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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