organic chemistry

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timer Asked: Apr 20th, 2017

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In the attachment several q and its answers. I need an explanation why they choose this answers with details please.

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Chapter 30—Orbitals and Organic Chemistry: Pericyclic Reactions SHORT ANSWER 1. a. b. c. d. There are three major classes of pericyclic reactions. Which of the following types of reactions is not an example of a pericyclic process? sigmatropic rearrangements cycloaddition reactions annulation reactions electrocyclic reactions ANS: c 2. The mnemonic phrase "The Electrons Circle Around", TECA, assists in predicting stereochemistry of pericyclic reactions. Describe application of this mnemonic. ANS: The mnemonic TECA assists us in remembering that "Thermal pericyclic reactions with Even numbers of electrons will occur with Conrotatory or Antarafacial stereochemistry". As a result, the following statements also apply: Photochemical pericyclic reactions with even numbers of electrons will occur with disrotatory or suprafacial stereochemistry. Thermal pericyclic reactions with odd numbers of electrons will occur with disrotatory or suprafacial stereochemistry. Photochemical pericyclic reactions with odd numbers of electrons will occur with conrotatory or antarafacial stereochemistry. Exhibit 30-1 To answer the following question(s), consider the  molecular orbitals of a conjugated diene shown below: 1 Orbitals and Organic Chemistry: Pericyclic Reactions 3. a. b. c. d. Refer to Exhibit 30-1. Which molecular orbital of a conjugated diene contains two nodes between nuclei? 1 2 3* 4* ANS: c 4. a. b. c. d. Refer to Exhibit 30-1. Which molecular orbital of a ground-state conjugated diene is the highest occupied molecular orbital (HOMO)? 1 2 3* 4* ANS: b 5. a. b. c. d. Refer to Exhibit 30-1. Which molecular orbital of an excited-state conjugated diene is the lowest unoccupied molecular orbital (LUMO)? 1 2 3* 4* ANS: d 6. 2 Draw the product(s) you would expect to obtain from photochemical cyclization of (2Z, 4E)hexadiene. Chapter 30 ANS: Exhibit 30-2 Consider the reaction below to answer the following question(s): 7. Refer to Exhibit 30-2. Has this reaction taken place in a conrotatory manner or in a disrotatory manner? ANS: In order for the hydrogens to be trans in the product, rotation must occur in a disrotatory manner. 8. Refer to Exhibit 30-2. Under what conditions, thermal or photochemical, would you carry out this reaction? Explain. ANS: For a disrotatory cyclization to occur the HOMO must have the symmetry pictured. For a 4  electron system this HOMO must arise from photochemical excitation of a pi electron (see questions 3-5 in printed test bank). To obtain a product with the correct stereochemistry, the reaction must be carried out under photochemical conditions. 3 Orbitals and Organic Chemistry: Pericyclic Reactions Exhibit 30-3 To answer the following question(s), consider the reaction below: 9. Refer to Exhibit 30-3. Two pericyclic reactions are involved in this synthesis of compound Y. Draw the structure of intermediate X and name it using IUPAC nomenclature. ANS: 10. a. b. c. d. Refer to Exhibit 30-3. The thermal reaction which generates X is an example of: an electrocyclic reaction a sigmatropic rearrangement a reverse cycloaddition reaction a cycloaddition reaction ANS: a 11. Refer to Exhibit 30-3. Explain the stereochemistry of compound Y. ANS: Thermal electrocyclic opening of the cis-substituted cyclobutene yields (1E, 3Z, 5Z)cyclodecatriene (X). Symmetry-allowed thermal cyclization of the cyclodecatriene occurs in a disrotatory fashion to yield the observed compound Y. 4 Chapter 30 Exhibit 30-4 Consider the reaction below to answer the following question(s): 12. a. b. c. d. Refer to Exhibit 30-4. What type of pericyclic reaction is involved in this transformation? sigmatropic rearrangement reverse cycloaddition reaction electrocyclic reaction cycloaddition reaction ANS: d 13. a. b. c. d. Refer to Exhibit 30-4. How many pairs of electrons are involved in this pericyclic reaction? two four eight sixteen ANS: c 14. Refer to Exhibit 30-4. Explain the stereochemistry of this reaction. ANS: The reaction of heptafulvene with tetracyanoethylene is a [14 + 2] cycloaddition that has occurred with antarafacial stereochemistry. 