of course i would want you to do all the problems at the end of...

C341/Spring 2010 Chapter 9

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Chapter 9 Outline: β-elimination

1. What is β-elimination?

2. Mechanisms of β-elimination?

3. Experimental Evidence for E1 vs. E2 Mechanisms?

4. Substitution vs. Elimination – practice, practice, practice…

5. Neighboring Group Effects

Of course I would want you to do ALL the problems at the end of the chapter, but your doing the following problems would be a good start: 9.37 – 9.40, 9.43 – 9.45, 9.47, 9.49, 9.51, 9.52, 9.54 – 9.57, and 9.61.


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1. What is β-elimination? (also called dehydrohalogenation)

Steps to identifying an elimination reaction:

• Find the sp3 C bonded to leaving group (C—X)

• Identify all β-carbons with H atoms

• Remove the adjacent H and X to form a π bond

• Usually the most stable pi bond is formed (Zaitsev’s rule)

What kind of bases will you be using?


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What are the possible products and which is the most stable product?


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2. Possible Mechanisms for β-Elimination E1 mechanism – involves forming a R+ intermediate


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Which is the rate determining step (RDS)? How does this compare to the SN1 mechanism? What change in the rate do you expect if the leaving group is changed from I to Br? What solvent, polar protic or polar aprotic do you expect will help facilitate this reaction mechanism? Do you expect that E1 might undergo rearrangements? Heat hastens elimination – Why?


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E2 mechanism – the leaving of the halide is concerted with the abstraction of the hydrogen.

What change in the rate do you expect if the leaving group is changed from Br to I? Would you expect that E2 will undergo rearrangements?


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The E2 mechanism removes only a stereospecific proton E2 favors when the –X and – H are oriented anti-periplanar to each other.

Observe:

Cl

NaOCH3

major product

CH3OH

Cl

NaOCH3

CH3OH


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3. Evidence for E1 & E2 a. kinetics (sound familiar?) E1 reactions → rate = E2 reactions → rate = b. regioselectivity o E1 reactions produce the most stable alkene (i.e. most substituted) – WHY?

o E2 reactions product ratios are dependent on the size of the base and the position of any β-protons

o Bulky bases lead to the least substituted alkene product, whereas small bases lead to the Zaitsev predicted product.

KO N N N N

DBN DBU


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For the following reaction, indicate which reaction mechanism (i.e. write E1 and/or E2 in the blank below) the following observations would support.

BrNaOCH3

A. the reaction rate increased when the [Base] was increased

B. the reaction rate decreased when the [RX] was decreased

C. the reaction showed a rearranged product

D. the reaction showed more than one product

E. the reaction rate decreased when the LG was changed to Cl

F. the product was the least substituted alkene

G. the product was an alkene


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Working problems Draw ALL elimination products expected for the following reactions. Circle the major product if more than one product is formed.


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Cl

NaOCH2CH3

CH3CH2OH

Cl

CH3CH2OH


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5. Substitution & elimination can occur simultaneously How does one predict which reaction and which mechanism will take place? This is not a trivial question! Back to our original example:

NaOCH3Br

H

nucleophilicsubstitution

β-elimination

Sodium ethoxide can act as a base (leading to elimination) or a nucleophile (leading to substitution) – which reaction predominates? Likely Mechanisms by which Haloalkanes React with Nucleophiles Type of weak base weak base strong base haloalkane poor NuӨ good NuӨ unhindered NuӨ e.g. H2O I-, CH3COO-

CH3O- Methyl NR SN2 SN2 Primary Unhindered NR SN2 SN2 Hindered NR SN2 E2 (e.g. KOtBu) Secondary SN1/ E1 SN2 (~pKa CA< 11) E2 (~pKa CA> 16) Tertiary SN1/E1 SN1/E1 E2


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The following substrate below will undergo both substitution and elimination under certain conditions. Circle the correct expectation for each reagent, indicating substitution (S), both with preference for substitution (B/S), both with a preference for elimination (B/E), or solely elimination (E).


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Predict the SUBSTITUTION and ELIMINATION mechanisms/products for the following reactions and draw the appropriate product(s). Indicate clearly the correct stereochemistry in your product(s).

BrNaOCH3

BrCH3OH


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Provide a viable step-by-step mechanism using correct mechanistic arrows:


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Provide BOTH substitution and elimination products for the following reaction. Indicate SN1 or SN2 or E1 or E2 under each product based on the mechanism it undergoes. Show stereochemistry as necessary and draw all products, even enantiomers.


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Provide viable step-by-step mechanisms using correct mechanistic arrows:

Br

CH3OHMechanism?

Br NaNH2

Mechanism?


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4. Neighboring Group Effects • What is unusual about the mustard gases is that they are primary halides

and yet undergo rapid hydrolysis in water, a very poor nucleophile.

• The reason is neighboring group participation by the adjacent heteroatom (S or N).

of course i would want you to do all the problems at the end of...

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