Allylic Free Radicals C C C

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

Free-radical stabilities are related to bond-dissociation energies CH 3 CH 2 CH 2 —H 410 kJ/mol CH 3 CH 2 CH 2 + H 368 kJ/mol + H CHCH 2 —H H2CH2CH2CH2C CHCH 2 H2CH2CH2CH2C C—H bond is weaker in propene because resulting radical (allyl) is more stable than radical (propyl) from propane

Allylic Halogenation

ClCH 2 CHCH 3 Cl addition 500 °C substitution CHCH 3 H2CH2CH2CH2C + Cl 2 CHCH 2 Cl H2CH2CH2CH2C + HCl Chlorination of Propene

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

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

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

Br reagent used (instead of Br 2 ) for allylic bromination + heat CCl 4 (82-87%) + N-BromosuccinimideOO NBr OO NHNHNHNH

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

Example Cyclohexene satisfies both requirements All allylic hydrogens are equivalent HHHH

Example Cyclohexene satisfies both requirements All allylic hydrogens are equivalent Both resonance forms are equivalent HH H HH H HHHH

Example 2-Butene All allylic hydrogens are equivalent Two resonance forms are not equivalent; gives mixture of isomeric allylic bromides. CH 3 CH CHCH 3 But CH 3 CH CH CH 2 CH 3 CH CH CH 2