Lecture #2

Lecture #2 - In Allylic substitution, the stability of the...

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In Allylic substitution, the stability of the allyl (allylic) radical is the most important feature of the reaction. Why is the allyl radical so stable? Can be explained in two ways: 1. Molecular Orbital Theory 2. Resonance 1
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Stability of the Allyl Radical Molecular Orbital Theory When a hydrogen atom is removed from propene to form the allyl radical, the hybridization at C 3 changes from sp 3 to sp 2 : C H H C H C H H H 1 2 3 -H . C H H C H C H 1 2 3 A new ! -system of three overlapping p-orbitals with three ! -electrons is formed. H In the extended !" system, the ! -electrons are delocalized over three atoms. In simple alkyl radicals, the single nonbonding electron is localized on one atom. This delocalization stabilizes the allyl radical. 2
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Calculations give the phasing (sign of the wave function) of the combining orbitals, and the relative energies of the three ! - molecular orbitals of the allyl radical, as shown below. ! -electron energy three isolated p-orbitals combine Bonding MO Nonbonding MO Antibonding MO CH 2 CH CH 2 + + + - - - ! 1 CH 2 CH CH 2 + + - - node ! 2 CH 2 CH CH 2 + + + - - - nodes ! 3 3
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The Reactivity of the Allyl Radical CH 2 CH CH 2 + + - - node ! 2 Nonbonding MO Therefore, a reasonable description of the allyl radical shows an equal partitioning of the single electron between C 1 and C 3. C C C H H H H H 1 3 1 2 . 1 2 . The dashed lines represent the two electrons in ! 1 and indicate partial double bonds for C 1 -C 2 and C 2 -C 3 . Molecular orbital theory predicts a symmetrical structure for the simple allyl radical with equivalent terminal carbons. Chemical studies support this picture. The reactivity of the allyl radical is associated with the highest filled !" molecular orbital, the nonbonding # 2 . The node at C 2 means the single electron in # 2 is only found at C 1 and C 3 . 4
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Resonance Theory Description of the Allyl Radical There are two equivalent resonance structures for the allyl radical: C C C H H H H H . C C C H H H H H . 1 1 2 2 3 3 A B Structures A and B interconvert by moving single electrons as shown by the single barbed arrows: C C C H H H H H . A B The hybrid of structures A and B is C , shown below. C C C H H H H H 1 3 1 2 . 1 2 . C 5
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Resonance Theory Description of the Allyl Radical C C C H H H H H . C C C H H H H H . 1 1 2 2 3 3 A B Remember: Resonance implies stabilization. Whenever equivalent resonance structures can be written for the same species, the species is much more stable than either resonance structure alone would indicate. CH H 2 C CH 2 What about the allyl cation? 6
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Molecular Orbital Description of the Allyl Cation The allyl cation is described in terms of the same set of ! -molecular orbitals as was noted for the allyl radical. However, the molecular orbitals are populated by only two rather than three electrons. ! -electron energy
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Lecture #2 - In Allylic substitution, the stability of the...

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