CHEM 210 Final, Fall 1995

Chemistry 210 FinalExam , _ Name. January 20, 1996 : : Prof. D. L, Naud LD. No. Section. 1. (6 points, no partial credit) Successful crystallization depends upon proper solvent choice. Order the following solvents in decreasing polarity. A.acetone B.toluene C. water D. methanol E. ethylacetate F. cyclohexane G. diethyl ether H. acetic acid Polar <---> Non-polar ould separate a mixture of p-methylphenol, benzoic 2. (8 points) Give a BRIEF step by step procedure how one ¢ (pKa of phenols is approx. 10; pKa of carboxylic acid, cyclohexylamine, and cyclohexanone by extraction only. acids is approx. 5) be \v 3. (5 points) The purity of a compound by melting point can be established by two ways. What are they? 2-one by distillation, however, the compound decomposes tions to succeed in purifying the compound. In the first, dis barely soluble in water at room temperature) or distill Which is better and state your reasoning. 4. (8 points) A student needs to purify 6-chloro-4-hexen- at its normal boiling point. He believes that he has two op he could co-distill the compound with water (the compoun: the compound under reduced pressure i.e. vacuum distillation). 5. (6 points) Calculate the mole fraction of compound A in the vapor phase (at 80°C) for the following solution: © = 760 torr at 80°C (vapor pressure of pure A) X= 0.40 (mol fraction in liquid phase), P 4’ Xp = 0.60, Py® = 180 torr at 80°C. Assume the solution behaves ideally. 6. (6 points) A student attempts to purify an organic product by recrystallizing it from water. After his first ~ recrystallization, the student filters his product from the mother liquor and immediately takes a melting point to determine purity. Unfortunately, his sample melts at 73°C, ten degrees below the substrate's literature melting point. He repeats the recrystallization and obtains a new melting point (using identical procedure) and finds the melting point didn’t change; the substrate still melts at 73°C. The students finally hits upon a bright idea and carries out a procedure. After the procedure, he finds the material melts at 83°C. ‘What procedure did the student perform? - 7.a.(4 points) What's the structure of isoamyl acetate? b. (12 points) Given the following equilibrium: ke carboxylic acid + alcohol Gu water + ester ky i) Upon addition of acid catalyst, how will kpbe affected? How will k, be affected? ii) How will the ratio kk, compare before and after addition of acid? iit) How will k, change when water is added to the above mixture initially at equilibrium? 8. (12 points) Briefly give an efficient method for separating a 50/50 mixture of: a) cyclohexene and benzene b) aniline and cyclohexene ¢) 2-aminophenol and 4-aminophenol A 9. (6 points) A good protecting group should meet three basic requirements. What are they? 10. (3 points) What is the electrophile in the reaction of acetanilide and mixed acide’): 11. (9 points) Compound A decomposes to compound C via B according to the following scheme: A—» B-——#C All reactions are first order in substrates. A solution of compound A was made (initial concentration is unknown) and the progress of reaction was followed. Four measurements were made 1500 seconds into the reaction, The concentration of A is 0.1396 M; concentration of B is 0.2651 M, the rate of formation of B is 25.9 uM per second, and the rate of formation of C is 92.81 1M per second. Calculate the rate constants, ky and k and the initial concentration of the solution. : 12. (9 points) Compound X decomposes to compound Y and Z following first order Knetics. Ail three compounds H are gases. The rate of reaction was determined by following the pressure of the gaseous mixture as a function of ‘ time. An initial pressure of pure X was measured at 35.15 torr. After 540 seconds, the pressure of the vessel measured 58.85 torr. Calculate for the rate constant, kx. Assume ideal gas behavior. Koxt X(gas) -—» Y(gas) + Z{gas) X {g) 13, (8 points) A student has in his laboratory silica gel TLC plates, cyclopentane, and ethyl acetate solvents only. cyclopentane and ethyl acetate and uses it to elute a sample The student prepares a solution made up of 50/50 (v/v) mixture containing three compounds of different polarities. The result of the TLC is shown in the figure below. What should the student do next in order to better resolve the mixture? [| __ Solvent front & __ Original spot f a new reagent on benzyl alcohol. Overall, the 14. (8 points) A researcher was studying the oxidizing ability 0. reaction is known to oxidize the alcohol to benzoic acid: reagent PhCH,OH ——~———-- PhCOOH In order to understand the oxidation mechanism a little better, the researcher followed the oxidation of benzyl alcohol at different stages of the reaction by TLC. These are the results: hol dissolved in ether. TLC plate A is the analysis of benzyl alco th the oxidizing agent at reaction time equal to zero. TLC plate B is the above ether solution wii TLC plate C is the same solution after one hour. analyses at reaction times of 2, 3, and 4 hours respectively. TLC plates, D, E, and F are the —— — ; Ca] 4 | : e @ co) e © @ ® @ fo} . @ <) @ ® @ 2 = = Ld — A B c D E F ulate as to what reactions are occurring in this oxidation reaction. From the TLC data, spec’