Wade08 - Reactivity of C=C Electrons in pi bond are loosely...

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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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Electrophilic Addition Step 1: Pi 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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Types of Additions =>
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Addition of HX (1) 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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Addition of HX (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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Regiospecificity 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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Regiospecificity Tertiary Bromide Secondary Bromide CH 3 C CH 3 C CH 3 H Br CH 3 C CH 3 CH CH 3 CH 3 C CH 3 CH CH 3 H CH 3 C CH 3 CH CH 3 H Br CH 3 C CH 3 CH H Br CH 3
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Regiospecificity Tertiary Bromide Secondary Bromide H Br H H H Br H Br
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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. =>
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Free Radical Initiation 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 =>
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Propagation Steps 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.
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Anti-Markovnikov ?? Tertiary radical is more stable, so that intermediate forms 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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Hydration of Alkenes Reverse of dehydration of alcohol Use very dilute solutions of H 2 SO 4 or H 3 PO 4 to drive equilibrium toward hydration. => C C + H 2 O H + C H C OH alkene alcohol
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Mechanism for Hydration + C H C + H 2 O C H C O H H + + H 2 O C H C O H H + C H C O H H 3 O + + => C C O H H H + + + H 2 O C H C +
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Orientation for Hydration Markovnikov product is formed. + CH 3 C CH 3 CH CH 3 O H H H + + H 2 O + H CH 3 CH CH 3 C CH 3 H 2 O CH 3 C CH 3 CH CH 3 H O H H + H 2 O CH 3 C CH 3 CH CH 3 H O H =>
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Indirect Hydration Oxymercuration-Demercuration Markovnikov product formed Anti addition of H-OH No rearrangements Hydroboration Anti-Markovnikov product formed Syn addition of H-OH =>
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Oxymercuration (1) Reagent is mercury(II) acetate which
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This note was uploaded on 01/12/2009 for the course CHE 201 taught by Professor Bong during the Fall '08 term at SUNY Buffalo.

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Wade08 - Reactivity of C=C Electrons in pi bond are loosely...

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