Starting materialII III IV VChemistry 14CL Practice Final Exam Key 1. The stoichiometric equation for the aldol condensation of benzaldehyde with acetone is given below. OHOO+2NaOH/H2OC2H5OHMW: 102g/mol 58g/mol 234g/mol (a) If a reaction of 1.00 mL of benzaldehyde (density = 1.0415 g/mL) is reacted with 0.360 mL of acetone (density =0.7899 g/mL), what is the theoretical yield (in grams) of product? 1.0415 g102 g/mol= 1.02 X 10−2mol benzaldehyde, which will give 5.1 X 10-3 mol product 0.360 mL X 0.7899 g/mL58 g/mol= 4.9 X 10−3 mol acetone Since two moles of benzaldehyde are required for each mole of product, acetone is the limiting reagent and the theoretical yield is (4.9 X 10-3 mol) (234 g/moL) = 1.15 g (b) If the actual experimental yield for the reaction was 89%, how many mL of acetone would be required to produce 4.00 grams of product? If the yield were 89% then in the previous example, 0.360 mL of acetone would give (1.15 g) (0.89) = 1.02 g of product. 1.02 g product0.360 mL=4.00 g productX mL X = 1.41 mL2. The chromium oxidation of compound I below yields a mixture of products (II – V). Identify the order of elution that would occur if you used an alumina stationary phase with diethyl ether to separate the mixture of products (II, III, IV, V) by column chromatography. (Assume there is no starting material in the products.) Starting material OHC6H5-CH-C(CH3)3 I Products: CH3CH3CH3-C-OHOHHC6H5COOC6H5CC6H5-C-C(CH3)3O II III IV V First eluted ______ II _______ Reason: The ketone is the least polar of the oxygen functional groups; in addition there are two nonpolar hydrocarbon group—an alkyl and an aryl group. Second eluted ____IV_______ Reason: The aldehyde is slightly more polar than the ketone and this compound has only one hydrocarbon group—a polarizable aryl group. Third eluted _____III________ Reason: The alcohol is more polar than the aldehyde or ketone; there is only one alkyl group. Fourth eluted ________V___________ Reason: The two oxygens of the acid group make this the most polar compound of the set. In addition the one hydrocarbon group is a polarizable aryl group. 23. (10 points) The molecular structure and the UV spectrum of caffeine are given below. (a) If the solution used for the spectrum had a concentration of 1mg/40mL of water, what is the molar extinction coefficient of caffeine at 275nm? (Assume a 1cm diameter UV cell). NCNCCCNCNCH3OOH3CHCH3Molecular formula: C8H10N4O2 (determined from the structure) Molecular weight = 194 g/mol Concentration of solution = 1 mg40 mL•1mol194 g/mol= 1.2 X 10−4 M A = εbc 0.84 = ε(1 cm)(1.29 X 10-4M) ε = 6770 (b) Briefly indicate what feature(s) of caffeine give(s) rise to the UV spectrum. This absorption is due to a π – π* transition of the chromophore of the conjugated diene with auxochromes in the adjacent nitrogen atoms. 34. (10 points) The 13C spectrum of caffeine is given below. The signals for each carbon have been identified and numbered. Draw the DEPT 90 and DEPT 135 caffeine spectra on the charts below the 13C spectrum. 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 2005010027.90(7)29.45(8)32.84(6)104.08(1)142.33(5)155.86(4)156.31(3)159.24(2)170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 200102030405060708090100110120170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 200102030405060708090100110120DEPT 90DEPT 13512N3N4NN5OOCH36CH37CH38One CH and 3 CH3’s Only one CH 45. (a) (9 points) The infra red spectrum of caffeine is given below. Identify the functional group or groups that give rise to the absorption(s) at the specified wavelengths and the type of vibrations involved. (i) 2960 cm-1 __________C-H stretch________________ (ii) 1740cm-1 or 1692 cm-1 ___________C=O stretch_______________ (iii) 1240 cm-1 ____C-C or C-N stretches_______________ 55. (b) (9 points) The mass spectrum of caffeine is given below. Identify the species that give rise to the m/z signals at (i) 194 ___________molecular ion___________________ (ii) 109 Fragmentation occurs next to a carbonyl groups. In this case the compound breaks on either side of the carbonyl carbons leaving the five-membered ring and one nitrogen with a methyl group (C8 in the NMR problem) attached (iii) 15 _______________methyl__________________________ 6. A buffer solution is 0.100 M in NaOAc and 0.200 M in HOAc. To 1.00L of this buffer, 25.0 mL of 0.500 M NaOH is added. Calculate the change in pH that occurs. Ka of acetic acid = 1.8 X 10-5 Original buffer: 1.8 X 10−5=[H+](0.1)(0.2) [H+] = 3.6 X 10-5 pH = 4.44 Moles of base added = (0.500M)(0.025L) = 0.0125 mol After addition of base: [H+] [OAc-] [HOAc] moles x 0.1 + 0.0125 mol 0.200-0.0125 mol =0.1125 =0.1875 molarities X 0.1125mol/1.025L 0.1875mol/1.025L [H+] = (1.8 X 10−5)(0.1875)(0.1125)= 3.0X10−5 pH = 4.53 pH change = 0.09 67. Picric acid and trinitrotoluene (TNT) form a simple two-component solid-liquid equilibria. The eutectic mixture, which is 64 mole % TNT, melts at 60oC. The melting point of picric acid is 122oC; the melting point of TNT is 80oC. (a) The solid-liquid phase diagram for this two-component system is shown on the next page. Mark on the graph the phases that exist in each region of the diagram. top (above 120oC - liquid left portion (60 oC - 120oC) solid picric acid and solution right portion (60 oC - 80oC) solid TNT and solution bottom (below 60 oC) solid picric acid and solid TNT (b) Describe the observations you would make if you cooled a mixture that was 50% in TNT from 120oC to 50oC. The mixture is a solution that cools until it reaches 101oC. At that point solid ( pure picric acid) begins to form and the solution becomes enriched in TNT. As the temperature continues to drop more solid forms. At 60oC, solid continues to form but the temperature does not change. This continues until all the solution has solidified. After that the temperature of the solid drops. 788. (a) Hexane (bp = 68oC) and octane (bp = 125oC ) form an ideal solution. Draw the liquid-vapor phase diagram for this system. Label the axes and the phases in each area. Indicate the temperature at which a 60% octane solution will begin to boil. What is the composition of the vapor in equilibrium with the solution? What is the composition of the distillate if there are two theoretical plates
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