Chapter 12

Chapter 12 - CHAPTER PREDI C TIN G PRODUCTS 12.1 GENERAL...

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Unformatted text preview: CHAPTER PREDI C TIN G PRODUCTS 12.1 GENERAL TIPS FOR PREDICTING PRODUCTS In the last several chapters, we saw the power of mechanisms in explaining the re- giochemistry and stereochemistry of every reaction. A solid understanding of these concepts gives us the power to predict the products of a reaction. That is the subject of this chapter. Generally. there are not many steps involved. You need to ask three questions whenever you are trying to determine the product of a reaction: 1. What kind of reaction is taking place? 2. What is the regiochemistry of that reaction? 3. What is the stereochemistry ol“ that reaction? By studying the mechanisms of every reaction and recording them at the end of Chapter 8, you will become familiar with these three aspects of every reaction and you will find it simple to predict products. Let‘s see an example of how this works. EXERCISE 12.1 Predict the product of the following reaction: (1) BHS/THF (2) H202 / 0H9 Answer If you have already learned this reaction and if you have recorded the mechanism at the end of Chapter 8, then you will already know the regiochemistry and stereochemistry of this reaction. Let’s answer all three questions: 1. What kind of reaction is taking place? —Addition of H and OH across a double bond 2. What is the regiochemistry of this reaction? —The OH goes on the less substituted carbon (anti—Markovnikov addition) 3. What is the stereochemistry of this reaction"? —syn addition 246 12.2 GE1TING PRACTICE 247 Now that we have answered all three questions. we are ready to draw the products (don‘t forget to draw both enantiomers that you get from syn addition): OH If you fail to ask all three questions, then you will not get the product correct. Matty students fail to ask questions 2 or 3 (or sometimes both). Clearly, the answer is cor— rect only if you get the stereochemistry and the regiochemistry correct. 1 2.2 GETTING PRACTICE This section offers one approach for practicing predicting products: For every reac- tion that you learn, we have already said that you should record it at the end of Chap- ter 8. Now we need to start a new list, right here in this chapter. Every time you learn a new reaction, you should write the reaction down using the arrows on the pages that follow, but do not show the mechanism or the product. Just show the starting compound and the reagents, like this: (1) BHg/THF (2) H202 / OHe As you learn more and more reactions. this list will grow. With every five new reac- tions. you should photocopy all of the reactions that you have recorded here. Then, start filling in the products on the photocopy. If you cannot fill them all in, go back to the end of Chapter 8 where you first recorded the reaction, and see how to answer the problem. If you did not fill out the end of Chapter 8. then look through your text— book and your class notes to determine the answer to all three questions (what kind of reaction? regiochemistry? and stereochemistry?). Repeat this procedure whenever you have entered five new reactions. If you keep up with this exercise as the course progresses, you will be in very good shape [or predicting products. The hardest challenge that you will face is keeping up with the work and not waiting until the night before the exam. If you wait (as most students do), you will find it very difficult to spend the time that it takes to master this material. Don’t make that mistake. The secret to success in this course is to do a little hit every night (rather than cramming on the night before the exam). Begin your list on the next page. 248 CHAPTER 12 PREDICTING PRODUCTS Remember not to fill in the products or the mechanisms. For each reaction, just draw the starting material in front of the anow and the reagents above the arrow. Leave the space for the product empty. You will fill in the products when you photocopy these pages: 12.2 GETTING PRACTICE 249 Now photocopy this page, and try to fill in the products on your photocopied page. Now, photocopy this page again, and fill in the products for every reaction on this page. 250 CHAPTER 12 PREDICTING PRODUCTS Now photocopy this page AND the previous page, and fill in all of the products. 12.2 GETTING PRACTICE 251 Now photocopy this page AND the previous page, and fill in all of the products. 252 CHAPTER 12 PREDICTING PRODUCTS Now photocopy this page AND the previous pages, and fill in all of the products. 12.2 GETTING PRACTICE 253 Now photocopy this page AND the previous pages, and fill in all of the products. 254 CHAPTER 12 PREDICTING PRODUCTS Now photocopy this page AND the previous pages, and fill in 111] of the products. 12.2 GETTING PRACTICE 255 Now photocopy this page AND the previous pages, and fill in all of the products. CHAPTER 12 PREDICTING PRODUCTS Now photocopy this page AND the previous pages, and fill in all of tho products. 12.2 GETTING PRACTICE 257 Now photocopy this page AND the previous pages. and fill in all of the products. 258 CHAPTER12 PREDICTING PRODUCTS If you cover more than 30 reactions and need more space to continue, then you can just use a regular piece of paper to keep your list going. 12.3 SUBSTITUTION VERSUS ELIMINATION REACTIONS Predicting products can be challenging when you have to consider substitution and elimination reactions simultaneously. So far, we have seen substitution and elimina- tion reactions separately. But now the truth comes out—substitution and elimination reactions are generally in competition with each other. To predict the products prop— erly, you need to compare all factors for substitution and elimination reactions, and you then need to decide which of the four mechanisms predominates (SNI or 8N2 or E1 or E2). The method for predicting products is still the same. We ask the same three questions: 1. What kind of reaction is taking place? 