CHEM 313 Final, Spring 1997

Chem 313 Final Exam, June 24, 1997 Prof D. Naud 1) The Ka (solvent is sulfuric acid) for removal of the first proton in 1,10-phenanthroline-2H* (A) is 35.5, but that for 2,2'-dipyridine-2H* (B) is 3.3. The Ka for removal of the proton in the N-methyl analogue of A is 129, for the N-methyl analogue of B, 0.49. Why does Ka go up for A (ratio is 3.6) but down for B (ratio is 0.15)? ave Qe “a * Not’ A ~ B B-CH, 2) The empirical resonace energy (ERE) of benzene is -36 kcal/mol, (The ERE of benzene is calculated from the hypothetical cyclohexa- 1,3,5-triene (A) with alternating bond lengths and localized double bonds. The heat of hydrogenation of A should be three times that of cyclohexene. The difference between this value and the heat of hydrogenation of benzene is the ERE. ) What should the ERE of biphenylene (B) be? The observed ERE of B is - ~ 10 kcal/mole, Expalin why there is a discrepancy between the observed and the expected. A OC Ta Teal Oo -s GO 3) In order for AAG® (or AAG*) of a series of reactants to be linearly proportional to the AA G° of another series of reactants, one of three conditions must be met. What are these three conditions? 4) Measurement of the heats of methylation of two compounds that give a common product is a useful procedure for determining relative stabilities. Consider the following data (kcal/mole) for reactions conducted in the vapor phase. 1) Which is the more stable compound of the isomer pair, A-B, and by how much? For C-D? ii) The pair C-D can be used as a model for the localized bond version of A-B. Calculate the difference in ERE between A and B. Which (A orB) is more stabilized according to this criterion? Give an energy level diagram to help out with your answers. Ss CL CH,FSO, SS) LHS | ~ n7 scu3?-®) (-40.2) 3 oe CH, N A CH, cH, 8B or CH,FSO, CH,FSO, paul alata ee n>scn, 23 . ~46.2 . | 5) A chemist wants to study the nucleophilic catalysis of an ester ()) and the base-catalyzed decomposition of nitramide (2) with a variety of bases. His studies will generate a Bronsted plot for each reaction. What would you advise him? oW Reactions 8 6 son a cube Crm, +H, ——e HONe * : NO. . 2 (2) Base + NH2NO2 —~ (BaseH)!* + NHNO}!- —-»> products Proposed bases: para-substited N-methyl-anilines, para-substituted pyridines and various 2-halo acetates and propanoates. 6) Pyridinium ion, pyr-Ht, has similar acidities in water and methanol. Acetic acid is three times more acidic than pyr-H* when in water, but is 10,000 times less acidic when in methanol, Explain this observation. Acid Solvent Ka Acetic Acid water 18x 1075 _ methanol 2.2 x 10°10 pyr-H* water 6.2x 10°6 methanol 2.8x 10° d. Write the rate law expression for the 7) The halogenation of acetone is general acid and base catalyze Spivey third-order term in the rate bromination of acetone buffered by acetate and acetic acid. Include the Dawson- law and describe its mechanism of reaction. 8) What kinetic isotope effects would one expect for these reactions? a) Cr + CL30Ts —-¥ CL3Cl + TsO” H20 b) (CH3)9CL0) ——> HI + (CH3)2CL(OH) Ht ¢) LyC=CHCHy + HOAc ——> L2CH-CH(OAc)CH3 ry oltns A 4 _—_—_ L a dy pstrate gH with A, B, and ical reactions of su 4. For the free radi c, the following data were obtained: Xy/ Ky AH, Z-H+A Z + AH 1.2 very exothermic z-H + B z + BH 7.1 moderately exothermic Z-H + C z + CH yittrle exothermicity a) Describe the transition state for each reaction (1.€+% early, jate etc.) b) predict the kinetic psotope effect for the third yeaction and ' 1G The formation of acetone oxime has the following generalized mechanism: k — + WON, te Se NOH 4 H,0 cH, e mechanistic reasons why Kops decreases ditions. At high pH, do you of oxime formation versus pH. Giv tions, and increases from neutral to basic con general acid (and/or base) catalyzed? The plot below is log kobs from acidic to neutral condi believe the reaction is specific or | f. 1-Aryl-2-propyl tosylates (A) can solvolyze by either solvent-assisted (kg) OF aryl-assisted (k,) pathways. &- oTs bs 7% OY Ac cn 2 Hon 3 4 C-F ko. The overall observed rate (Kops) is: ” Kops = ks * FKa n of aryl-assisted intermediate that goes on to product. A Hammet plot of kobs From the data in this plot, develop a method for determining the percent of ary! 5) and calculate these percentages (for each substituent). in which F is the proportio: for acetolysis is nonlinear. participation (100 Fk,/Kob: N Pal o x ° fo} 4