Lecture_25

Lecture_25 - Lecture 25 Ch. 8 Subs1tu1on Ch. 9...

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Unformatted text preview: Lecture 25 Ch. 8 Subs1tu1on Ch. 9 Elimina1on Which of the following can't undergo nucleophilic subs1tu1on reac1ons? Which of the following can't undergo nucleophilic subs1tu1on reac1ons?  II cannot because it is a vinylic halide Conjugated Halides  Vinyl and aryl halides do not go SN1 or SN2 –  Electron cloud repulsion (SN2) –  Instability of carboca>on (SN1) What was the mechanism for this reac1on?  A) SN1  B) SN2  C) E1  D) E2  E) none of the above What was the mechanism for this reac1on?  A) SN1  B) SN2  C) E1  D) E2  E) none of the above What was the mechanism for this reac1on?  A) SN1  B) SN2  C) E1  D) E2  E) none of the above What was the mechanism for this reac1on?  A) SN1  B) SN2  C) E1  D) E2  E) none of the above Compe11on Compe11on Intermolecular and Intramolecular  Which do you get? –  Depends on concentra>on and size of ring Intermolecular and Intramolecular  Low concentra>on –  Favors intramolecular  High concentra>on –  Favors intermolecular  If it is 5 or 6 membered, favored Chapter 9: Elimina1on Compe11on with Subs1tu1on  E2: Elimina>on Bimolecular  Called dehydrohalogena>on, 1,2- elimina>on, or beta- elimina>on E2 is Regioselec1ve  Which proton do I take? –  2- bromopropane has only 1 choice –  2- bromobutane, however, has two choices E2 is Regioselec1ve  Answer lies in the reac>on coordinate diagram  Zaitsev’s Rule- proton removed from β- carbon with fewest hydrogens E2 is Regioselec1ve  Based on stabili>es of resul>ng alkenes:  Zaitsev’s rule sucks? –  Can’t always predict more stable alkene –  Doesn’t account for conjuga>on E2 is Regioselec1ve  By the way…just to confuse you… –  Even if there isn’t conjuga>on, some>mes more stable product isn’t major product E2 is Regioselec1ve  If RX is not hindered, then you would get expected products (more stable ones) E2 is Regioselec1ve  Wait…there are more excep>ons… –  Fluorine doesn’t behave like the others –  Fluroine is bad leaving group, so as base pulls hydrogen off, neg charge builds up on carbon E2 is Regioselec1ve  Stabili>es for carbanions are opposite of carboca>ons E2 is Regioselec1ve  Summary of sec>on 9.2: Major product is most stable alkene unless… –  Reactants are sterically hindered –  Conjuga>on results from another pathway –  Poor leaving group (fluorine) Ques1on:  What would be a good base to get the an>- Zaitsev product? E1  Elimina>on unimolecular  How is it possible for H2O to pull that proton off? Isn’t the pKa of an alkane too high? E1  pKa is greatly reduced due to neighboring posi>ve charge and hyperconjuga>on E1  Major product is more stable alkene usually –  Due to small difference in 2nd transi>on state E1  Rate of E1 depends upon ease of leaving group and the stability of carboca>on E1  Rearrangements Compete1on Between E1 and E2   Primary RX only go E2 (primary C+ too unstable) –  Secondary and Ter>ary go E1 or E2  E2 (and SN2) favored by: –  High concentra>on of strong base –  Polar apro>c solvent (DMF, DMSO)  E1 (and SN1) favored by: –  Weak base –  Polar pro>c solvent (H2O, ROH) Elimina1ons are Stereoselec1ve  We have seen that they are regioselec>ve: –  Let’s look at E2 stereoselec>vity for a couple slides Elimina1ons are Stereoselec1ve  Two ways to elimina>on Elimina1ons are Stereoselec1ve   So more E than Z in 2- bromopentane elimina>on Elimina1ons are Stereoselec1ve   E2 elimina>on of chiral centers ...
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