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Ch 8 Notes - Acid-Base Reactions pKa 4.82 2.86 Ka 1.5 x...

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Inductive effect (through bond) falls off with distance (bonds) pK a K a Δ G° (kcal/M) 4.82 2.86 4.05 4.52 1.5 x 10 -5 139 x 10 -5 8.9 x 10 -5 3.0 x 10 -5 6.7 4.0 5.7 6.3 Acid-Base Reactions CO 2 CO 2 H + H + CO 2 CO 2 H + H + Cl Cl CO 2 CO 2 H + H + CO 2 CO 2 H + H + Cl Cl Cl Cl
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H 3 C C O O H H 3 C C O O + H+ H 3 C C O O pKa = 4.76 H 3 C C O O H 3 C C O O RESONANCE: 1. When a molecule can be represented by 2 or more structures that differ only in the arrangement of electrons, there is resonance. 2. The real molecule or resonance hybrid is more stable than any of the contributing structures. Acid-Base Reactions H 3 C H 2 C O H H 3 C H 2 C O + H+ pKa = 17
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R C LG R R ! + ! - Nuc + R C LG R R + Nuc Electron rich reactant Nucleophile base Electron poor reactant Electrophile acid A group leaves in both, the more stable, the more readily it leaves B N : N B - + : Substitution Reactions H O + + O H H O CH 3 O H H H O O CH 3 ! + ! -
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C H OH CH 2 N H H HO HO NORADRENALINE C H OH CH 2 N Me H HO HO ADRENALINE (EPINEPHRINE) S NH 3 + -O 2 C Me O BASE HO OH H H H S-ADENOSYLMETHIONINE (SAM) S NH 3 + -O 2 C O BASE HO OH H H NORADRENALINE AND ADRENALINE ARE HORMONES THAT ARE RELEASED IN RESPONSE TO STRESS (FIGHT OR FLIGHT). ADRENALINE IS THE MORE POTENT.
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H C Br H H ! + ! - H C H H Br + H C Br H H ! + ! - H C H H N C H C H H C N H C H H C N ! + ! - carbocation (a reactive intermediate) 1) 2) slow fast S N 1 Rate = k [MeBr] SM I P Different Mechanisms H C Br H H ! + ! - H C Br H H + C N C N H C Br H H ! - N C ! - S N 2 Rate = k [MeBr] [CN-] 2 in rds SM P Note: as reaction starts, lone pair attack at sigma star
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EXAMPLE: KOtBu Br H H H Me Me H H Me Me OtBu + + + H KBr LG H R 1 R 2 R 3 R 4 R 1 R 2 R 3 R 4 B + + + LG: H on C next to (vicinal) leaving group Base (not always charged) new pi system leaving group B: H GENERAL REACTION: Lewis acid-base reaction; same as in S N 1 slow + (1) ! + trans. state E1 Br H H H Me Me Br H H H Me Me !" Br (2) :OtBu + KOtBu Bronsted acid base reaction + fast H K+ 1. Possible mechanisms (as before unimolecular and bimolecular): rate = k [tBuBr] E1 elimination unimolecular (one molecule in TS) RDS is the first stp 2 stp 1 Elimination Reactions
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LG H R 1 R 2 R 3 R 4 R 1 R 2 R 3 R 4 H R 1 R 2 R 3 R 4 H R 1 R 2 R 3 R 4 Nuc H R 1 R 2 R 3 R 4 LG H R 1 R 2 R 3 R 4 LG H R 1 R 2 R 3 R 4 Nuc LG H R 1 R 2 R 3 R 4 ! - LG H R 1 R 2 R 3 R 4 LG ! - ! - ! - ! + Base Base H Base LG S N 1 (step 1) LG ! + Nuc S N 2 E1(step 1) E2 ! - ! - Nuc E1 (step 2) S N 1 (step 2) S N 1 S N 2 E1 E2 Big Picture of Ionic Reactions
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C H H R H H H H H H H H H H p p pi pi 50% 50% a b c a b c H H H H H H H H H H p p pi pi 50% 50% a b c a b c C H H R Normal carbonium Csp 2 These considerations also apply to other intermediates next to pi systems: Delocalized C+, no change in hybridization Delocalized radical Csp 2 Normal radical shallow pyramid Also for other atoms in pi systems: H H H H H Allyl carbanion H H H H H H H O H H H O H H H O H less more enolate anion Why more e-density on oxygen?
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Please build a model… Carbanion cannot overlap with adjacent pi system H H H H H H H H H H H H H H H H H H C 1 2 3 1 2 3 View along C2C3 Note: resonance is only possible if orbitals properly overlap. The carbanion below is adjacent to a
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