Chapter 8 - Wade(1)

Chapter 8 - Wade(1) - Introduction to General and Organic...

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Chapter 8 Reactions of Alkenes Dr. David P. Brown Department of Chemistry Introduction to General and Organic Chemistry III
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Chapter 8 2 Reactivity of C=C Electrons in pi bond are loosely held. Electrophiles are attracted to the pi electrons. Carbocation intermediate forms. Nucleophile adds to the carbocation. Net result is addition to the double bond. =>
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Chapter 8 3 Electrophilic Addition
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Chapter 8 4 Addition of HBr (1) Protonation of double bond yields the most stable carbocation . Positive charge goes to the carbon that was not protonated.
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Chapter 8 5 Addition of HBr (2) 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 6 Regiospecificity Markovnikov’s Rule : The proton of an acid adds to the carbon of 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 7 Examples:
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Chapter 8 8 Cl HCl In some cases, product(s) resulting From rearrangement of the initially Formed carbocation is observed! 2 o R +
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Chapter 8 9 ~CH 3 Cl Rearrangement occurs by a methyl shift. 2 o R + 3 o R + Product of rearrangement
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Chapter 8 10 Free-Radical Addition of HBr In the presence of peroxides ( ROOR ), 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. =>
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Chapter 8 11 Mechanism Peroxide O-O bond breaks easily to form free radicals. + R O H Br R O H + Br O O R R + R O O R heat Hydrogen is abstracted from HBr. Electrophile => Initiation:
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Chapter 8 12 Bromine adds to the double bond. + C Br C H Br + C Br C H Br Electrophile => C Br C C C Br + Hydrogen is abstracted from HBr. Propagation Steps:
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Chapter 8 13 Why Anti-Markovnikov ? Tertiary radical is more stable, so that intermediate is formed faster . CH 3 C CH 3 CH CH 3 Br + CH 3 C CH 3 CH CH 3 Br CH 3 C CH 3 CH CH 3 Br X
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14 Hydration of Alkenes Reverse of dehydration of alcohols. Use very dilute solutions of H
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Chapter 8 - Wade(1) - Introduction to General and Organic...

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