2. What is the regiochemistry of that reaction? 3. What is the stereochemistry of that reaction? The reason these problems get a bit more challenging is that question 1 (what kind of reaction) is a bit more tedious to work through. You need to analyze all four factors: the substrate. the reagent (which could be a nucleophile or a base). the leav— ing group, and the solvent. Once you have analyzed all four factors, you will be able to determine which mechanisms are operating. Then for each mechanism that is op— erating, you need ask the remaining two questions (regiochemistry and stereochem— istry) to draw the products. Let‘s quickly review the regiochemistry and stereochemistry of each of the four mechanisms: SNl Stereochemistry = racemization Regiochemistry = not applicable (the nucleophile simply attacks at the car- bon connected to the leaving group) For example, H20 8N1 i3] Stereochemistry = inversion of configuration Regiochemistry = not applicable (the nucleophile simply attacks at the car- bon connected to the leaving group) 12.3 SUBSTITUTION VERSUS ELIMINATION REACTIONS 259 For example, 0H6 '3, S 2 H CI N El Stereochemistry = if there are cis/trans isomers, you will get both Regiochemistry = form the more substituted double bond (Zaitsev product) H20 / + E1 / Stereochemistry = if there are cis/trans isomers, you will get the one deter— mined by the H and LG being antiperiplanar (draw Newman projections) Regiochemistry = form the more substituted double bond (Zaitsev prod» not). If you are using a strong, sterically hindered base, then form the less substituted double bond (Hoffmann product) For example, e E2 For example, OH E2 i’CI It is OK to have more than one product for a reaction. For example. it is OK to determine that 8N2 and E2 are competing and that you get products from both mechanisms at the same time. There is nothing wrong with that. There does not have to be only one answer. But there are a few extra pieces of information that will help you determine if there is one or more than one mechanism operating: 1. Some reagents are excellent nucleophiles, but are not good bases. Some reagents are excellent bases, but are not good nucleophiles. We discussed this in Chapter 10 (Section 10.3). If you do not understand how a reagent can be a good nucleophile and not a good base, go back to this section and review it. You should be familiar with certain reagents: - Nucleophiles (that do not act as bases): F‘, Cl‘, Br‘, 1‘, CN‘. RS‘, RSH. When you see these reagents, you do not need to worry about elim— ination reactions. - Bases (that do not function as nucleophiles): H‘. When you see this reagent. you do not need to worry about substitution reactions. 0 Most other reagents that you will see can function as either a base or a nucleophile. 260 CHAPTER 12 PREDICTING PRODUCTS 2. Look at the temperature, if it is given. High temperature (like 1003C or higher) will favor elimination products. Low temperature (like room tem- perature) will favor substitution products. 3. Bimolecular reactions are generally faster than unimolecular reactions. So, all things being equal, 8N2 will be faster than SN], and E2 will be faster than E1. Let’s do some examples and see how it works. EXERCISE 12.2 Predict the products of the following reaction: 6 Br DMSO Answer We need to ask our three questions: 1. What kind of reaction is taking place? We see that we have a substrate with a leaving group, so we need to con— sider substitution and elimination reactions. The reagent is a good uucle~ ophile, but it is not a good base, so we only need to worry about substitution reactions. We need to determine whether we have 8N1 or 8N2. Let’s go through all four factors: - The substrate is secondary, so that doesn‘t help us much. - The nucleophile is pretty good (it has negative charge) so that favors 8N2. - The leaving group is good (but not excellent and not bad) so that doesn’t tell us much. - The solvent is a polar aprotic solvent. This tells us that we have an 5N2 reaction. Now we know what reaction is taking place (8N2). so we ask our last two questions: 2. What is the regiochemistry? Not applicable. The nucleophile attacks at the carbon that bears the leaving group. 3. What is the stereochemistry? Inversion. Now we are ready to draw our product. We replace the Br with 21 CN group and we invert the stereocenter: CN 12.3 SUBSTITUTION VERSUS ELIMINATION REACTIONS 1 This reaction had only one product. There are times when you can get more than one product. PROBLEMS Predict the products of each of the reactions below. 9 OH 123 MBr (9 /Y OH 12.4 Br 12.5 M 0' OHe M heat 12.6 9 /Y CI 12.7 Br DMSO 9 C)—§—c|—13 CH30H O 12.8 S? o—§—Ph H28 0 12.9 DME OH H2804 heat 12.10 262 CHAPTER 12 PREDICTING PRODUCTS 9 pl OH heat 12.1 1 Hint: You will need to draw a Newman projection for this one. 12.12 1 2.4 LOOKING FORWARD You must train yourself to ask the following three questions: 1. What kind of reaction is taking place? 2. What is the regiochemistt'y of that reaction? 3. What is the stereochemistry of that reaction? These questions will help you to predict products for all of the reactions that you will see for the rest of the course. Remember that there are three critical ques— tions to ask: If you keep this in mind as you go through the course. you will find that predicting products is not so difficult after all. ...
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This note was uploaded on 07/31/2010 for the course CHEM 241 taught by Professor Burnett during the Fall '07 term at UVA.

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Chapter 12 - CHAPTER PREDI C TIN G PRODUCTS 12.1 GENERAL...

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