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

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Unformatted text preview: Conjugation in Alkadienes and Allylic Systems Chapter 10 The Allyl Group 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. 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. Allylic Carbocations S N 1 Reactions of Allylic Halides CH 2 CH C X R R Allylic Halide X = Cl, Br, I, etc. R R slow CH 2 CH C X R CH 2 CH C 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 Cl CH CH 3 CH CH 3 CH CH 3 C Cl Cl CH CH 3 CH CH 3 C H 2 C CH CH Cl Cl CH CH 3 CH CH 3 C H 2 C CH CH H 2 C CH CH relative rates: (ethanolysis, 45°C) 123 1 CH CH 3 CH CH 3 H 2 C CH CH + C CH CH 3 CH CH 3 H 2 C CH CH + C Allylic Carbocations S N 1 Reactions of Allylic Halides 3 3 C CH CH 3 CH CH 3 H 2 C CH CH δ + δ + H 2 O Cl Cl CH CH 3 CH CH 3 C H 2 C CH CH CH CH 3 CH CH 3 H 2 O Cl Cl CH CH 3 CH CH 3 C H 2 C CH CH Cl Cl CH CH 3 CH CH 3 C H 2 C CH CH H 2 C CH CH CH CH 3 CH CH 3 CH CH 3 CH CH 3 Allylic Carbocations S N 1 Reactions of Allylic Halides (85%) (85%) OH OH CH CH 3 C H 2 C CH CH (15%) (15%) + CH CH 3 HO HO CH CH 2 CH CH C (85%) (85%) OH OH CH CH 3 C H 2 C CH CH OH OH CH CH 3 C H 2 C CH CH H 2 C CH CH (15%) (15%) + CH CH 3 HO HO CH CH 2 CH CH C (15%) (15%) + CH CH 3 HO HO CH CH 2 CH CH C (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 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 Allylic Systems Stability from Conjugation Allylic Radicals 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 ....
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This note was uploaded on 12/22/2011 for the course CHEM 3371 taught by Professor Anderson,k during the Fall '08 term at Southern Methodist.

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

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