Page 1 EXAM ONE CHEMISTRY 0310 PROF WILCOX October 4 2012 NAME PeopleSoft Number Signature Instructions 1 Write your name above and or initials on the second and last pages of the exam 2 Read all questions carefully There are questions and there are pages to this exam Check to be sure that you have all pages 3 Check the blackboard at the front from time to time to see any late bulletins about the exam Stay calm and stay in your seat Raise your hand if you have a question Ask for help if you feel there is an error in the exam or if a question is unclear 1 12 8 8 2 8 9 10 3 4 10 5 4 4 11 5 5 6 12 8 6 8 13 8 7 8 14 6 TOTAL 100 Page 2 1 Use the table of bond dissociation energies to calculate the H enthalpy of reaction for the following processes Show your work In c one of the needed bond energies is provided in the figure CH3 Br CH2 H Br2 H H Br CH3 CH3 a CH3 CH3 101 46 70 87 Total 146 157 11 kcal mol H H OH OH b 98 5 119 Total 98 5 119 20 5 89 kcal mol CH3 HH H CH3 CH3 CH3 c CH3 HH Break C CH3 88 kcal mol Break Primary C H 101 kcal mol Total 1 5 kcal mol Make C CH3 89 kcal mol Make Secondary C H 98 5 kcal Page 3 2 For each pair of structures shown indicate whether the two species are Enantiomers Diastereomers or Identical molecules You may use the abbreviations E D or I respectively OH OH CH3CH3a E H Br HBr b HBr HBr I CH3 F HHF H3C Cl c Cl E CH3 F HH d F Cl Cl CH3 I 3 Circle each chiral molecule Put a star next to each stereocenter and label it as R or S H3CCHCl2CCl3CH2BrHOHHHHClH3CCHCl2CCl3CH2ClHOHHHH3CClSSR Page 4 4 Show how you would calculate the equilibrium constant for the following process Include the appropriate data numbers required for the calculation and set up the equation HBr H2O Br H3O pKa 4 7 1 7 Keq 103 difference in pKa 3 equilibriumfavors weak acid side 5 Use Newman projections to illustrate three different staggered Cl conformations of 2 chlorobutane as seen looking down the C2 C3 bond Cl Cl Cl CH3H3C CH3 CH3 CH3 CH3 6 Use the table of Change in Free Energy on Flipping from the CyclohexaneConformer with the Indicated Substituent Equatorial to the Conformer with the SubstituentAxial page 2 to calculate the free energy change in each of the following reactions Show your work a H3C 1 7 kcal mol CH3 Cl H3C b Cl 1 7 for CH3 0 52 for Cl Total 1 18 kcal mol CH3 Page 5 7 Redraw the following molecules as Fischer projections Br H C2H5H H H Br H3CF Cl F C2H5 Br HH3C Cl CH3 Br H CH3 8 For each of the following sentences circle the word or item that best completes atrue statement a When comparing two acids an acid with the higher lower pKa is the stronger acid b Bond angles around sp2 hybridized atoms will be close to 90 109 120 degrees c It is true false that water H2O can act as a base d The transition state for the first propagation step in radical bromination is earlier later than the corresponding step in chlorination e A molecule that has a plane of symmetry is chiral not chiral 9 Give a correct equation that describes the relationship between the standard freeenergy change in a reaction DG and the reaction equilibrium constant Keq G RT Keq e G RTlnKeq Page 6 10 Give the mechanism for the free radical chlorination of methane Show initiation propagation and at least one termination step CH4 Cl2 CH3Cl HCl Cl Cl Initiation Cl2 Two propagation steps CH4 Cl CH3 HCl ClCH3 Cl CH3 Cl2 Examples of termination Cl Cl Cl2 ClCH3 CH3 Cl 11 Fill in the blank Show your work Set up the equations no need to use a calculator a If the specific rotation of a mixture of enantiomers is 12 0 and the pure enantiomeric excess of the mixture is 60 and the S R or S isomer is the major component b A solution of substance C with a concentration of 0 2 g mL in a polarimeter with a path length of 10 cm rotates 589 nm polarized light 10 0 degreesclockwise The specific rotation a of C is 50 R isomer has a specific rotation of 20 0 the optical purity also called Page 7 i 12 Monochlorination of isobutane 2 methylbutane affords fourdifferent products ignore stereochemistry The relative reactivity of primary secondary and tertiary C H bonds toward achlorine radical is 1 4 5 1 2 3 Write the four possible products of monochlorination of 2 methylbutane Cl ii A B C D Calculate the relative amount ratio of the four products expected in this chlorination Show your work No calculator is needed Whole number ratios are fine No need to convert to percentages If you cannot do thenecessary math by hand set up the equations with all necessary numbersincluded ClClClA31 B24 C15 D61 D61 D61 Page 8 CHALLENGE 13 The heats of combustion of the following molecules are different The difference is 0 8 kcal mol 1 2 a Which molecule has the larger heat of combustion NUMBER 1 b Briefly explain why the difference is 0 8 kcal mol A correct answer can be based on your knowledge of cyclohexane derivatives and butane conformations The axial methyl when all alone on the ring and not near another group will be 1 7 kcal lessstable than the equatorial In this case in 2 the equatorial methyl is not alone It is gauche to the other methyl group that is always axial This gauche interaction causes strain in the 2 Not as much strain as in 1 but significant Gauche methyl methyl interactions give 0 9 kcal mole of strain Butane number So there is 1 7 kcal mol strain in 1 and 0 9 kcal mol strain in 2 The difference is 0 8 kcal mol