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Chapter 8 Outline: Alkenes: Structure and
Preparation via β-elimination
1. What is β‐elimination? 2. Alkenes: structure, steroisomerism and stability 3. Elimination Reactions o E2 Mechanism o E1 Mechanism 4. Substitution vs. Elimination o Identifying the reagent o Identifying the mechanisms o Predicting the products 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: 8.50, 8.52, 8.54‐58, 8.61‐8.63, 8.65 – 8.69, 8.71, 8.72, 8.75‐80, 8.82, 8.84, 8.85, 8.87 Page 1 of 32
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1. What is ß‐elimination? • Elimination reactions often compete with substitution reactions. • What are the two main ingredients for a substitution? – A nucleophile and an electrophile with a leaving group • What are the two main ingredients for an elimination? – A base and an electrophile with a leaving group. • How is a base both similar and different from a nucleophile? Attack at the α‐
carbon
Attack at the β‐
hydrogen
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2. Alkenes: structure, stereoisomerism and stability Alkenes in nature: Why might it be important to know these structures? Page 3 of 32
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Alkene are good starting materials • Alkenes are also pivotal compounds in the chemical industry. • 70 billion pounds of propylene (propene) and 200 billion pounds of ethylene (ethene) are both made from cracking petroleum each year. Page 4 of 32
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Alkene Isomerism & Stability Page 5 of 32
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Place the following alkenes in order of stability. Rank them in order of 1‐
5 (1 = most stable; 5 = least stable). If two molecules are equally stable, then indicate this by using equivalent numbers. Indicate if the following double bonds are E, Z or neither (N). Page 6 of 32
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Alkenes & Physical Properties Saturated fats = animal products Polyunsaturated fats = vegetable products What is the difference between oils and fats with respect to fats? Why are trans fatty acids bad for you? Page 7 of 32
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3. Elimination Reactions: Preparation of Alkenes Recall the four mechanistic steps we learned in Chapter 7: – Nucleophilic attack – Loss of a leaving group – Proton transfer – Rearrangement Elimination reactions use these same steps and may be concerted (one‐
step) or two step; which is demonstrated below? What is the rate law? Is this reaction E1 or E2? Page 8 of 32
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E2 Mechanism • The kinetics of E2 and SN2 are quite similar. WHY? • However, tertiary substrates are unreactive toward SN2 while they react readily by E2. WHY? • 3° substrates are more reactive toward E2 than are 1° substrates even though 1° substrates are less hindered. • The 3° substrate should proceed through a more stable transition state (kinetically favored) and a more stable product (thermodynamically favored). Page 9 of 32
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E2 Mechanism • How would both the transition state energy and the product energy be different if the substrate were 1°? Page 10 of 32
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E2 Mechanism: regioselectivity • If there are multiple reactive sites or regions on a molecule, multiple products are possible. • What is the relationship between the alkene products? • REGIOSELECTIVITY occurs when one product is formed predominantly over the other. Page 11 of 32
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E2 Mechanism Good sterically‐hindered, non‐nucleophilic bases: Is the Zaitsev product kinetically favored, thermodynamically favored, or both? Page 12 of 32
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Draw ALL elimination products expected for the following reactions. Circle the major product if more than one product is formed. Page 13 of 32
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Elimination by E2 ‐ Stereoselectivity Page 14 of 32
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E2 Mechanism What is the difference between STEREOSELECTIVE and STEREOSPECIFIC? What product will be formed via elimination? What factors dictate when there are more than proton to abstract? Page 15 of 32
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E2 mechanisms with chair conformations Which of the two possible chair conformations will allow for the elimination to occur? Which of the two molecules below will NOT be able to undergo an elimination reaction? WHY? Page 16 of 32
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Elimination by E2 Draw all the possible products and circle the most stable: Work with your neighbor and provide products for the following: Cl
NaOEt
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NaOtBu
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E1 Mechanism Page 18 of 32
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E1 Mechanism How does the substrate reactivity trend for E1 compare to the trend we discussed in Chapter 7 for SN1? WHY? Page 19 of 32
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E1 Mechanism • Because E1 and SN1 proceed by the same first step, their competition will generally result in a mixture of products. • How might you promote one reaction over the other? Page 20 of 32
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E1 Mechanism Page 21 of 32
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Provide the mechanism for the following reaction: What other product(s) is (are) possible? Page 22 of 32
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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. Br
NaOCH3
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 Page 24 of 32
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Substitution vs. Elimination Substitution and elimination are almost always in competition. 1. Determine the function of the reagent. Is it more likely to act as a base, a nucleophile, or both? – Kinetics control nucleophilicity. WHY? HOW? – Thermodynamics control basicity. WHY? HOW? 2. Analyze the substrate and predict the expected mechanism (SN1, SN2, E1, or E2). 3. Consider relevant regiochemical and stereochemical requirements. Page 25 of 32
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1. Determine the function of the reagent: assess the strength of a nucleophile: – The greater the negative charge, the more nucleophilic it is likely to be. – The more polarizable it is, the more nucleophilic it should be. – The less sterically‐hindered it is, the more nucleophilic it should be. WHY? – The weaker the reagent, the more likely it is to promote SN1 or E1. WHY? Page 26 of 32
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2. Determine the function of the reagent: assess the strength of a base: – Assess the strength of its conjugate acid quantitatively using the pKa of its conjugate acid. – Which is a stronger base, Cl‐ or HSO4‐? – Are Cl‐ and HSO4‐ relatively strong or weak? – Which is a stronger base, Cl‐ or CH3O‐? – Compare CH3OH and CH3NH2. – Reagents that act only as nucleophiles are highly polarizable and/or they have very strong conjugate acids. Why are DBN and DBU especially stable bases?
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Predicting reactions: Page 28 of 32
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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). Page 30 of 32
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Provide viable step‐by‐step mechanisms using correct mechanistic arrows: CH3OH
Br
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Mechanism?
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Predict the products for the following reaction: OH
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Conditions?