Due: Monday, April 24, 2006

 

1. Compound A, three spectra of which are shown on the right, is converted to compound B with LiAlH4. Compound B reacts with tosyl chloride/pyridine to form C, which upon exposure to EtONa/EtOH, affords compound D (major) and compound E (minor). Hydrogenation of D and E both produce compound F. Compound E liberates 30.0 kcal/mol of heat while compound D is exothermic by 26.8 kcal/mol. What are the structures A-F? Explain how you interpreted the spectra and determined the structures. What is the difference in the heat of formation of D and E? Click on spectra for larger versions.

2. Three isomers A, B and C (C6H10) have IR and 13C spectra displayed below. All three undergo hydrogenation to produce the same alkane D (C6H14). Heat liberated upon hydrogenation (kcal/mol): A: -69.1; B: -65.1; C: -60.3. What are the structures A-D? Explain your reasoning. Help is here. Click on spectra for larger versions.

3. In the E2 elimination of 2-bromo-2-methylbutane with NaOMe/HOMe, the ration of Zaitsev/Hofmann product is 70/30. when KO-t-Bu/HO-t-Bu is employed, the ratio is 27/73. a) Calculate the selectivity for each type of hydrogen in each experiment. b) Why is the the ratio different even though the energy of the two products does not change. c) How much heat is liberated in the hydrogenation of each isomer? Construct a standard state diagram to illustrate the thermochemistry.

4a) Using the Heat of Formation tables, determine the heat of hydrogenation of (Z)-2-butene. b) Repeat part a) for (Z)-cyclohexene. c) Knowing that cyclohexane is strain free (how do you know this from its ΔHfo?), what can be said about strain in (Z)-cyclohexene. d) If cycloheptane and (Z)-cycloheptene were strain free, what would be there respective heats of formation? What would be the heat of hydrogenation? e) The ΔHfo for cycloheptane is -28.2 kcal/mol and the ΔHfo for (Z)-cycloheptene is -1.8 kcal/mol. What is the heat of hydrogenation of (Z)-cycloheptene? f) Draw a standard state diagram illustrating the three hydrogenations b), d) and e). Your diagram should confirm that the 7-membered ring compounds are less stable than the strain free 7-membered ring compounds. Which 7-membered ring, (Z)-cycloheptene or cycloheptane, is more responsible for the smaller heat of hydrogenation relative to the 6-membered compounds. Illustrate and explain. g) (E)-Cycloheptene has never been isolated (put in a bottle) but it has been detected as an intermediate in reactions. Its ΔHfo has been calculated as +18.7 kcal/mol. What is its heat of isomerization to (Z)-cycloheptene? Show work. h) What is its calculated heat of hydrogenation?