Chemistry 250-02 Organic Chemistry I Fall, 2014 Day 32 Chapter 11 Organic Chemistry I Fall, 2014 Day 32 Chapter 11 Substitution and Elimination Chem Act 13 C & D (Today) then 14 A & B (Wed)

McMurry Problem 11.26 A “Walden” cycle What is the purpose for making B ? Why is [α]D for B positive ? What kind of reaction is B -> C and what is the mechanism ? What is the optical purity of C and what are the %’s of the two enantiomers of C ? How could you make C from B with 100 % enantiomeric excess (i.e. optically pure) ? What kind of reaction is D -> E and what is the mechanism ? How can you convert A into CH3-CHCl-CH2 -Phenyl ?

SN2 It’s not always straight inversion. R. Wester, M. Weidem-ller (U SN2 It’s not always straight inversion ! R. Wester, M. Weidem-ller (U. Freiburg), W. Hase (Texas Tech) Science 2008, 319, 183

Chem Activity 14A Elimination as the reverse of addition

An Elimination Reaction

Two-Step Elimination (E1) Carbocation intermediate formed (r.d.s.) Rate = k [RX] 1 Unimolecular Unimolecular elimination = E1 Must have β-hydrogen available Weak “patient” base Regiochemistry: Zaitsev’s rule Stereochemistry: E/Z alkenes formed (slight preference for E)

SN1 vs. E1

Chem Activity 14B

One-Step Elimination (E2) Rate = k [RX]1 [Base]1 Bimolecular Bimolecular elimination = E2 Must have β-hydrogen available Strong “impatient” base (RO-, R2N-) Regiochemistry: Zaitsev product with small base Hofmann product with large base Stereochemistry: anti elimination (usually)

The E2 Elimination Reaction

E2 Eliminations

What are the products ?

What are the products ? E1 K+ E2

SN1 SN2 E1 E2 or E2cB ? What other products are possible ?

SN2 vs. E2

What is the product and why ?

What is the product and why ?

What is the product? Points to consider: What kind of substrate? Do temperature conditions favor E vs. S? Good or poor nucleophile? Strong or weak base? Big or small base?

Hint: you’ve seen this behavior from carbocations before… What happened here? Hint: you’ve seen this behavior from carbocations before…

Assess the possibilities Points to consider: What kind of substrate? Do temperature conditions favor E vs. S? Good or poor nucleophile? Strong or weak base? Big or small base?

Summary

Chem Activity 14C E2 via anti periplanar conformer

E2 via Anti Stereochemistry

Which compound would serve as the best starting material for the transformation shown below? 1. 2. 3. 4. 5.

Including stereoisomers, how many products are possible from the following reaction? 1 2 3 4 5

What is the major elimination product? B C D

What is the major elimination product? B C D

What is the major elimination product? Hint: Memorization Task 14.4 B C D

What is the major product of the following reaction? 1. 2. 3. 4. 5.

What is the best base for this reaction? NaOH NaOCH3 NaOCH2CH3 Na

E1 and E2 Eliminations

What is the most likely mechanism for this reaction? SN1 SN2 E1 E2

CA 14C Stereochemistry of E2 Read Model 8 and answer CTQ 36.

T/F: Configurational stereoisomers can interconvert via a chair flip. True; a chair flip converts cis to trans. True; a chair flip converts axial to equatorial but not tBu. False; a chair flip does not convert cis to trans. False; configurational stereoisomers are constitutional isomers.

CA 14C Stereochemistry of E2 Read Model 9 and answer CTQ 37-47. Note: Pg. 215, CTQ 45 the last product has an –OCH3 group, not –OH.

Explain this result

What product is formed? A B C

What is the most likely product of the following reaction? 1. 2. 3. 4. 5.

What mechanism is most likely to operate for the following reaction? SN1 SN2 E1 E2 E1cB

Chem Act 14 Table page 220 Favored Mech. Base/Nuc R-X Rate dependent on [??] Solvent Temp SN1 SN2 E1 E2