文档介绍：Hammond’s Postulate
Because hydrogen abstraction for chlorination is exothermic:
the transition state resembles the alkn, O2, and no other oxidizing agent
occurs by a radical chain mechanism similar to that for allylic halogenation
in this section, we concentrate on autoxidation of the hydrocarbon chains of polyunsaturated triglycerides
the characteristic feature of the fatty acid chains in polyunsaturated triglycerides is the presence of 1,4-dienes
radical abstraction of a doubly allylic hydrogen of a 1,4-diene forms a particularly stable radical
Radical Autoxidation
autoxidation begins when a radical initiator, X•, abstracts a doubly allylic hydrogen
this radical is stabilized by resonance with both double bonds
Radical Autoxidation
the doubly allylic radical reacts with oxygen, itself a diradical, to form a peroxy radical
the peroxy radical then reacts with another 1,4-diene to give a new radical, R•, and a hydroperoxide
vitamin A, a naturally occurring antioxidant, reacts preferentially with the initial peroxy radical to give a resonance-stabilized phenoxy radical, which is very unreactive, and scavenges another peroxide radical
Radical Autoxidation
vitamin E as an antioxidant
Radical Addition of HBr to Alkenes
Addition of HBr to alkenes gives either Markovnikov addition or non-Markovnikov addition depending on reaction conditions
Markovnikov addition occurs when radicals are absent
non-Markovnikov addition occurs when peroxides or other sources of radicals are present
Radical Addition of HBr to Alkenes
addition of HCl and HI gives only Markovnikov products
to account for the the non-Markovnikov addition of HBr in the presence of peroxides, chemists proposed a radical chain mechanism
Chain initiation
Radical Addition of HBr to Alkenes
Chain propagation
Radical Addition of HBr to Alkenes
Chain termination
This pair of addition reactions illustrates how the products of a reaction can often be changed by a change in experimental con