15. Refer to Exhibit 30-4. Would you expect this reaction to occur under photochemical or thermal conditions? 5 Orbitals and Organic Chemistry: Pericyclic Reactions ANS: Since this reaction is a [14 + 2] cycloaddition with eight electron pairs, we would expect the thermal reaction to occur with antarafacial stereochemistry and the photochemical reaction to occur with suprafacial stereochemistry. The product has antarafacial stereochemistry resulting from a thermal reaction. Exhibit 30-5 To answer the following question(s), consider the reaction below: 16. a. b. c. d. Refer to Exhibit 30-5. What type of pericyclic reaction is involved in this transformation? sigmatropic rearrangement reverse cycloaddition reaction electrocyclic reaction cycloaddition reaction ANS: d 17. a. b. c. d. Refer to Exhibit 30-5. How many pairs of electrons are involved in this pericyclic reaction? two four eight sixteen ANS: a 6 Chapter 30 18. Refer to Exhibit 30-5. Propose a mechanism for the reaction that fully accounts for the formation of both products. ANS: This reaction occurs with suprafacial stereochemistry that would be expected for a photochemical [2 + 2] cycloaddition. However, suprafacial cycloaddition can occur in two different wayswith the methyl groups of both alkenes on the same side or on opposite sides. Exhibit 30-6 Consider the reaction sequence below to answer the following question(s): 19. a. b. c. d. Refer to Exhibit 30-6. Step 1 is an example of: an electrocyclic reaction a cycloaddition reaction a sigmatropic rearrangement a reverse cycloaddition reaction ANS: a 7 Orbitals and Organic Chemistry: Pericyclic Reactions 20. Refer to Exhibit 30-6. Explain the stereochemistry of product B. ANS: Step 1 is the thermal electrocyclic reaction of a conjugated triene (three electron pairs) that occurs with disrotatory stereochemistry to yield a cis-fused cyclopropane. 21. a. b. c. d. Refer to Exhibit 30-6. Step 2 is an example of: an electrocyclic reaction a cycloaddition reaction a sigmatropic rearrangement a reverse cycloaddition reaction ANS: c 22. Refer to Exhibit 30-6. Explain the stereochemistry of product C. ANS: Step 2 is a thermal [1, 5] sigmatropic rearrangement (three electron pairs) that occurs with suprafacial stereochemistry. 23. Refer to Exhibit 30-6. Show how C is formed from B. ANS: Exhibit 30-7 Consider the Sommelet-Hauser rearrangement below to answer the following question(s): 8 Chapter 30 24. a. b. c. d. Refer to Exhibit 30-7. The Sommelet-Hauser rearrangement is an example of a [2, 3] sigmatropic rearrangement. How many pairs of electrons are involved in this reaction? two three four five ANS: b 25. Refer to Exhibit 30-7. Would you expect the Sommelet-Hauser rearrangement to be antarafacial or suprafacial? Explain. ANS: Since three pairs of electrons are involved in the rearrangement, the Sommelet-Hauser rearrangement should occur with suprafacial geometry. Exhibit 30-8 Classify each of the following sigmatropic reactions by order [x, y]. 26. ANS: [3, 3] 27. 9 Orbitals and Organic Chemistry: Pericyclic Reactions ANS: [1, 7] 28. ANS: [1, 5] Exhibit 30-9 Vitamin D3, cholecalciferol, is synthesized under the skin by the photochemical pericyclic process shown below. Answer the following question(s) about these reactions. 