Chap-8A - Chapter 8 Reactions of Alkenes Chapter 82 8-1...

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Unformatted text preview: Chapter 8 Reactions of Alkenes Chapter 82 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. Chapter 83 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. CC + E + C E C + C E C ++ Nuc: _ C E C Nuc • Step 2: Nucleophile attacks the carbocation. Chapter 84 Types of Additions Chapter 85 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 CHCH 3 H CH 3 C CH 3 CHCH 3 H HB r CH 3 C CH 3 CHCH 3 Chapter 86 CH 3 C CH 3 CHCH 3 HB r CH 3 C CH 3 CHCH 3 H + + Br _ CH 3 C CH 3 CHCH 3 H + Br _ CH 3 C CH 3 CHCH 3 H Br Chapter 87 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. Chapter 88 What is the product? Chapter 89 •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 Chapter 810 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. Chapter 811 Free Radical Initiation •Peroxide O-O bond breaks easily to form free radicals. + RO HB r ROH + Br OO RR + RO OR heat • Hydrogen is abstracted from HBr. Electrophile Chapter 812...
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Chap-8A - Chapter 8 Reactions of Alkenes Chapter 82 8-1...

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