L13a - - Elimination Reactions: OH H2SO Product...

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1 1 Product distribution X Y . X Z .. Z 2% 1% 0% 0% 95% 2% H 2 SO 4 140-170 o OH 1. X Y Z 2. X Z Y 3. Y X Z 4. Y Z X 5. Z X Y 6. Z Y X Major minor middle XY Z 2 © Kay Sandberg β - Elimination Reactions: Section 5.13 Rearrangements in alcohol dehydration CH 3 CH 2 CH CH 2 12% C C H 3 C H CH 3 H 32% C C H 3 C H H CH 3 56% + + regioselective stereoselective CH 3 CH 2 CH 2 CH 2 OH H 2 SO 4 140-170 o 3 © Kay Sandberg H C H 3 C C H CH 3 Notice the spatial relationship of methyl groups H C H 3 C C CH 3 H Major minor G o cis - trans - reaction progression 4 ©Dr. Kay Sandberg Last lecture This lecture Alkene preparations Alkene reactions E1 mechanism dehydration of alcohols E2 mechanism dehydrohalogenation of alkyl halides 5 © Kay Sandberg Summary: Transformations involving alcohols Alkene formation: alcohol dehydration Alkyl halide formation CC H OH α β + H OH H + R —OH R —X + H —OH H —X In both (for 2 o & 3 o ROH), RDS is dissociation of alkyloxonium ion (unimolecular) forming H 2 O and C + . 6 © Kay Sandberg - Elimination Reactions: Section 5.14 De hydro halogenation of alkyl halides HX + base + Na X Na OCH 2 CH 3 + H OCH 2 CH 3 “” O ppp product prediction purposes alkyl halide
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2 7 © Kay Sandberg β - Elimination Reactions: Section 5.14 De hydro halogenation of alkyl halides CC HX α β + base Na OCH 2 CH 3 HOCH 2 CH 3 , 55 o ““ sodium ethoxide ethanol cyclohexyl chloride Cl Cl H H H H H H H H H typical reagent system typical reagent system © Kay Sandberg Reactions: Preparation of alkenes Section 5.15 H X Base - Elimination Reactions: De hydro halogenation HS B + + X SB grab “good” corresponds to “lower energy” Do you see anything “bad” that results? “bad” corresponds to “higher energy” Do you see anything “good” that results? slam boot 9 © Kay Sandberg Section 5.15 H X Base H B + + X SB C-H bond breakage π bond “ makeage C-X bond breakage B-H bond “ H X δ− B δ− Good? Look at the species with the negative charge. Bad? σ -electrons are converted to π -electrons. 10 © Kay Sandberg Section 5.15 H X Base H B + + X B H X δ− B δ− E 2 mechanism: elimination bimolecular R X & B - 11 © Kay Sandberg Reactions: Preparation of alkenes Section 5.15 H X Base - Elimination Reactions: De hydro halogenation H B + + X B E 2 mechanism: elimination bimolecular 1) Rate is second order: 1st order in R X & 1st order in B - R = k [R X ][ B - ] 2 nd order kinetics 12 © Kay Sandberg Reactions: Preparation of alkenes Section 5.15 H X Base - Elimination Reactions: De hydro halogenation H B + + X B E 2 mechanism: elimination bimolecular 2) Rate depends on halogen of alkyl halide The weaker the C-X bond the easier to break the faster the rxn R F << R Cl < R Br < R I Iodide is the best leaving group
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3 13 H C H C Br H H H O C C C C H H H H H H H H H © Kay Sandberg 14 H H C H C Br H H δ - O C C C C H H H H H H H H H δ - © Kay Sandberg 15 H H C H C Br H H δ - O C C C C
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L13a - - Elimination Reactions: OH H2SO Product...

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