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Unformatted text preview: Name _______________________________________________________________ Chemistry 171Q, Preliminary Exam #1 September 23, 2004 8:00 - 9:30 a.m. There are five (5) questions to this exam. Before you begin, please make sure that your exam copy contains all five. If not, please notify the instructor immediately. Note that the last two pages are deliberately left blank for use as scratch paper. You may carefully remove them to use as scratch paper. Good Luck!!! Problem 1. 2. 3. 4. 5. Score /25 points /10 points /25 points /10 points /20 points ____________________ TOTAL /90 points -1- 1. (25 points) Complete each of the following: (a) (10 points) Methyleneimine, which has molecular formula CNH3 , is of current interest in astrophysics and has been observed in interstellar dust clouds. Draw a Kekul structure for methyleneimine. Use lines for all covalent bonds and show all nonbonding electrons. Specify the hybridization you expect for the carbon and nitrogen atoms in your structure. (b) (5 points) The octane rating of gasoline is related to the content of the C8 H18 hydrocarbon with the trivial name isooctane, which has the condensed formula (CH3 )3 CCH2 CH(CH3 )2 . Write the IUPAC name for isooctane. ______________________________________________________ (c) (10 points) It is a common misconception that the pKa of H2 O is 14.0 (deduced from Kw = [H3 O+][HO ] = 10-14; pKw = 14.0). The actual pKa for H2 O is 15.75. For a generic acid HA in aqueous solution, recall that K a(HA) is defined as follows:
HA + H2O [H3O H3O ] [A ] + A Ka = [HA] -2- Write the expression for Ka(H2 O) in aqueous solution in terms of the concentrations of all relevant species. From your expression above, show how pKa(H2 O) is related to pKw. 2. (10 points) Alcohols typically show broad characteristic infrared (IR) absorptions at 3400 cm-1 due to the O-H stretching vibration. However, if the OH group is intramolecularly hydrogen bonded to a Lewis basic group within the molecule, the IR absorption is typically shifted to slightly higher energy. The IR spectra of the two stereoisomers of butane-2,3-diol (CH3 CHOHCHOHCH3 ) both show two IR absorptions in the OH stretching region. Interestingly, the intensity of IR resonance assigned to the intramolecular, hydrogen-bonded OH group is significantly larger in one of the stereoisomers, indicating a greater fraction of the isomer exists in the hydrogen-bonded state. (a) Using Newman projections, draw the most stable conformers of the two stereoisomers of butane-2,3-diol that would be expected to show an intramolecular, hydrogen bonded OH group. It may be helpful to show the hydrogen bond in each structure with a dashed line (i.e. O-----H). A B -3- (b) Which of the two isomeric conformers drawn above (A or B) would you expect to be more stable? (Hint: A CC/CC gauche interaction is more destabilizing than a CC/CO gauche interaction.) Explain your reasoning. (c) Which stereoisomer of butane-2,3-diol (A or B) would you expect to show the larger fraction of the IR resonance due to the intramolecular, hydrogen-bonded OH group? Briefly explain your reasoning. 3. (25 points) Predict whether Keq is > 1 or < 1 for each of the following reactions. Provide a brief but clear explanation for each of your predictions. (This question is about thermodynamics, therefore, you do not have to worry about how a reaction occurs or whether it occurs at a significant rate.) For part (a), estimate the free energy change for the reaction (G) at 25 C. (a) (11 points)
pKa = 5 + OCH3 NH2 + HOCH3
pKa = 15 Keq G 1 kcal/mol -4- (b) (7 points)
Keq H3C O Keq 1 (c) (7 points)
H H3C H CH3
Keq H CH3 H CH3 Keq 1 -5- 4. (10 points) Predict the products of each of the reactions and use the electron-pushing formalism to show the flow of electrons as reactants are being converted to products. Make sure your arrows clearly show which bonds are being made and/or broken. Also, indicate any formal atomic charges in the reaction products. (a) (5 points)
H2SO4 + CH3OCH3 (b) (5 points)
H3C C H3C H + HOCH3 -6- 5. (20 points) Propose structures for A-D that are consistent with the experimental observations. Your observation/deduction reasoning will be considered for partial credit. Compounds A, B, and C have formula C 6 H12. A and C add one equivalent of H2 in the presence of a catalyst to give D (C 6 H14). The infrared (IR) spectrum of C has an absorption at 1646 cm-1. The 13C NMR spectra of A and D each show two resonances (i.e. two kinds of carbons). The 13C NMR spectrum of B shows one resonance. Observation Deduction A fl B fl C fl D fl -7- -8- -9- ...
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This test prep was uploaded on 04/17/2008 for the course CHM 171Q taught by Professor Goodman during the Fall '04 term at Rochester.
- Fall '04