Exam 1 Chem 3045x Monday, October 1, 2001

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1 Exam 1 Chem 3045x Monday, October 1, 2001 Instructions: This is a closed book examination. Please print your name and social security number on the front page of the examination. Be sure to allot your time in a manner that is related to the point value of the question. Be sure to show your reasoning wherever possible for partial credit. All material to be graded must be on one of the pages of the exam with your name and social security number on the front page. If you need more space than is available on the page with the questions, use the back page or the previous page and label the number of the question on that page. Your Name: Your Soc. Sec. Number: Time for the exam: 50 minutes (10:00 to 10:50 AM). Question 1: 10 points Question 2: 10 points Question 3: 10 points Question 4: 10 points Question 5: 10 points Question 6: 10 Points Question 7: 10 Points Question 8: 20 Points Question 9: 10 Points Total: 100 Points Exam 1/3045/2001 p. 1

2 1. (10 Points). Consider the composition C 2 N 2. Draw the Lewis structures of 5 constitutional isomers which possess this composition. The structures must obey the octet rule and explicitly indicate the formal charges on any of the atoms. 2 C N N 2 C N N contributing forms not isomers 2 C N N C N N C N N C N N 2 N C N 2 2 C N N 2 C N N 2 C N N C N N C N N C N N 2 C N N 2 N C N N C N 2 N C N 2 N C N Exam 1/3045/2001 p. 2

3 2. (10 Points). Draw a circle around each of the following molecules that possess a net dipole moment C 2 F 2 CO 2 CF 4 C 3 OC 3 cyclohexane 3. (10 Points) Write a Lewis structure for the composition C for which (a) All the carbon atoms are methylene groups (one structure). -C 2 -C 2 -C 2 -C 2 - (b) None of the carbon atoms are methylene groups (two structures). 4. (10 Points) Draw a circle around any of the following compounds which are isomers. 3 C 3 C C 3 3 C Cyclohexane C 6 12 exane C 6 14 Methyl cyclopentane C ,1,2-trimethylcyclopropane C 6 12 Exam 1/3045/2001 p. 3

4 5. (10 Points) Conformational analysis of cyclohexane. In general, the chair conformation can be assumed to be the most stable shape of a monosubstituted or disubstituted cyclohexane. This assumption is true for trans-1,4-ditert-butylcyclohexane (A). owever, cis-1,4-cis-di-tert-butylcyclohexane (B) adopts a twist boat or skew boat conformation rather than the chair conformation. Draw the chair form of both structures and suggest a reason why the cis isomer is more stable in the twist (skew) conformation rather than the usual chair conformation, while the trans isomer A is more stable in the chair conformation. In the A chair conformation the large tert-butyl groups are in the equatorial position and A do not interact strongly with any other groups. The chair form is stable. In the B chair form the axial tert-butyl group interacts strongly with the 1,3-hydrogens in the axial position. These are strong interactions and cause the chair to twist away from the chair conformation. This is the skew chair conformation. B Exam 1/3045/2001 p. 4

5 6. (10 points) Knowledge of the magnitude of energies and dynamics is important for qualitative thinking in organic chemistry. (a) What is the approximate energy difference between a staggered and an eclipsed conformation of ethane (indicate the energy units explicitly)? 3 kcal/mole (b) What is the length of the C-C bond in ethane is approximately (indicate the energy units explicitly)? 1.5 A (c) At room temperature ethane undergoes approximately how many full rotations about the C-C bond in one second? sec -1 (d) At room temperature for every 100 ethane molecules in the staggered configuration, approximately how many molecules are in the eclipse conformation? 1 (e) What is the approximate value of kt, the average kinetic energy of a molecule at room temperature (indicate the energy units explicitly)? 1 kcal/mole Exam 1/3045/2001 p. 5

6 7. (10 Points) The reaction of 2-methylbutane with chlorine produces nearly equal amounts of two isomeric (C 5 11 ) monochlorination products A and B. The reaction of 2-methylbutane with bromine produces essentially only one monobrominated product (C 5 11 Br) C. (a) Draw the structures of the products A, B, and C. 2 /hv Br 2 /hv Br (b) Explain why chlorination of 2-methylbutane produces two monochlorinated products, A and B, but the bromination of 2-methylbutane produces only one monobrominated product, C. Both reactions involve attack of a halogen atom on a C- bond. radical is very reactive and non-selective. Br radical is less reactive and more selective. A RC 2 - bond is stronger than a R 3 C- bond. Br radical attacks the R 3 C bond preferentially. Exam 1/3045/2001 p. 6

7 8. (20 Points). Consider the composition C512. (a) Write the Lewis structure for all of the possible constitutional isomers of this composition. (b) Predict which of the isomers will have the highest boiling point. Linear chains have more contacts than branched chains. This leads to greater van der Walls attractions. (c) Predict which of the isomers will have the lowest boiling point. More spherical shape-smallest number of attractive van der Waals interactions (d) Briefly explain the reasoning behind your prediction. Exam 1/3045/2001 p. 7

8 9. (10 Points) assify each of the following pairs of structures as (a) conformers; (b) constitutional isomers; (c) stereoisomers; (d) identical structures; (e) compositoins. Put your answer in the box to the right of the structures. C 3 C 2 C 3 3 C C 2 C 3 constitutional isomers C 3 C 3 C 3 C 3 C 3 C 3 3 C C 3 C 3 3 C 3 C C 3 stereoisomers constitutional isomers conformers Exam 1/3045/2001 p. 8