Section_7_Alkenes_Alkynes_post

Section_7_Alkenes_Alkynes_post - Vollhardt: 12-1 to 12-13...

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Sec. 7 - alkenes/alkynes 1 Hydrogenation of Alkenes (Stereospecific) C C R R R R Pd / H 2 syn addition (same side) C C R R R R H H CH 2 CH 3 CH 3 H 2 , PtO 2 , EtOH, syn addition (same side) CH 2 CH 3 CH 3 H H H 3 C H 3 CH 2 C H H + 82% cis-1-ethyl-2-methylcyclohexane (Racemic) In the absence of a catalyst, hydrogenations of alkenes, although exothermic, do not spontaneously occur, even at high temperatures. In the presence of a catalyst , the same hydrogenations proceed at a steady rate, even at room temperature. The most frequently used catalysts for hydrogenation reactions are: Palladium dispersed on carbon (Pd-C) Collodial platinum (Adam’s catalyst, PtO 2 ) Nickel (Raney nickel, Ra-Ni) C C R R R R Vollhardt: 12-1 to 12-13
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Sec. 7 - alkenes/alkynes 2 The primary function of a catalyst in hydrogenation reactions is to provide metal-bound hydrogen atoms on the catalyst surface: Common solvents used for hydrogenations include methanol, ethanol, acetic acid, and ethyl acetate (very polar solvents).
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Sec. 7 - alkenes/alkynes 3 Addition Reactions to Alkenes Addition of Hydrogen Halides to Alkenes CC H H H H R 2 C=CR 2 + HX R 2 H C-CR 2 X Nucleophile (weak base) + H + Electrophile (acid) H H H H H + General Reaction X Nucleophile (base) Electrophile (acid) H H H H H X Order of reactivity = HI > HBr > HCl > HF Why? Slow Fast Reactivity parallels acid strength. The alkene is a very weak base so you need a very strong acid to get a reaction. Why? Unsymmetrical alkene
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Sec. 7 - alkenes/alkynes 4 Markovnikov’s Rule In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon atom of the double bond forming the more stable carbocation as an intermediate (you already know this concept from S N 1 reactions: the most stable carbocation intermediate forms). OR Hydrogen becomes attached to the carbon atom of the double bond with the greater number of hydrogens (for addition of HX)
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Sec. 7 - alkenes/alkynes 5 Examples C C H CH 3 H 3 C H 3 C H Br + nucleophile (weak base) electrophile (acid) C C H CH 3 H 3 C H 3 C H + C CH 2 CH 3 H 3 C H 3 C Br sp 2 sp 2 sp 3 sp 3 major product CH 3 CH 3 H Br + CH 3 CH 3 H H + CH 3 CH 3 Br Br CH 3 CH 3 (R) (R) (R) (S) diastereomers bottom top top bottom +
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Sec. 7 - alkenes/alkynes 6 C H 3 C H 3 C CH 2 Examples I – Cl δ - δ + C H 3 C H 3 C CH 2 I + Cl C CH 3 H 3 C CH 2 I Cl nucleophile (weak base) electrophile (acid) C H 3 C H 3 C CH 2 dilute H 2 SO 4 hydration water present S O O O H OH O H H H or + C H 3 C H 3 C CH 3 + S O O OO H + OH 2 H 2 O - weak base poor nucleophile but better than HSO 4 ¯ also in greater concentration C O H 3 C CH 3 CH 3 H H + H 2 O C OH H 3 C CH 3 CH 3 H 3 O + +
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Sec. 7 - alkenes/alkynes 7 Examples What is the product of the following reactions dilute H 2 SO 4 heat concentrated H 2 SO 4 NaBr HO concentrated H 2 SO 4 H Br A) B) C) D) E) H HO Br OH H Text-alkene-1-3
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Sec. 7 - alkenes/alkynes 8 Alkene Hydration and Alcohol Dehydration are Equilibrium Processes All steps are reversible in the hydration of alkenes. The proton serves as a catalyst only; i.e. it is regenerated in the reaction.
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Section_7_Alkenes_Alkynes_post - Vollhardt: 12-1 to 12-13...

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