CHEMISTRY 241 Section 004 EXAMINATION I TUESDAY, October 11, :30-11:50 AM Professor William P. Dailey NAME: QUESTIONS POINTS SCORE

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1 CEMISTRY 241 Section 004 EXAMIATI I TUESDAY, ctober 11, :30-11:50 AM Professor William P. Dailey AME: Student ID number : QUESTIS PITS SCRE TTAL READ ALL QUESTIS CAREFULLY BEFRE ASWERIG TEM.

2 1. (16 Points) Provide correct IUPAC names for the following compounds. 2-Methyl-1-(1-methylcyclopentyl)heptane 2,3,6,7,7-pentamethyloctane 5-(1,1-dimethylethyl)-1-methyl-2-(1-methylethyl)cyclohexane Bicyclo[1.1.1]pentane 2. (10 Points) The value for the heat of combustion of cycloheptane is kcal/mol while the molar heat of combustion of cyclohexane is kcal/mol. Recall that cyclohexane has no strain energy. Calculate the total amount of strain energy found in cycloheptane. Propose the reason(s) for this ring strain. Enthalpy per C2 for cyclohexane = /6 = kcal/mol Enthalpy per C2 for cycloheptane = /7 = kcal/mol Difference =0.9 kcal/mol/per C2 So strain in cycloheptane = 7x0.9 = 6.3 kcal/mol Models show some eclipsing C- bonds and some ring strain. 3. (12 Points) Consider the reaction between 2-methylpropane and chlorine (Cl 2 ). Draw all of the monochlorinated products. Assume that the relative reactivity of primary:secondary:tertiary C- bonds towards chlorination is 1:3:8. Predict the relative amounts of each different monochlorinated product and clearly show your calculations. 2

3 Cl Cl primary tertiary 9 equivalent primary 's so relative amount product is 9x1 = 9 1 tertiary, so relative amount of tertiary product is 1x8 = 8 So % primary = 9/(98)% = 53% % tertiary = 8/(98)% = 47% 4. (12 Points) Consider the following trimethyl-substituted cyclohexane shown below. It can exist is two chair conformations that differ in energy by 4.4 kcal/mol. Draw the two possible chair conformations, note all the axial methyl groups and predict which one of the chair conformations is the most stable. In one of these conformations, there exists a severe 1,3-diaxial methyl-methyl interaction, a numerical value for which we have not determined. owever you can derive a value for this interaction by carefully analyzing the difference in energy of the two conformations. two gauche 3 C C 3 C 3 gauche gauche 3 C C 3 gauche C 3 C3 C 3 1,3-diaxial methy-methyl gauche all methyls are equatorial C 3 All methyls are axial The chair with all methyls equatorial is most stable. The energy of the left chair is due to 4 gauche interactions and one 1,3-diaxial methyl-methyl interaction. The energy of the right chair has 2 gauche. So equating the two with the 4.4 kcal/mol energy difference gives 4 gauche 1,3-diaxial = 2 gauche 4.4 kcal/mol 4(0.9) 1,3-diaxial = 2(0.9) 4.4 kcal/mol 1,3-diaxial = 2.6 kcal/mol 3

4 5. (12 Points) Draw representations of the following a. A ewman projection along the central C-C bond of trans-decalin 2 C C 2 2 C C 2 b. A ewman projection along one of the C-C bonds in cyclopropane C 2 (eclipsed) c. A ewman projection along the 3,4 C-C bond in the most stable conformation of 3- ethyl-4-methylpentane. 3 C C 3 3 C 2 C C 2 C 3 d. A ewman projection of the highest energy conformation of 1,2-dichloroethane. Cl (eclipsed) 6. (8 Points) umber the following compounds from 1 to 5 in order of increasing boiling point. (1 is lowest boiling, 5 is highest) Clearly state the reason(s) for this ordering. 1 Greater surface area (more van der Waals interaction) makes 2 higher boiling than 1. Greater dipole moment makes 3 boil higher than 2 and 1. Even greater dipole moment makes 4 boil higher than 3. ydrogen bonding in 5 makes it boil highest. 4

5 7. (9 Points) Write the best Lewis structure for the following molecules and show all formal charges. a). C3 b) c) C 3 C C 8. (9 Points) a) Draw a qualitative (no numbers needed) energy profile for rotation about the C-C bond of 1,2-dichloroethane, Cl2CC2Cl. Draw the conformation that corresponds to each minimum and maximum on the diagram. Label all the transition states. Same relative energy diagram as butane but without any energy values. The transition states are the three maxima. b) The experimental dipole moment of 1,2-dichloroethane is 1.0 D. Which of the following statements is false? Explain. a. 1,2-Dichloroethane exists totally in the gauche form. b. 1,2-Dichloroethane exists totally in the anti form. c. 1,2-Dichloroethane exists as a mixture of the gauche and anti forms. In the anti form there is no dipole moment. Since the molecule has a dipole moment, it must therefore exist in some other conformation at least partially. 9. (15 points) Label the reactants in these acid-base reactions as Lewis acids or Lewis bases. Complete the reactions using the curved arrow formalism. a. 5

6 F B F LA F LB F F B - F F - LB LA F - - LA LB b. Predict the products of the following reactions using the curved arrow formalism. 3 C C 3 BF 3 3 C 3 C BF 3 C 3 3 C (8 points) Chlorination and bromination of alkanes can be useful reactions in organic synthesis. n the other hand, iodination is generally not successful. Using the table below, as well as what you know about the mechanism of halogenation, offer an explanation for the lack of success of this reaction. Cl-Cl 58 kcal/mol -Cl 103 C-Cl C- 70 I-I 36 -I 71 C-I 56 For the two propagation steps in radical halogenation, the sum (the overall reaction) is endothermic for iodination. So it usually doesn't work. 11. (8 Points) Compare and contrast the structure and bonding in carbocations with that found in carbanions. Describe how alkyl subsitution affects the stability of each species. 6

7 Carbocation stability follows the trend that tertiary is more stable than secondary is more stable than primary. Carbanion stability follows the opposite trend. Primary is more stable than secondary than tertiary. Electron-donation via hyperconjugation from alkyl groups will stabilize a positive charge but destabilize a negative charge. 12. (12 points) a. Predict which one of the following compounds will have the more Lewis basic oxygen atom. Clearly explain your reasoning. The dipolar resonance structure for the amide is much more important than the corresponding form of ketone, so the oxygen will have a greater negative charge and be more basic. b. Predict which one of the following alkenes will have the most Lewis acidic carbon atom and predict which carbon atom it is. Clearly explain your reasoning. The most important resonance structure that has a positive charge on carbon is shown. c. Predict which one of the following compounds containing - bonds will be the stronger onsted acid. Clearly explain your reasoning. This resonance structure is more important with C= rather than C=C since is more electronegative. This makes the hydrogen more acidic. 13. (10 points) The simplest example of a compound known as an allene is 2C=C=C2. Draw the Lewis structure for this compound, predict its geometry, draw and note the hybridization 7

8 of the orbitals on the carbon atoms, and draw where the valence electrons are located in these orbitals. sp sp2 C C C 14. (9 points) Draw the mirror image of each compound below and determine whether it is the same or different from the original structure. Same Cl Cl Different (enantiomer) Different (enantiomer) 8