. Diene conjugation is worth much less than allylic conjugation.
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