Ch 10 (1) - Chapter 10 Conjugation in Alkadienes and...

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Unformatted text preview: Chapter 10 Conjugation in Alkadienes and Allylic Systems allylic carbocation The Double Bond Substituent allylic carbocation The Double Bond Substituent allylic radical allylic carbocation The Double Bond Substituent allylic radical conjugated diene 10-5 vinylic carbons allylic carbon Allylic and Vinylic Vinylic hydrogens (halogens, etc.) are bonded to vinylic carbons Allylic hydrogens (halogens, etc.) are bonded to allylic carbons A tertiary allylic halide undergoes solvolysis (S N 1) faster than a simple tertiary alkyl halide Allylic Carbocations Relative rate 123 1 Why?? Allylic Carbocations Consider the intermediate a carbocation The allylic halide reacts faster because it forms the intermediate faster The intermediate (allylic carbocation) is more stable H 2 C=CH- group can stabilize the cation better than CH 3- Allylic Carbocation Stability Delocalization of electrons in the double bond stabilizes the carbocation Resonance structures Orbital overlap Allylic S N 1 Reactions Two different alkyl halides give the same products with the same distribution Why? Allylic S N 1 Reactions These are the same cation: Allylic rearrangement the movement of the double bond of an allylic group during reaction Allylic S N 2 Reactions Allylic halides also undergo S N 2 reactions, faster than simple primary alkyl halides Relative rate 80 1 Why?? Allylic S N 2 Reactions Two reasons: Sterics Trigonal carbon vs. tetrahedral carbon sp 2 hybridized sp 3 hybridized Allylic S N 2 Reactions Two reasons: Electronics Delocalization of electrons lowers LUMO energy This lowers the activation energy Allylic Free Radicals Stabilized by resonance Compare the bond energies for allylic vs. alkyl groups: CH 3 CH 2 CH 2 H 410 kJ/mol CH 3 CH 2 CH 2 + H 368 kJ/mol + H CHCH 2 H H 2 C CHCH 2 H 2 C The C-H bond in propene is easier to break because it leads to a more stable radical ClCH 2 CHCH 3 Cl 500 C CHCH 3 H 2 C + Cl 2 CHCH 2 Cl H 2 C + HCl Chlorination of Propene At low temperatures, addition across the double bond At high temperatures, substitution via radical mechanism Scope of Allylic Halogenations Only really useful when: All allylic hydrogens are equivalent All resonance forms of allylic radical are equivalent Otherwise, a mixture of products results Allylic Anions Anion is stabilized by resonance the negative...
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This note was uploaded on 03/08/2012 for the course BIOL 200 taught by Professor Jimlara/stevehauska/hannalerhoula-baker during the Spring '09 term at University of Washington.

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Ch 10 (1) - Chapter 10 Conjugation in Alkadienes and...

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