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Unformatted text preview: NAME: CEM 850 Exam-1 September 20, 2003 This exam consists of 16 pages including the Table at the end. Be sure to write your name on all pages of the exam. Please write legibly and draw all structures clearly. Good luck. 1. (10 pts) Consider the following eight representations of brominated hydrocarbons: CH3CHBrCH=C(CH3)2 1-Bromo-3-methylcyclopentane A Br B (CH3)2CHCH=CHCH2Br C 2-Bromo-2,2-dimethylpropane D 2-Bromo-4-methyl-4-pentene E H H H3C H Br CH3 Br F G H In this collection, identify... a) Two compounds that could have cis and trans isomers: ______ and ______. b) A chemically meaningful but IUPAC incorrect name: ______; write the correct name here: _________________________________________________. c) A chemically impossible structure (violates octet rule): _______. d) Two representations of the same compound: ______ and ______. e) All but one of the chemically reasonable structures [i.e. not the answer to question (c)] have the same formula, which is: ___________. Which is the one with a different formula (hint: think of degrees of unsaturation): ______. 1 NAME: 2. (10 pts) (a) (1 pt each) Form acceptable Lewis structures for the following uncharged compounds by adding the necessary nonbonding electrons and multiple bonds: O H N H C N H H H C N (b) (1 pt each) Assign charges (if any) to the atoms in the following structures: H H B H (c) (2 pts) Among the three proposed resonance structures below, one is not acceptable. Circle the bad structure and briefly explain what's wrong with it. H O N O O H2C N O H2C N O H2C O N O O Explanation: 2 NAME: (c) (4 pts) Ions H2+ and He2+ have similar bond strengths (ca. 60 kcal/mol) but neutralization of their positive charges by adding an electron results in bond strengthening for one and dissociation for the other. Label the simple MO interaction pictures below, and use them to help explain the similarity of the cations and the difference between the neutral systems. Explanation: 3. (10 pts) Consider the four Newman projections below. H3 C H H H H3 C CH3 H H3 C CH3 H3C H3 C H H CH3 CH3 A H B CH3 H H H CH3 C H D H CH3 (a) (2 pts) These diagrams depict a compound of formula _____________ whose IUPAC name is _________________________________. (b) (2 pts) Which two structures represent eclipsed conformations? _____ and _____. 3 NAME: (c) (2 pts) Which two structures represent energy minima (low points) on the torsional potential energy surface? _____ and _____. (d) (2 pts) For two of the structures there are additional conformations of equal energy. Which ones are they? _____ and _____. Draw their same-energy conformations by completing the Newman projections below. H3 C H3 C H CH3 H CH3 (e) (2 pts) Place structures A-D on the potential energy diagram shown below. E N E R G Y -180 -120 -60 0 60 120 180 4. (10 pts) (a) (3 pts) Fill in > or < symbols to rank the following pairs of groups according to the CahnIngold-Prelog priority rules: Example: Br C(CH3)3 C H2Br C H2CH=CH2 _>_ H ___ ___ ___ CC H CH=O C H2C H2C H3 4 NAME: (b) (4 pts) In the following molecules, (i) identify (circle) all stereogenic centers; (ii) use the Cahn-Ingold-Prelog convention to prioritize the groups on the stereogenic centers (label them 1,2,3,4); (iii) assign these centers' configurations (R/S); and (iv) name one of the compounds (you choose) according to IUPAC. (a) Cl Cl Cl Cl (b) (c) (3 pts) Which of the following pairs represent identical molecules (I), diastereomers (D), enantiomers (E), or structural isomers (S)? Please circle your answer: (a) Cl Cl Cl Cl and compound (a) above I D E S (b) and compound (b) above I D E S (c) Br H Br H O OH HO O H Br I D E S Br H 5 NAME: 5. (4 pts) Gibbs free energy differences between axial-substituted and equatorial-substituted chair conformations of cyclohexene are given in the attached Table. Calculate the ratio of equatorial to axial vinylcyclohexane at 25 oC. NED: NOTE THE TYPO ABOVE (CYCLOHEXENE VS CYCLOHEXANE) WAS BROUGHT TO THE STUDENT'S ATTENTION DURING THE EXAM 6. (3 pts) Calculate the difference in Gibbs free energy in kilocalories per mole between the alternative chair conformations of the following: a) trans-4-methylcyclohexanol b) cis-4-methylcyclohexanol c) trans-1,4-dicyanocyclohexane 6 NAME: 7. (5 pts) There are four cis, trans isomers of 2-isopropyl-5-methylcyclohexanol. a) Using a planar hexagon representation for the cyclohexene ring, draw structural formulas for the four cis, trans isomers. b) Draw the more stable chair conformation for each of your answers in part a. c) Of the four cis, trans isomers, which is the most stable? 