Chemistry 3719 Fall 2000 Exam 1 Name: KEY. Anti Gauche Eclipsed 1 Eclipsed 2

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1 hemistry 3719 Fall 2000 Exam 1 Name: KEY This exam is worth 100 points and you have 50 minutes to complete it. You may use molecular models to help you with any of the problems. Good luck. 1. (8 pts) 1,2-Dibromoethane has the formula 2 2. Looking down the - bond axis, draw Newman projections that depict the anti, gauche and the two possible eclipsed conformations. Which of the eclipsed conformations is the least stable and why? Anti Gauche Eclipsed 1 Eclipsed 2 Eclipsed 2 is the least stable due to the two large atoms getting very close to each other. 2. (6 pts) Give the hybridizations for each of the carbon, oxygen and nitrogen atoms in the following molecules. sp 2 sp N 3 N sp 3 sp sp 3 sp 2

2 3. (10 pts) Each of the two following compounds is shown in one of the possible chair forms. Do a ring flip and draw the other chair form, then indicate which of the two conformations is more stable. Explain your reasoning. ( 3 ) 2 ( 3 ) 2 ( 3 ) 2 ( 3 ) 2 In the first conformer, the two large groups are axial and 1,3-diaxial interactions will destabilize this compared to the second conformer in which the large groups are equatorial. ( 3 ) ( 3 ) 3 The large t-butyl group is axial in the conformer on the left and thus interacting with hydrogens across the ring. In the second conformer the larger group gets to be equatorial which is favourable. 4. (6 pts) Using the δ+/δ- notation indicate the polarization (if any) within each of the following molecules: δ+ δ- δ- δ+ δ+ δ- δ+ F Li

3 5. (15 pts) Give each of the following compounds acceptable IUPA names: a. b. 3,4,5-Trimethyldecan-2-ol 5-Isopropylnonane c. 3 3 cis-1,2-dimethylcyclohexane d e. cis-1,3-diethylcyclobutane l 2-hloro-4-isopropylheptane

4 6. (6 pts) ompare cis-1,3-diethylcyclohexane with trans-1,3-diethylcyclohexane. Draw each of the compounds in its most stable chair conformation, then decide whether the cis or the trans isomer is more stable and explain why is-1,3-diethylcyclohexane Trans-1,3-diethylcyclohexane The cis isomer has both substituents equatorial and thus pointing away from the rest of the ring and avoiding any 1,3-diaxial interactions. The trans isomer has to have one ethyl substituent axial which will destabilize it relative to the cis isomer. 7. (6 pts) onvert each of the following line representations into formula representations. Propane is shown as an example or ( 3 ) 2 2 ( 3 ) or ( 3 ) 2 ( 3 ) or ( 3 ) 2 ( 3 ) 2 ( 3 )( 3 ) 2

5 8. (5 pts) Pentane and 1-pentanol are close in molecular weight but 1-pentanol boils at 138 o whereas pentane boils at 36 o. Explain. Pentane molecules interact with each other through relatively weak van der Waal s forces whereas pentanol possesses a hydroxyl group () which is polarized and capable of hydrogen bonding with other pentanol molecules. These extra intermolecular attractive forces will require more energy to break and thus 1-pentanol will boil higher than pentane. 9. (6 pts) lassify each of the highlighted carbon atoms in the molecules below as being 3 o, 2 o or 1 o. ( 3 ) 3 1 o 3 2 ( 3 ) 3 1 o 3 o 10. (10 pts) Draw all of the isomers of Be careful not to repeat structures.

6 11. (8 pts) Penicllin is an important drug used to treat microbial infections. Label any of the following functional groups that appear in Penicillin : 2 = 2 S 2 N S 3 alkene amide N 3 carboxylic acid 12. (6 pts) Draw the second resonance form for each of the following species. Show all of the lone pairs in your answer.

7 13. (4 pts) What shape does methane adopt and why? Draw a wedge/dash depiction of methane and show the bond angles and hybridization of the carbon atom. sp 3 hybridization at bond angles 109 o 28' sp 3 hybrid carbon has four sigma bonds which, in accordance with VSEPR theory, will get as far as possible from each other, i.e. by pointing to the four corners of a regular tetrahedron. 14. (4 pts) Draw representations of cyclopentane in the planar and envelope conformations. Planar Envelope