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Chap-8A - Chapter 8 Reactions of Alkenes 8-1 Reactivity of...

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Chapter 8 Reactions of Alkenes
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Chapter 8 2 8-1 Reactivity of the C=C Bond • Since σ bonds are stronger than π bonds, double bonds tend to react to convert the double bond into σ bonds. Electrophiles are attracted to the π electrons. Carbocation intermediate forms. Nucleophile adds to the carbocation. Net result is addition to the double bond. Addition reactions are typically exothermic.
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Chapter 8 3 8-2 Electrophilic Addition to Alkenes • The π bond is localized above and below the C-C σ bond. The π electrons are relatively far away from the nuclei and are therefore loosely bound. Step 1: π electrons attack the electrophile. C C + E + C E C + C E C + + Nuc: _ C E C Nuc Step 2: Nucleophile attacks the carbocation.
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Chapter 8 4 Types of Additions
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Chapter 8 5 8-3 Addition of Hydrogen Halide to Alkenes Protonation of double bond yields the most stable carbocation. Positive charge goes to the carbon that was not protonated. X + Br _ + + CH 3 C CH 3 CH CH 3 H CH 3 C CH 3 CH CH 3 H H Br CH 3 C CH 3 CH CH 3
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Chapter 8 6 CH 3 C CH 3 CH CH 3 H Br CH 3 C CH 3 CH CH 3 H + + Br _ CH 3 C CH 3 CH CH 3 H + Br _ CH 3 C CH 3 CH CH 3 H Br
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Chapter 8 7 8-3A Orientation of Addition: Markovnikov’s Rule Markovnikov’s Rule: The proton of an acid adds to the carbon in the double bond that already has the most H’s. “Rich get richer.” More general Markovnikov’s Rule: In an electrophilic addition to an alkene, the electrophile adds in such a way as to form the most stable intermediate. HCl, HBr, and HI add to alkenes to form Markovnikov products.
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Chapter 8 8 What is the product?
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Chapter 8 9 • The electrophile adds to the sp 2 carbon that is bonded to the greater number of hydrogens • Addition is regioselective Regioselective reaction : a reaction in which one direction of bond-forming or bond-breaking occurs in preference to all other directions
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Chapter 8 10 8-3B Free-Radical Addition of HBr In the presence of peroxides, HBr adds to an alkene to form the “anti-Markovnikov” product. Only HBr has the right bond energy. HCl bond is too strong. HI bond tends to break heterolytically to form ions. The free radical initiators change the mechanism of addition from electrophilic addition to free radical addition. This change of mechanism gives rise to the opposite regiochemistry.
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Chapter 8 11 Free Radical Initiation Peroxide O-O bond breaks easily to form free radicals.
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