Third Midterm Exam CHEM 255 Organic Chemistry I Prof. Bastin November 19, 2009

Transcription

1 Third Midterm Exam CEM 255 Organic Chemistry I Prof. Bastin November 19, 2009 Name Section Provide CLEAR, CONCISE answers using unambiguous, carefully drawn structures and arrows for all of the appropriate questions. Be sure to indicate stereochemistry where appropriate. Be sure to read each question VERY CAREULLY. Be sure to show your logic for partial credit. Good luck and enjoy! 1) /15 pts 2) /10 pts 3) /8 pts 4) /10 pts Total: /100 pts 5) /10 pts 6) /8 pts 7) /9 pts 8) /8 pts 9) /12 pts 10) /10 pts

2 Bastin all ) (15 Pts) What is the relationship (enantiomers, diastereomers, identical/homomers) of the following sets of compounds? Also indicate whether each molecule is chiral or achiral. Assume the molecules are frozen in the given conformations. C 3 C 3 3 C C 3 0 o 30o (d) Ph 3 C C C C C 3 Ph Ph 3 C C C C 3 C Ph (e) O O O O

3 Bastin all ) (10 pts) Many biochemical processes occur by reaction pathways analogous to those carried out in the laboratory. Some toxic compounds are very good S N 2 substrates because they transfer alkyl groups to enzymes, thus deactivating them. Enzyme N 2 + R X Enzyme NR + X Mustard gas in an infamous toxic thioalkylating agent, C 2 C 2 SC 2 C 2, that was responsible for 500,000 casualties in World War I. The overall reaction for mustard gas with an enzyme is as follows, and requires two S N 2 reactions and proceeds through an intermediate cation. C 2 C 2 SC 2 C 2 S N 2 intermediate Enzyme N 2 S N 2 C 2 C 2 SC 2 C 2 N + 2 Enzyme Use curved arrows to clearly show both S N 2 reactions and identify the intermediate. 3) (8 pts) Indicate whether each of the following statements is true or false. Write T or in the blanks below. Nucleophiles are always negatively charged. Enantiomers have the same pk a. Enantionmers have the same rate of reaction with methanol. (d) The (S)-enantiomer of a molecule always rotates plane-polarized light in a counterclockwise direction. (d)

4 Bastin all ) (10 pts) Provide all of the symmetry operations (rotation axes, mirror planes, and inversion centers) present in each of the following molecules. Assume the molecules are frozen in the conformations shown. P 3 C C 3 3 C C 3 5) (10 pts) I have two pairs of molecules, A and A as well as B and B. One is a pair of constitutional isomers and the other is a pair of diastereomers, but I do not know which is which. Using a melting point apparaturs, a polarimeter, and an acute sense of smell can you think of a way of determining which pair are the constitutional isomers, and which pair are the diastereomers? ief answer preferred.

5 Bastin all ) (8 pts) Provide the absolute configuration (R/S) of each stereogenic center in each of the following molecules. C 3 O O (d) O C 3 C 2 C 2 O C 3 7) (9 pts) Provide either the name or structure for the following molecules. C 3 O (S)-1-Methyl-2-cyclohexenol

6 Bastin all ) (8 pts) Alcohols undergo an oxidation reaction to yield carbonyl compounds on treatment with CrO 3. or example, 2-tert-butylcyclohexanol gives 2-tert-butylcyclohexanone. If axial O groups are generally more reactive than their equatorial isomers, which do you think would react faster, the cis or trans isomer of 2-tert-butylcyclohexanol? Explain. O O CrO 3 C(C 3 ) 3 C(C 3 ) 3 2-tert-butylcyclohexanol 2-tert-butylcyclohexanone

7 Bastin all ) (12 pts) Predict the product(s) for each of the following S n 2 reactions. Also indicate the OMO and LUMO of each reaction. inally, provide the curved arrow mechanism for each reaction. N 3 + C 3 O + CN + (d) I O + C 3

8 Bastin all ) (10 pts) Draw both chair conformations for the following molecules. Which groups are axial and which groups are equatorial (label them on your diagrams)? Explain which conformation is more stable for each molecule. By how much energy? Some axial strain energies that might be useful are: 0.45 kcal/mol for an O/, 3.7 kcal/mol for a Me/Me, 0.9 kcal/mol for a Me/, 2.6 kcal/mol for a Me/, 0.25 kcal/mol for a /, and 1.1 kcal/mol for a ipr/ PER 1,3-diaxial interaction. C 3 C(C 3 ) 2 C 3 C(C 3 ) 2