Chapter 10 Conjugation in Alkadienes and Allylic Systems conjugare is a Latin verb meaning "to link or yoke together" 1

The Double Bond as a Substituent C + allylic carbocation 2

The Double Bond as a Substituent C + • C allylic carbocation allylic radical 2

The Double Bond as a Substituent C + • C allylic carbocation allylic radical C conjugated diene 2

10.1 The Allyl Group C H 3

Vinylic versus Allylic carbon vinylic carbons 4

Vinylic versus Allylic H C C H C H vinylic hydrogens are attached to vinylic carbons 4

Vinylic versus Allylic H allylic hydrogens are attached to allylic carbons 4

Vinylic versus Allylic X C C X C X vinylic substituents are attached to vinylic carbons 4

Vinylic versus Allylic X X C C X C allylic substituents are attached to allylic carbons 4

10.2 Allylic Carbocations C + 5

relative rates: (ethanolysis, 45°C) Allylic Carbocations the fact that a tertiary allylic halide undergoes solvolysis (SN1) faster than a simple tertiary alkyl halide Cl CH3 C H2C CH CH3 C CH3 Cl CH3 123 1 relative rates: (ethanolysis, 45°C) 6

Allylic Carbocations provides good evidence for the conclusion that allylic carbocations are more stable than other carbocations CH3 CH3 H2C C + CH CH3 C + CH3 CH3 formed faster 6

H2C=CH— stabilizes C+ better than CH3— Allylic Carbocations provides good evidence for the conclusion that allylic carbocations are more stable than other carbocations CH3 H2C CH + CH3 C CH3 C + CH3 H2C=CH— stabilizes C+ better than CH3— 6

Stabilization of Allylic Carbocations Delocalization of electrons in the double bond stabilizes the carbocation resonance model orbital overlap model 9

Resonance Model CH3 H2C CH + C CH3 CH3 H2C CH + C 10

Resonance Model CH3 H2C CH + C CH3 CH3 H2C CH + C CH3 d+ d+ H2C CH C 10

Orbital Overlap Model d+ d+ 12

Orbital Overlap Model 12

Orbital Overlap Model 12

Orbital Overlap Model 12

Hydrolysis of an Allylic Halide Cl CH3 C H2C CH H2O Na2CO3 CH3 OH CH3 C H2C CH + HOCH2 CH C CH3 (15%) (85%) 16

Corollary Experiment CH3 ClCH2 CH C H2O Na2CO3 CH3 HOCH2 CH C OH CH3 C + (15%) (85%) 16

give the same products because they form the same carbocation Cl CH3 C H2C CH CH3 ClCH2 CH C and give the same products because they form the same carbocation 16

give the same products because they form the same carbocation Cl CH3 C H2C CH CH3 ClCH2 CH C and give the same products because they form the same carbocation CH3 CH3 + H2C C + CH H2C CH C CH3 CH3 16

more positive charge on tertiary carbon; therefore more tertiary alcohol in product + H2C CH C + H2C CH C CH3 CH3 16

(85%) (15%) CH3 CH3 C + C H2C CH OH HOCH2 CH CH3 CH3 more positive charge on tertiary carbon; therefore more tertiary alcohol in product CH3 CH3 + H2C + CH C H2C CH C CH3 CH3 16

10.3 Allylic Free Radicals C • 5

Allylic free radicals are stabilized by electron delocalization • C • 22

Free-radical stabilities are related to bond-dissociation energies 410 kJ/mol • CH3CH2CH2—H CH3CH2CH2 + H• 368 kJ/mol • CHCH2—H H2C CHCH2 H2C + H• C—H bond is weaker in propene because resulting radical (allyl) is more stable than radical (propyl) from propane 23

10.4 Allylic Halogenation 5

Chlorination of Propene addition ClCH2CHCH3 Cl CHCH3 H2C + Cl2 CHCH2Cl H2C 500 °C + HCl substitution 25

selective for replacement of allylic hydrogen free radical mechanism Allylic Halogenation selective for replacement of allylic hydrogen free radical mechanism allylic radical is intermediate 26

Hydrogen-atom abstraction step Cl : . .. H 410 kJ/mol 368 kJ/mol H allylic C—H bond weaker than vinylic chlorine atom abstracts allylic H in propagation step 27

Hydrogen-atom abstraction step Cl : .. • C C H H C 410 kJ/mol 368 kJ/mol H 27

reagent used (instead of Br2) for allylic bromination N-Bromosuccinimide reagent used (instead of Br2) for allylic bromination Br O NBr O NH heat + + CCl4 (82-87%) 29

Allylic halogenation is only used when: Limited Scope Allylic halogenation is only used when: all of the allylic hydrogens are equivalent and the resonance forms of allylic radical are equivalent 30

Cyclohexene satisfies both requirements Example H Cyclohexene satisfies both requirements All allylic hydrogens are equivalent 31

Cyclohexene satisfies both requirements Example H Cyclohexene satisfies both requirements All allylic hydrogens are equivalent H • H • Both resonance forms are equivalent 31

All allylic hydrogens are equivalent 2-Butene CH3CH CHCH3 Example All allylic hydrogens are equivalent 2-Butene CH3CH CHCH3 But • CH3CH CH CH2 • CH3CH CH CH2 Two resonance forms are not equivalent; gives mixture of isomeric allylic bromides. 31