NAME (print): ______________________________________________________________
TA:_____________________ Section Day:_________________ Section Time:__________
Take a few moments to look over the exam. Important points and unknowns are in bold type. Do all preliminary work on the worksheets. The worksheets will not be graded. There is a Periodic Table (page 16) and a BDE Table (page 17) at the end of the exam. The exam is 2 to 2-1/2 hours with 1/2 hour for review. STOP writing when you are asked to do so. Remember: Neatness is to your advantage. Have a GREAT winter break! Best of Luck in Chem. 221!
1. Potpourri I (30 pts) ______ 5. Mechanisms (20 pts) _______
2. Structure Proof (30 pts) ______ 6. Organic Reactions (40 pts) _______
3. Synthesis (30 pts) ______ 7. Kinetics (28 pts) _______
4. Free Radicals (40 pts) ______ 8. Potpourri II (32 pts) _______
TOTAL: (250 pts) ___________
1. Potpourri I: (30 pts) Circle the best answer for each of the following:
a) The strongest acid
b) The hexene with the greatest heat of combustion
c) The closest value (kcal/mol) for DGo between the chair conformations of trans-1,2-dimethylcyclohexane
1.8
0.9
0
2.7
indeterminate
d) The ratio of molecular ions in the mass spectrum of 1,2,3-tribromopropane
1:1:1
9:6:1
3:1
1:3:3:1
1:2:1
e) The bond with the lowest energy IR absorption
f) The compound with three singlets in its proton decoupled 13C NMR spectrum
2. Structure Proof: (30 pts) An unknown compound A reacts with catalytic OsO4 and stoichiometric H2O2 to give a meso diol B. Ozonolysis and zinc reduction of A affords a single compound C, whose 1H NMR spectrum is the following : d 0.99 (3H, t, J = 7 Hz), 2.05 (3H, s) and 2.40 (2H, q, J = 7 Hz). Compound C displays 4 singlets in its proton decoupled 13C NMR spectrum. One of the singlets is of weak intensity and it is located at 207 ppm. Compound C also displays a signal at 1710 cm-1 in its IR spectrum and it has a molecular ion in its mass spectrum of m/z = 72. What are the structures of A-C? Show how the data leads to your answers.
A -----> _________________________
B -----> _________________________
C -----> _________________________
3. Synthesis: (30 pts) Design a synthesis of 3-methyl-3-pentanol from (E)-2-butene as the only source of the carbon skeleton. All necessary reagents are available to you.
[ Note that the product contains six carbons and the starting material has four carbons.]
4. Free Radicals: (40 pts) Write the radical chain reaction for the monochlorination of 2,2-dimethylpropane (neopentane) [ORGO, Alkanes #1]. Provide the initiation step, each of the propagation steps and the overall reaction. Calculate the DHo for each propagation step and the DHo for the overall reaction. Provide an energy vs. reaction co-ordinate diagram for the reaction designating the energies you have calculated. Do justice to the Hammond Postulate. BDEs are on page 17 (last page).
initiation step --->
1st propagation step --->
DHo = ____
2nd propagation step --->
DHo = ____
Overall reaction --->
DHo = ____
5. Mechanisms: (20 pts) Write a mechanism (curved arrows, charges, etc.) for one of the three reactions shown below. Do not do more than one!
6. Organic Reactions: (40 pts) Draw the structure of the major product in each of the following reactions. Pay attention to stereochemistry.
7. Kinetics: (28 pts) In each of the following examples, one of the reactants reacts faster than the other one with the designated reagent. Circle the faster reacting compound, draw the structure of the product and, in as few words as possible, justify your answer.
8. Potpourri II: (32 pts) A chemist requires a sample of (S)-2-chlorobutane (6). She designs and executes the following synthesis from (+)-ethyl lactate (1).
a) (8 pts) She chose not to form diol 2 by catalytic osmylation of propene. Dihydroxylation with KMnO4, although inexpensive relative to OsO4, was also rejected. Why?
b) (8 pts) The reaction of diol 2 with p-toluenesulfonyl chloride/pyridine gave mostly monotosylate 3a and little monotosylate 3b. In her enthusiasm to proceed, she used a mixture of 3a and 3b to form the chiral epoxide 4 by an intramolecular Williamson ether synthesis, the optical purity of 4 was less than when 3a itself was used in the reaction. Why was the optical purity of 4 diminished when the mixture of 3a and 3b was used?
c) The enantiomerically-pure epoxide 4 was added to CH3MgBr in ether. The unpurified products of the reaction were reacted directly with thionyl chloride as described. Careful distillation of the reaction products provided the desired alkyl chloride (S)-6 and a small amount of an optically inactive isomer 7, whose 1H NMR spectrum is shown below. [The proton decoupled 13C NMR spectrum of 7 showed three singlets. Mass spectrum molecular ion of 7: m/z = 92 and 94].
i) (8 pts) What is the structure of 7?
ii) (4 pts) How was isomer 7 formed?
iii) (4 pts) What are the relative intensities of m/z = 92 and 94 ?
