8. What is the slow, rate-determining step, in the acidcatalyzed dehydration of 2-methyl-2-propanol?

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1 CHEMISTRY MIDTERM # 2 answer key October 25, 2011 Statistics: Average: 68 pts (68%); Highest: 100 pts (100%); Lowest: 30 pts (30%) Number of students performing at or above average: 56 (54%) Number of students performing below 55%: 27 (26%) Number of students performing at or above 90%: 12 (12%) PART I: MULTIPLE CHOICE (30 questions, 2 points each) 1. Carbon-carbon double bonds do not freely rotate like carbon-carbon single bonds. Why? a) The double bond is much stronger and thus more difficult to rotate. b) Overlap of the two 2p orbitals of the bond would be lost. c) The shorter bond length of the double bond makes it more difficult for the attached groups to pass each other. d) Overlap of the sp2 orbitals of the carbon-carbon bond would be lost. 8. What is the slow, rate-determining step, in the acidcatalyzed dehydration of 2-methyl-2-propanol? a) Protonation of the alcohol to form an oxonium ion. b) Loss of water from the oxonium ion to form a carbocation. c) Loss of a -hydrogen from the carbocation to form an alkene. d) The simultaneous loss of a -hydrogen and water from the oxonium ion. 9. Which of the following most readily undergoes an E2 reaction with sodium ethoxide (NaOCH 2 CH 3 )? A) (CH 3 ) 3 CF B) (CH 3 ) 3 CCl C) (CH 3 ) 3 CBr D) (CH 3 ) 3 CI 10. Which of the following does not give 1,2- dimethylcyclohexene as one of the acid-catalyzed dehydration products? 2. What is the IUPAC name of the following compound? a) 2-methyl-3-propyl-2-pentene b) 3-ethyl-2-methyl-2-hexene c) 4-ethyl-5-methyl-4-hexene d) 2,3-dimethyl-2-hexene 3. How many isomeric alkenes of formula C 5 H 10, including stereoisomers, are possible? A) three B) four C) five D) six 4. What is the IUPAC name of the following compound? a) 3-bromo-2-methylcyclohexene b) 1-bromo-2-methyl-2-cyclohexene c) 6-bromo-1-methylcyclohexene d) 2-bromo-1-methylcyclohexene 11. How many isomeric alkenes are possible, including stereoisomers, in the following reaction? A) two B) three C) four D) five 12. Which of the following cannot undergo an E2 reaction? 5. What is the IUPAC name of the following compound? a) (E)-3-bromo-1-fluoro-2-methylpropene b) (Z)-3-bromo-1-fluoro-2-methylpropene c) (E)-1-bromo-3-fluoro-2-methylpropene d) (Z)-1-bromo-3-fluoro-2-methylpropene 6. Which of the following alkenes exhibit E-Z isomerism? I. 1-chloropropene II. 2-chloropropene III.3-chloropropene A) only I B) I and II C) II and III D) I and III 7. Identify the major organic product expected from the acid-catalyzed dehydration of 2-methyl-2-pentanol. a) 2-methyl-1-pentene b) 2-methyl-2-pentene c) 3-methyl-1-pentene d) cis-3-methyl-2-pentene A) only I B) only II C) only III D) I and III 13. Which of the following will give 2-methyl-1-butene as the only alkene product on treatment with KOC(CH 3 ) 3 in dimethyl sulfoxide? a) 2-bromo-3-methylbutane b) 1-bromo-3-methylbutane c) 2-bromo-2-methylbutane d) 1-bromo-2-methylbutane 14. When a strong base is used in the elimination reaction of an alkyl halide the mechanism, in general, is a) E1. b) E2. c) E1 for tertiary halides, E2 for primary and secondary halides. d) E2 for tertiary halides, E1 for primary and secondary halides.

