Chemistry
Zaitsev Rule
The Zaitsev rule is a principle in organic chemistry that predicts the preferred outcome of a dehydrohalogenation or beta-elimination reaction. It states that the major product of a reaction will be the alkene with the most substituted double bond. This rule is based on the stability of the resulting alkene.
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3 Key excerpts on "Zaitsev Rule"
- David R. Klein(Author)
- 2017(Publication Date)
- Wiley(Publisher)
Since the base is not sterically hindered, we expect the Zaitzev product (more substituted alkene). Formation of the Zaitzev product requires deprotonation at the following, highlighted position: This position bears only one proton, so the reaction is expected to be stereospecific. That is, only one particular stereoisomeric product will be obtained. To determine which product to expect, we can rotate the central C–C bond so as to place the proton and the leaving group in the plane of the page. But in this case, that is not necessary, because the proton and the leaving group are already anti-periplanar to one another (one is on a dash and the other is on a wedge): In such a case, it is relatively easy to draw the product, because the carbon skeleton is simply redrawn without the proton and without the leaving group (with a double instead). Note that a double bond has planar geometry, so the methyl group is on a straight line (not a wedge) in the product: CHAPTER 7 213 (b) The problem statement indicates that the major product is obtained via an E2 process. There are three positions, but only two of them bear protons, so there are two possible regiochemical outcomes. Since the base is not sterically hindered, we expect the Zaitzev product (more substituted alkene). Formation of the Zaitzev product requires deprotonation at the following, highlighted position: This position bears only one proton, so the reaction is expected to be stereospecific. That is, only one particular stereoisomeric product will be obtained. To determine which product to expect, we can rotate the central C–C bond so as to place the proton and the leaving group in the plane of the page.
eBook - PDF- David R. Klein(Author)
- 2020(Publication Date)
- Wiley(Publisher)
The π bond can be formed inside the ring or outside the ring. The first outcome gives a trisubstituted alkene, while the second outcome gives a disubsti- tuted alkene. The more substituted alkene (trisubstituted) is the Zaitsev product, and the disubsti- tuted alkene is the Hofmann product. This example does not employ a sterically hindered base, so we expect that the Zaitsev prod- uct will be the major product: Cl Major Minor + NaOMe 8.15 Predict the major and minor products for each of the following E2 reactions: (a) Cl ? NaOEt (b) Cl ? t-BuOK (c) ? NaOH (d) ? t-BuOK (e) Br ? NaOH (f ) Br ? t-BuOK 8.16 For each of the following reactions, identify whether you would use hydroxide or tert- butoxide to accomplish the desired transformation: (a) Cl (b) Br STEP 1 Identify all β positions bearing protons. Cl β β β STEP 2 Draw all possible regiochemical outcomes. STEP 3 Identify the Zaitsev and Hofmann products. STEP 4 Analyze the base to determine which product predominates. PRACTICE the skill 346 CHAPTER 8 Alkenes: Structure and Preparation via Elimination Reactions 8.17 Show two different methods for preparing each of the following alkenes using a sterically hindered base: (a) (b) Try Problems 8.62, 8.63b–d, 8.66, 8.68a,d, 8.76 Stereoselectivity of E2 Reactions The examples in the previous sections focused on regiochemistry. We will now focus our atten- tion on stereochemistry. For example, consider performing an E2 reaction with 3-bromopen- tane as the substrate: Br This compound has two identical β positions so regiochemistry is not an issue in this case. Deprotonation of either β position produces the same result. But in this case, stereochemistry is relevant, because two possible stereoisomeric alkenes can be obtained: Br Major Minor + NaOEt Both stereoisomers (cis and trans) are produced, but the trans product predominates. Consider an energy diagram showing formation of the cis and trans products (Figure 8.14).
eBook - PDF- David R. Klein(Author)
- 2021(Publication Date)
- Wiley(Publisher)
STEP 2 Draw all possible regiochemical outcomes. STEP 3 Identify the Zaitsev and Hofmann products. STEP 4 Analyze the base to determine which product predominates. Cl Zaitsev Hofmann Zaitsev Hofmann Not sterically hindered Sterically hindered Major Minor Major Minor β β β Try Problems 7.12–7.14, 7.58, 7.59a,d, 7.62, 7.63b, 7.66, 7.86 SECTION 7.9 • Substitution and elimination reactions often compete with each other. To predict the products, three steps are required: 1) determine the function of the reagent; 2) analyze the substrate and determine the expected mechanism(s); and 3) consider any relevant regiochemical and stereochemical requirements. SECTION 7.10 • Alkyl halides and alkyl sulfonates undergo similar reactions. • Alcohols react with HBr to give alkyl halides, either via an S N 2 pathway (for primary and secondary substrates) or via an S N 1 pathway (for tertiary substrates). • When treated with concentrated sulfuric acid, tertiary alcohols are converted into alkenes via an E1 process. Primary alcohols are also converted into alkenes, likely via an E2 process. SECTION 7.11 • A retrosynthetic analysis shows the product first, followed by reagents that can be used to make that product. A wavy line indicates a disconnection, which identifies the bond that can be made by the reaction. • Planning a retrosynthesis requires that we identify a suitable nucleophile and electrophile that will react with each other to give the target molecule (the desired product). SECTION 7.12 • Protic solvents contain a hydrogen atom connected directly to an electronegative atom, while polar aprotic solvents lack such a hydrogen atom. • A polar aprotic solvent will speed up the rate of an S N 2 pro- cess by many orders of magnitude. • S N 1 processes are favored by polar protic solvents. SkillBuilder Review 347 7.4 PREDICTING THE STEREOCHEMICAL OUTCOME OF AN E2 REACTION STEP 1 Identify all β positions bearing protons.
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