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ALLYL - Conjugated systems Compounds that have a p orbital...

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© E.V. Blackburn, 2008 Conjugated systems Compounds that have a p orbital on an atom adjacent to a double bond
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© E.V. Blackburn, 2008 Ionic addition However, we have seen that X 2 reacts with alkanes, by a free radical mechanism, to form substitution products: Perhaps we can brominate at the methyl position of propene ..... C H + X 2 250-400 o or h ν C X + HX CH 3 CH=CH 2 Br 2 CH 3 CHBrCH 2 Br
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© E.V. Blackburn, 2008 Free radical substitution We must use conditions which favor free radical substitution reactions and are not favorable to ionic addition: C C H C C C Br C + Br 2 + HBr h ν or
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© E.V. Blackburn, 2008 Free radical substitution v ionic addition ionic addition 500 - 600 o CH 2 ClCH=CH 2 (gas phase) radical substitution CH 3 -CH=CH 2 Cl 2 low T CCl 4 CH 3 CHClCH 2 Cl
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© E.V. Blackburn, 2008 N -bromosuccinimide N -bromosuccinimide (NBS) is used for the specific purpose of brominating alkenes at the allylic position. N Br O O N H O O + + h ν , CCl 4 Br
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© E.V. Blackburn, 2008 N -bromosuccinimide How does it work? NBS provides a low concentration of Br 2 which is produced by reaction between HBr and NBS: N Br O O N H O O HBr + Br 2 + CH 2 =CHCH 3 + Br CH 2 =CHCH 2 + HBr + Br CH 2 =CHCH 2 + Br 2 CH 2 =CHCH 2 Br
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© E.V. Blackburn, 2008 Orientation and reactivity allylic hydrogens are particularly reactive. the order of ease of hydrogen abstraction is: allylic > 3 o > 2 o > 1 o >CH 4 > vinylic How can we explain the stability of allylic radicals ? vinyl hydrogens undergo very little substitution.
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© E.V. Blackburn, 2008 Properties of allylic radicals Allylic radicals can rearrange: We will find the answer in the concept of resonance. Let us start by examining some of the properties of allylic radicals: CH 3 CH 2 CH=CH 2 NBS CH 3 CHBrCH=CH 2
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© E.V. Blackburn, 2008 Properties of allylic radicals Allylic radicals can rearrange: We will find the answer in the concept of resonance.
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