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Unformatted text preview: Chemistry 303 fall, 2006 FIRST EXAMINATION 7:00 PM, OCTOBER 17TH, 2006 Duration: 2.0 hr Name____________________________________________________________ Lab TA___________________________________________________________ (if you do not know his/her name, give day of lab section) This is an "open book" examination; you may use anything that is not alive. Note: if you do not know the complete or specific answer, give a partial or general answer-- We love to give partial credit. If there seems to be more than one good answer, explain your thinking. If you invoke resonance delocalization as part of your answer, draw the relevant resonance structures WRITE SOMETHING BUT: write legibly! We will not work too hard to decipher sloppy writing. Write only in the space provided for each question. Score: p2___________/13 p6___________/09 p3___________/12 p7___________/06 p4___________/15 p8___________/13 p5___________/14 p10___________/06 p9___________/13 Total: /101 There are 10 pages in this exam; please check now to be sure you have a complete set. Please be aware that a small number of students will be taking the exam at different times up until the morning on Tuesday. It would be well not to discuss the exam until after that time. Pledge:_________________________________________________________________________________ 1 I. (13 pts). Consider the molecules, A, B, C. H3C O H CH3 H3C
tert-butyl alcohol [2-hydroxy-2-methylpropane] A H3C H3C O H CH3 H3C C H2 H2 C C H2 OH isopropyl methyl ether [2-methyoxypropane] B n-butanol [1-hydroxybutane] C A. (5 pts). Write here the list of molecules in order of increasing bp: _____ < lowest _____ < _____ highest B. (4 pts). Give the single most important reason for the difference in bp for the one with highest bp and the one that is second highest. Use words and pictures. C. (4 pts). Give the single most important reason for the difference in bp for the one with second highest bp and the one that is lowest. Use words and pictures. 2 II. (12 pts). Consider the pair of molecules below. A. (02 pts). Draw in all of the non-bonded electron pairs (lone pairs). B. (04 pts). Both molecules have two more-or-less basic sites. Draw the conjugate acid of 1 and explain why the site you chose is the most basic. C. (02 pts). Draw the conjugate acid of (2). N 1 NH2 N NH2 2 D. (04 pts). Which is more basic, (1) or (2)? Give the single most important reason for the difference in basicity; explain. 3 III. (15 pts). Consider the molecule, A. A. (06 pts). For your benefit, on the structure shown here, draw all C and H atoms, and lone pairs. Label each carbon with the correct hybridization: sp, sp2, or sp3. O A B. (09 pts). 1. (06 pts) Identify the two most acidic protons in the molecule and circle them on the structure. Explain carefully why they are more acidic than any other H in the molecule. 2. (03 pts) Between the two acidic protons, one has pKa ca 18 and the other has pKa ca 22. Identify which is which and explain the single most important reason for the difference. 4 IV. (14 pts). Consider the molecule for which X is a resonance structure. A. (6 pts) Draw the two most significant additional resonance structures related to X, and label them Y and Z. Show all nonbonding electrons and formal charges, if any.
H B N H X B. (4 pts) Rank your three resonance structures, X, Y, and Z in terms of stability/significance. Explain the main reasons behind your ranking. C. (4 pts) Would you expect the molecule represented by resonance structures X,Y,Z to have a greater or smaller molecular dipole than Q? Explain. H N N H Q 5 V. (15 pts). When phenol is mixed with D2O (water in which the protons are replaced with isotope H, deuterium) and a little triethylamine is added, a new product is formed, D. The molecular ion region of the mass spectrum of D is shown. Phenol has molecular weight = 94.
D2O Et3N: [D]
m/z 95 2 OH phenol MW 94 A. (05 pts). Draw the structure of D and explain how it is consistent with the base peak (a) in the mass spectrum. B. (04 pts). Explain the origin of the peak (b) at m/z 96.
(b) 90 m/z continued... 6 C. (06 pts). Write a reaction sequence (two steps) which shows how D is formed from phenol and makes clear the role of the triethylamine. A complete answer will include the use of the arrow formalism to show the steps. 7 VI. (13 pts). (The preceptors think this is too tricky since it took them 5 min to figure it out; please prove them wrong) A. (10 pts). Imagine a molecule with five carbon atoms, any number of hydrogens (no other atoms) which has three (and only three) sp hybridized carbons. 1. (06 pts) There are two isomers that fit the criteria. Draw them here in large format. 2. (03 pts) Label clearly each of the sp hybridized carbons. 3. (01 pts) Circle the carbon-carbon bond that is overall shortest, considering both structures. [one circle] [Note: you may request from the preceptor one or both of the structures needed for this question. If you obtain one structure, the maximum score for part A will be 6 pts; if you request two structures, the maximum score for part A will be 2 pts. ] B. (03 pts). Which of the two isomers is more acidic? Show the conjugate base for that isomer and give the single most important reason for the difference. 8 V. (19 pts). Consider the two reactions shown here. A. (04 pts). Complete each reaction by showing the conjugate base for each.
(1) OH H+ + (2) SH H+ + B. (05 pts). The pKa for eq (1) in water is 16, while the pKa for eq (2) is 11. Give the primary reason for the difference in acidity. C. (04 pts). Illustrate the difference in acidity for the two reactions by drawing an energy diagram for each reaction. Indicate the free energy difference (G) for each reaction and indicate clearly which is larger. Explain any ambiguity. FREE ENERGY DIAGRAM or REACTION COORDINATE DIAGRAM Rxn 1 Rxn 2 E E reactants products extent of reaction reactants products extent of reaction continued.... 9 D. (06 pts). The pKa for eq (1) in acetonitrile is 25, while the pKa for eq (2) is 15 in acetonitrile. 1. Why is the pKa for eq (1) 9 units higher in acetonitile compared to water? H3C C N acetonitrile 2. The effect of the solvent change on eq (2) is much smaller, increasing the pKa in acetonitrile by only 4 units. Why is the effect smaller for eq (2) compared to eq (1)? end exam 10 ...
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