12 - Chapter 12 Addition reactions to alkenes: Structure...

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Chapter 12 – Addition reactions to alkenes: Structure and reactivity with more details Now that we have a basic understanding of addition reactions to alkenes, we can look at variations of this reaction to gain further insight into some important structure and reactivity principles of organic chemistry. Upon closer look at addition to alkenes, we will discover an alternative mechanism and we will use thermochemical principles to determine how alkene substitution influences the driving force of the reaction. First, let’s look at an alternative mechanism for addition of H-Br to an alkene. Alkene Addition Reactions via Radical Intermediates As previously mentioned in Chapter 9, most organic reactions involve the movement of electron pairs. For radical reactions, however, the electron reconfiguration involves the movement of single electrons. Whereas heterolytic bond cleavage leads to ion pairs, homolytic bond cleavage results in unpaired electrons – or free radicals. Some weak bonds have a tendency to fragment homolytically (e.g., peroxides, halogens). Chemists use a slight variation of curved arrow notation to show the movement of single electrons. For single electron movment, “fishhook” arrows, i.e., single headed arrows are drawn. The difference in these two notations is illustrated below for heterolytic vs. homolytic fragmentation of Br 2 . When peroxides are heated, they fragment homolytically. This chemical behavior can be used to initiate addition reactions that proceed via radical intermediates. A consequence of the different reaction mechanism is that the distribution of products can vary from that seen for the heterolytic reaction case, as the comparison below illustrate.
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On the Stability of Carbon-Centered Radicals In order to rationalize the reactivity trends seen above, we must first understand how radical stability depends on structure. The radical is a half-occupied electron pair domain.
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12 - Chapter 12 Addition reactions to alkenes: Structure...

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