Third Midterm Exam
CHEM 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 CAREFULLY. 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
5) _______/10 pts
6) _______/8 pts
7) _______/9 pts
8) _______/8 pts
9) _______/12 pts
10) _______/10 pts
Total: ________/100 pts
Bastin
Chem 255-Exam 3
Fall 2009
2
1) (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.
(a)
Br
CH3
H
H
H3C
CH3
Br
Br
Br
CH3
H
H
(b)
Cl
H
Cl
Cl
H
Cl
H
H
(c)
30o
0o
(d)
Ph
H 3C
C C C
CH3
Ph
CH3
Ph
C C C
H 3C
Ph
(e)
H
OH
H
H
OH
H
OH
OH
Bastin
Chem 255-Exam 3
Fall 2009
3
2) (10 pts) Many biochemical processes occur by reaction pathways analogous to those
carried out in the laboratory. Some toxic compounds are very good SN2 substrates
because they transfer alkyl groups to enzymes, thus deactivating them.
Enzyme–NH2
+
Enzyme–NHR
R–X
+
HX
Mustard gas in an infamous toxic thioalkylating agent, ClCH2CH2SCH2CH2Cl, 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 SN2 reactions and proceeds through an
intermediate cation.
ClCH2CH2SCH2CH2Cl
SN2
intermediate
Enzyme–NH2
SN2
ClCH2CH2SCH2CH2N+H2–Enzyme
Use curved arrows to clearly show both SN2 reactions and identify the intermediate.
3) (8 pts) Indicate whether each of the following statements is true or false. Write T or F in
the blanks below.
(a) Nucleophiles are always negatively charged.
(b) Enantiomers have the same pKa.
(c) 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.
(a)_____(b)_____(c)_____(d)_____
Bastin
Chem 255-Exam 3
Fall 2009
4) (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.
(a)
F
F
P
F
F
F
(b)
H
H3C
CH3
H3C
CH3
H
(c)
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? Brief answer preferred.
4
Bastin
Chem 255-Exam 3
Fall 2009
5
6) (8 pts) Provide the absolute configuration (R/S) of each stereogenic center in each of the
following molecules.
(a)
(b)
H
CH3
OH
Br
O
H
(c)
(d)
H
HO
CH3
CH3
H
CH2CH2Cl
HO
7) (9 pts) Provide either the name or structure for the following molecules.
(a)
H
Br
(b)
CH3
OH
(c)
(S)-1-Methyl-2-cyclohexenol
Bastin
Chem 255-Exam 3
Fall 2009
6
8) (8 pts) Alcohols undergo an oxidation reaction to yield carbonyl compounds on treatment
with CrO3. For example, 2-tert-butylcyclohexanol gives 2-tert-butylcyclohexanone. If
axial –OH 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
OH
CrO3
C(CH3)3
C(CH3)3
2-tert-butylcyclohexanol
2-tert-butylcyclohexanone
Bastin
Chem 255-Exam 3
Fall 2009
9) (12 pts) Predict the product(s) for each of the following Sn2 reactions. Also indicate the
HOMO and LUMO of each reaction. Finally, provide the curved arrow mechanism for
each reaction.
(a)
NH3
+
CH3Br
(b)
OH
+
Br
(c)
Br
CN
+
(d)
I
OH
+
CH3
7
Bastin
Chem 255-Exam 3
Fall 2009
10) (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 OH/H, 3.7 kcal/mol for a
Me/Me, 0.9 kcal/mol for a Me/H, 2.6 kcal/mol for a Me/Cl, 0.25 kcal/mol for a Cl/H, and
1.1 kcal/mol for a iPr/H PER 1,3-diaxial interaction.
(a)
CH3
CH(CH3)2
Cl
(b)
CH3
CH(CH3)2
Cl
8