Ch_322a_11.013

Ch_322a_11.013 - The zinc chloride ( a Lewis acid )...

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The Reaction of Methyl and Primary Alcohols with HX Reagents: An S N 2 Mechanism The Reaction of Methanol and conc. HBr (1) CH 3 OH + H-O-H H : + fast CH 3 -O-H H + + H 2 O Fast and reversible protonation of the hydroxyl group produces a better leaving group for the S N 2 reaction with bromide ion. (2) CH 3 -O-H H + Br : - + Br-CH 3 + H 2 O slow S N 2 protonation nucleophilic substitution The S N 1 mechanism is energetically unfavorable because of the high energy methyl carbocation intermediate. The S N 2 reaction proceeds well with methyl and primary systems because of favorable steric factors.
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Electrophilic Assistance with Zinc Chloride The addition of zinc chloride (ZnCl 2 ) greatly accelerates the reaction of primary and secondary alcohols with HCl.
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Unformatted text preview: The zinc chloride ( a Lewis acid ) catalyzes the reaction by coordinating to the oxygen and enhancing the rate of the key bond heterolysis step, as shown below. R-O H : : + ZnCl 2 Lewis acid R-O H ZnCl 2 +-fast R-O H ZnCl 2 +-Cl :-good leaving group + slow Cl-R + Zn(OH)Cl 2-Zn(OH)Cl 2-+ H + ZnCl 2 H 2 O + overall ROH + HCl ZnCl 2 RCl + H 2 O Quiz 11.013 Provide the products of the following reactions. CH 3 CH 2 CH 2 OH conc. HBr CH 3 CHCH 3 OH conc. HCl ZnCl 2 (CH 3 ) 3 C CH 3 OH conc. HCl CH 3 CH 2 CH 2 Br CH 3 CHCH 3 Cl t-Bu CH 3 Cl + t-Bu CH 3 Cl...
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Ch_322a_11.013 - The zinc chloride ( a Lewis acid )...

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