2 15. Zaitsev's rule can be used to predict the major product for which of the following reactions? a) 2-methylpentane + Br 2 (with light) b) 2-bromo-2-methylpentane +NaOCH 2 CH 3 (in ethanol) c) 2-methyl-2-pentanol + PBr 3 d) 2-methyl-2-pentanol + HCl 16. What is the major product of the reaction sequence shown below? a) 2-methyl-1-butene b) 2-methyl-2-butene c) 3-methyl-1-butene d) 2-methylbutane a) 1-methylcyclobutanol b) trans-2-methylcyclobutanol c) cis-2-methylcyclobutanol d) equal amounts of cis- and trans-2-methylcyclobutanol 23. The hydroboration-oxidation reaction can be characterized as the to an alkene. a) anti-markovnikov syn addition of water b) anti-markovnikov anti addition of water c) Markovnikov syn addition of water d) Markovnikov anti addition of water 24. Which of the following series of reactions would convert cyclohexanol to 1,2-epoxycyclohexane? 17. What is(are) the product(s) in the Pd-catalyzed hydrogenation of 1,2-dimethylcyclopentene? a) trans-1,2-dimethylcyclopentane b) cis-1,2-dimethylcyclopentane c) a mixture of trans and cis-1,2-dimethylcyclopentane d) 1,1-dimethylcyclopentane 18. Which of the following alkenes is expected to have the highest heat of hydrogenation? a) 1-pentene b) trans-2-pentene c) cis-2-pentene d) 2-methyl-2-butene 19. The product(s) in the following reaction is(are) a) only trans-1-4-dimethylcyclohexane. b) only cis-1-4-dimethylcyclohexane. c) both trans and cis-1-4-dimethylcyclohexane. d) methylcyclohexane. 20. Which of the following IS NOT a possible reaction of a carbocation? a) addition of a nucleophile b) rearrangement to a more stable carbocation c) addition of a proton to form an alkane d) loss of a -hydrogen to form an alkene 21. Which species below is the intermediate in the free radical addition of HBr to 1-butene? 25. The reaction of 1-butene with bromine, Br 2, in aqueous solution gives primarily 1-bromo-2-butanol. Identify the nucleophilic species in the reaction. A) Br 2 B) Br - C) H 2 O D) HOBr 26. Which of the following gives acetone, (CH 3 ) 2 C=O, as one of the products of its ozonolysis? 27. Which of the following does not give 1-bromo-1- methylcyclopentane as the major product? 22. What is(are) the product(s) of the following hydroboration-oxidation reaction?

3 28. The following alkene(s) is(are) suitable starting material(s) for the preparation of 3-methyl-2-butanol using hydroborationoxidation: A) 3-Methyl-1-butene C) 2-Methyl-1-butene B) 2-Methyl-2-butene D) Both (A) and (B) 29. Which point on the potential energy diagram corresponds to the carbocation intermediate, (CH 3 ) 3 C +, for the reaction shown below? A) A B) B A C C) C B D) D D 30. The polymerization of styrene gives a polymer that has which structure? PART II: TRUE (T) / FALSE (F) QUESTIONS (5 questions, 2 points each) Fill in A for true (T) and B for false (F). 31. (T) E1 dehydrohalogenations are stepwise; 32. (T) Carbocations are not formed in S N 2 and E2 reactions; 33. (T) Hydrogenation of alkenes is always exothermic; 34. (T) Trans-cyclohexene is less stable than cis-cyclohexene; 35. (T) Hydroboration-oxidation is a syn-addition process; PART III: REGULAR (LONG SOLUTION) PROBLEMS 36. (13 pts) Predict the principal organic product(s) in each of the following reactions. Draw the correct stereoisomeric product, wherever applicable.

4 37. Suggest an appropriate set of reagents/conditions and write an equation for each of the following transformations: A. (2 pts) 2-Propanol to propene; B. (2 pts) Cyclohexene to bromocyclohexane; C. (2 pts) 1-pentene to 1,2-epoxycyclopentane; D. (2 pts) 1-pentene to 1-pentanol; 1. BH 3.THF 2. H 2 O 2 /OH - OH 38. (4 pts) Provide a detailed mechanism (Structures and equations, not verbal!) of the following reaction:

5 39. (4 pts) The two isomeric structures (1 and 2) given below behave very differently when reacted with NaOCH 3 /CH 3 OH. Compound 1 yields two elimination products, while 2 gives only one. Provide a detailed structural rationalization to account for the difference (No lengthy verbal explanations please! More structures, fewer words!). 40. (1 pts) Provide the structural formula of MCPBA. 41. (2 pts) BONUS PROBLEM (In order to receive credit for this problem, it has to be solved entirely!!). Trifluoroiodomethane undergoes addition to alkenes in the presence of light, following a free-radical chain mechanism. Provide plausible initiation and propagation steps for the reaction below (Hint: C I bonds are relatively weak).