7 NAME: 8. (4 pts) Select the member of each pair that shows the greater rate of SN2 reaction with KI in acetone. (a) Cl or Cl (b) Cl or Br (c) Cl or Cl (d) Br or Br 9. (4 pts) Which compound in each set undergoes more rapid solvolysis when refluxed in ethanol? Show the major product formed from the more reactive compound. Br (a) or Br (b) Cl or Br Br (c) Br or (d) Cl or Cl 8 NAME: 10. (4 pts) The following nucleophilic substitution occurs with rearrangement. Suggest a mechanism for formation of the observed product. If the starting material has the S configuration, what is the configuration of the stereocenter in the product? N Cl NaOH H2O N OH 9 NAME: 11. (5 pts) Solvolysis of the following bicyclic compound in acetic acid gives a mixture of products, two of which are shown. The leaving group is the anion of a sulfonic acid, ArSO3H. A sulfonic acid is a strong acid, and its anion, ArSO3, is a weak base and a good leaving group. Propose a mechanism for this reaction. O OSO2Ar H3C O OH H H + H H O CH3 10 NAME: 12. (5 pts) A comparison of the rates of S N1 solvolysis of the bicyclic compounds 1 and 2 in acetic acid shows that compound 1 reacts 1011 times faster that compound 2. Furthermore, solvolysis of 1 occurs with complete retention of configuration: the nucleophile occupies the same position on the one-carbon bridge, as did the leaving OSO2Ar group. OSO2Ar OSO2Ar 1 2 (a) Draw the structural formulas for the products of solvolysis of each compound. (b) Account for the difference in rate of solvolysis of 1 and 2. (c) Account for complete retention of configuration in the solvolysis of 1. 11 NAME: 13. (6 pts) When cis-4chlorocyclohexanol is treated with sodium hydroxide in ethanol, it gives only the substitution product trans-1,4cyclohexanediol 1. Under the same conditions, trans-4-chlorocyclohexanol gives 3-cyclohexenol 2 and the bicyclic ether 3. (a) Propose a mechanism for formation of product 1, and account for its configuration. (b) Propose a mechanism for formation of product 2. (c) Account for the fact that the bicyclic ether 3 is formed from the trans isomer but not from the cis isomer. OH NaOH CH3CH2OH Cl OH 1 OH cis-4-chlororcyclohexanol OH NaOH CH3CH2OH Cl OH + O 3 2 trans-4-chlororcyclohexanol 12 NAME: 14. (8 pts) Using correct arrow notation, provide mechanisms for the following transformations. (a) HBr CH3 Br CH3 (b) 1. Hg(OAc)2 OH 2. NaBH4 O 13 NAME: 15. (8 pts) Provide the reagents/conditions that would enable the following transformations. (a) Br OCH3 (b) Br OH D 14 NAME: 16. (4 pts) Consider the two reactions shown below and answer the following questions: CH3CH2OH HCl (catalytic) CH3CH2OH O HCl (catalytic) (a) What are the expected products from the reaction of the cycloalkenes? (b) Which cycloalkene reacts faster? Use chemical structures to explain your choice. Bonus Question: (2 pts) The hydroboration of alkenes is presented in Chapter 8 of Jones. A pioneer of organoborane chemistry is Professor H.C. Brown of Purdue University. In 1979 Brown was awarded the Nobel Prize for his work in the area. Brown's decision to study organoboranes came many years before when shortly before beginning graduate school he read an early book on the subject. His girlfriend as a graduation present gave the book to him. Why did she decide to give H.C. a book about organoborane chemistry? a. She was an inorganic chemist herself and found the subject facinating. b. It was the cheapest book in the store c. The author of the book was her Uncle. d. Her favorite color was red and the book had a red cover. e. She hoped that after reading the section on sodium borate Brown would quit graduate school and take over the family Borax concession. 15 NAME: 16 ...
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This note was uploaded on 07/25/2008 for the course CEM 850 taught by Professor Borhan during the Fall '05 term at Michigan State University.
- Fall '05