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Ch_322a_8.01 - relatively weak-bond(~60 kcal/mol is...

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Chapter 8 Alkenes and Alkynes II: Addition Reactions Overview Alkenes typically undergo addition reactions . C=C + A-B C-C A B In these reactions the ! -bond is destroyed as two new " -bonds are formed. Examples C=C H-X C-C H X C-C H OSO 3 H H-OSO 3 H H-OH C-C H OH In these examples, H + , an electrophile , adds first followed by a nucleophile.
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General Features of Alkene Reactivity Addition reactions of alkenes are typically two-step processes . Addition of an electrophile produces a cation intermediate that then reacts with a nucleophile . These reactions have the following features: (1) The addition reaction transforms the ! -bond of the alkene and one " -bond in A-B into two " -bonds in the product. C=C + A-B ! - bond " - bond bonds lost C-C A B ! - bonds bonds gained The overall energy change is usually favorable because a
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Unformatted text preview: relatively weak !-bond (~60 kcal/mol) is exchanged for a strong "-bond. (2) The !-electrons are exposed and are highly polarizable. Electrophiles are attracted to the exposed negative charge. C C + E + electrophile C C E + electrophilic attack on the easily accessible and polarizable !-electrons Typical electrophiles are positively charged species such as H + , R + , and metal cations, and Lewis acids such as BF 3 and AlCl 3 . (3) Carbocation intermediates are formed after electrophilic attack. C C : + H-X C C + H + X-carbocation The carbocation rapidly reacts with the nucleophile : C C + H X: + C X C H C X C H-...
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Ch_322a_8.01 - relatively weak-bond(~60 kcal/mol is...

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