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Chapter 10 - Conjugation in Alkadienes and Allylic Systems...

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Conjugation in Alkadienes and Allylic Systems Chapter 10 The Allyl Group
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CH 3 Circle all allylic carbons The Allyl Group CH 2 C CH 3 CH 3 CH 2 CH 2 CH 2 CH CH 2 1 o carbocation (highly unstable) allyl carbocation (resonance stabilized) Allylic Carbocations Conjugation Stability of the allyl cation is more comparable to a 2° carbocation.
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Allylic Carbocations Conjugation Allyl carbocation has three adjacent p orbitals. All three p orbitals overlap. The total number of electrons in all three orbitals are shared. The systems is conjugated . Conjugation = overlap of p orbitals on three or more adjacent carbons. Allylic Carbocations Conjugation More representative picture of the allylic carbocation. The 2 π electrons are shared among the three overlapping p orbitals. The system is conjugated . Conjugation = overlap of p orbitals on three or more adjacent carbons.
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Allylic Carbocations S N 1 Reactions of Allylic Halides CH 2 CH C X R R Allylic Halide X = Cl, Br, I, etc. R CH CH C R slow CH 2 CH C X R 2 R - X Nu CH 2 CH C Nu R R Mechanism for S N 1 of an allylic halide For all S N 1 reactions, rate increases with increasing stability of the carbocation. S N 1 mechanism of solvolysis in EtOH ( i.e. , ethanolysis) R C X R R R C R R - X R C O R R slow EtOH Et H X R C O R R Et Allylic Carbocations S N 1 Reactions of Allylic Halides Cl Cl CH CH 3 CH CH 3 CH CH 3 C Cl CH CH 3 CH CH 3 C H 2 C CH CH relative rates: (ethanolysis, 45°C) 123 1
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CH 3 CH 3 H 2 C CH + C CH 3 CH 3 H 2 C CH + C Allylic Carbocations S N 1 Reactions of Allylic Halides C CH 3 CH 3 H 2 C CH δ + δ + H 2 O Cl Cl CH 3 CH 3 C H 2 C CH CH CH 3 CH CH 3 Allylic Carbocations S N 1 Reactions of Allylic Halides (85%) (85%) OH CH 3 C H 2 C CH CH (15%) (15%) + CH CH 3 HO HO CH CH 2 CH CH C CH CH 3 CH CH 3 H 2 C CH CH + C CH CH 3 CH CH 3 H 2 C CH CH + C 3 o 1 o The major resonance contributor has the + charge on the more highly substituted carbon
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Allylic Systems Stability from Conjugation Allylic Radicals
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Allylic Halogenation • Because allylic C—H bonds are weaker than other sp 3 hybridized C—H bonds, the allylic carbon can be selectively monohalogenated. • Bromination of allyl compounds usually employs N -bromosuccinimide (NBS) instead of Br 2 . Allylic Halogenation Bromination with NBS
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Allylic Halogenation Limited Scope Allylic halogenation only used when both of these conditions are true: (1) All of the allylic hydrogens are equivalent (2) The resonance forms of the allylic radical are equivalent. Example Cyclohexene All allylic H H satisfies both requirements. Both resonance forms are equivalent. H H H H H H hydrogens are equivalent. H H Allylic Halogenation Limited Scope Example
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Dienes (Alkadienes) Dienes Molecules with more than one C=C double bond are called dienes (or alkadienes) Three major classes •Isolated diene •Conjugated diene (also called: 1,3-dienes) •Cumulated diene (also called: allenes) C
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Choose the longest continuous chain that contains both double bonds.
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