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UK CHE 230 - Exam

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1 Organic Chemistry I (230-001) Examination II October 27, 2004 Name (PRINT LEGIBLY): Please provide clear and concise answers to all of the following questions. Use equations and/or drawings to support your answers where appropriate. Please answer questions in the area provided, the back of an exam page or on the clearly labeled spare page. Problem Score 1. (a-e) /25 2. (a-e) /25 3. (a-c) /18 4. (a-b) /10 5. (a-b) /10 6. (a-c) /12 Total /100 PL EA S E observ e the following. 1) Write LARGE and LEGIBLY. If I can’t read what you write, I can’t give you full or partial credit. 2) Read each question carefully before answering. If you don’t answer the question I posed, you will not receive credit.2 1. (25 pts) (a, 4 pts) Assign the configurations of each stereo center in the compound below: CH3HHO2C CH3 (b, 4 pts) Draw a configurational enantiomer of the compound above (c, 4 pts) Identify each of the stereocenters in the structure below with an asterisk CH3OHHO3 (d, 6 pts) Circle the chiral compounds in the following list: H3CHHCH3H3CHH3CHCH3CH3H3CHOHOHOHOHOHOHBr (e, 7 pts) Indicate whether each of the following is true or false. 1. If a compound has an enantiomer it must be chiral 2. If a compound has a diasteromer it must be chiral 3. Any molecule containing only one stereocenter must be chiral 4. Some chiral molecules are optically inactive 5. Some diastereomers have a mirror-image relationship 6. If a structure has no plane of symmetry it is chiral 7. Mirror-image molecules are in all cases enantiomers.4 2. (25 pts, 5 pts) Describe the relationship of each pair of structures as specifically as possible and in no more than two words (a) ClClClCl (b) NH2NH2MeMeMeMe (c) C C CHCl ClHCCCHClClH5 (d) CO2HNH2HOOHNH2HO2C (e) MeMeClClMeMeClCl6 3. (18 points) (a, 6 pts) Draw trans- 1,4-dimethylcyclohexane in its lowest energy conformation (b, 6 pts) Draw cis-1,4-dimethylcyclohexane in its lowest energy conformation7 (c, 6 pts) Based on the structures you drew in parts (a) and (b), which is lower in energy, cis- or trans-1,4-dimethylcyclohexane? Briefly explain why. 4. (10 points, 5 points each) (a) While reorganizing his laboratory, Joe Greenhorn found an enantioenriched sample of 2-butanol containing a 3:1 mixture of enantiomers. What is the ee of Joe’s sample of 2-butanol?8 (b) Joe measured the specific rotation, [α], of his sample of 2-butanol to be -7˚. Enantiopure (S)-2-butanol is known to have [α]25D= +14˚. What is the configuration at the stereogenic C atom in the major enantiomer of 2-butanol in Joe’s sample? 5. (10 pts) Living systems put glucose into other forms for specific purposes. The molecules are A: glucopyranose α-anomer, B: glucopyranose β-anomer and C: glucofuranose (below). Using what you know about ring strain and the conformation of substituted cycloalkanes, briefly explain (no more than two sentences): OHOOHHOHOHOAOHOHOHOHOBOHOHOHOOHHHOC (a) Which molecule is most stable and why? (b) Which molecule is least stable and why?9 6. (12 points, 4 points each) (a) Circle the two chair structures of cis-1-bromo-3-methylcyclohexane. BrMeADBrMeBrCMeBBrMe (b) One structure is more stable than the other by 3.7 kcal/mol. Which is it? Briefly explain why. (c) Using the table below, compute the approximate energy cost of a 1,3-diaxial interaction between a bromide and a methyl group (show your work). Group Cost of single 1,3-diaxial Cost of a gauche interaction with H interaction with CH3 . Br 0.25 kcal/mol 0.25 kcal/mol CH3 0.85 kcal/mol 0.85 kcal/mol OH 0.50 kcal/mol 0.50 kcal/mol Approximate energy cost of a 1,3-Me/Br diaxial interaction =10 Spare


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UK CHE 230 - Exam

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