10 Chapter 30 29. a. b. c. d. Refer to Exhibit 30-9. The first step in this process is a(n): sigmatropic rearrangement cycloaddition reaction annulation reaction electrocyclic reaction ANS: d 30. a. b. c. d. Refer to Exhibit 30-9. The second step in this process is a(n): sigmatropic rearrangement cycloaddition reaction annulation reaction electrocyclic reaction ANS: a 31. Predict the product (including stereochemistry) when the following substance reacts under thermal conditions. ANS: 32. Predict the product of the following reaction. 11 Orbitals and Organic Chemistry: Pericyclic Reactions ANS: 33. Define a pericyclic reaction. ANS: A concerted reaction through a cyclic transition state. Occurs in one step without the formation of intermediates. 34. ANS: [4+2] 12 Draw the structure of the product when the following substance reacts with maleic anhydride. Classify the reaction according to the number of  electrons that interact. Atoms other than carbon and hydrogen are labeled. Chapter 30 35. The photochemical cycloaddition of the structure below is related to DNA mutations causing skin cancer. Draw the structure of this dimer. Atoms other than carbon and hydrogen are labeled. ANS: [2+2] cycloaddition occurs due to photochemical conditions. MULTIPLE CHOICE 1. a. Which of the following is not an electrocyclic reaction? b. c. 13 Orbitals and Organic Chemistry: Pericyclic Reactions d. e. All are electrocyclic reactions. ANS: E 2. For an even number of electron pairs (double bonds), which of the following rules is correct?: a. thermal conditions - controtatory, photochemical conditions - disrotatory b. thermal conditions - disrotatory, photochemical conditions - conrotatory ANS: A 3. Consider the following image which shows the two outermost lobes a polyene. For a bond to form which of the following is correct? a. Disrotatory motion is required. b. Both must rotate clockwise. c. One must rotate clockwise and the other counterclockwise. d. Conrotatory motion is required. e. a and c f. b and d ANS: D 4. Under thermal conditions, the following substance will undergo: a. disrotatory cyclization. b. conrotatory cyclization. c. no reaction. ANS: A 14 Chapter 30 5. Under photochemical conditions, the following substance will undergo: a. disrotatory cyclization. b. conrotatory cyclization. c. no reaction. ANS: A 6. How many  MO’s does the following compound have? a. 2 b. 4 c. 6 d. 8 ANS: C 15 Orbitals and Organic Chemistry: Pericyclic Reactions 7. Consider the following MO diagram for a conjugated triene. When irradiated with light of the appropriate wavelength, how many electrons will be in bonding MO’s? a. 6 c. 4 b. 5 d. 3 ANS: B 8. For an odd number of electron pairs (double bonds), for cycloadditions reactions which of the following rules is correct?: a. thermal conditions - antarafacial, photochemical conditions - suprafacial b. thermal conditions - suprafacial, photochemical conditions - antarafacial ANS: B 9. a. b. c. d. e. In which type of pericyclic reaction is a  bond broken and a new  bond formed? cycloaddition photochemical electrocyclic thermal electrocyclic sigmatropic rearrangement. all of the above ANS: D 16 Chapter 30 10. Consider the reaction given below. Which of the following is correct? a. [1,5] antarafacial b. [3,3] antarafacial c. [1,5] suprafacial d. [3,5] antarafacial ANS: B 11. a. b. c. d. e. f. Which of the following reaction types are pericyclic reactions? Diels-Alder reaction Cope rearrangement Claisen rearrangement Stork reaction All except d are pericyclic reactions. All are pericyclic reactions. ANS: E 12. Classify the following reaction. a. electrocyclic reaction b. cycloaddition c. sigmatropic rearrangement ANS: C 13. Classify the following reaction. a. electrocyclic reaction b. cycloaddition c. sigmatropic rearrangement ANS: A 17 Orbitals and Organic Chemistry: Pericyclic Reactions 14. Classify the following reaction. a. electrocyclic reaction b. cycloaddition c. sigmatropic rearrangement ANS: B 18

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