CHE 230-001 Organic Chemistry Exam 2 (9 total pages including periodic table)
1.
-1-
(10 pts.) Rank the following molecules by
their energy of hydrogenation. Label the starting
material of the hydrogenation reaction that has the
A
D
most negative G 1, the next most negative G label
2 etc. An example of a hydrogenation reaction is
CH2=CH2 + H2 --> CH3-CH3
B
Put your answers on the line below.
A_2__
B_1__
2.
C_4__
D_5__
E_6__
F_3__
C
E
F
When compound 2x below is treated with triethylamine (a base) it
looses a proton and becomes conjugate
O
base 2y. When 2y is treated with lithium
OH
diethylamide (a much stronger conjugate
2x
base than triethylamine) 2y looses another
proton and becomes conjugate base 2z.
a. (5 pts) Draw the structure of 2y and label it.
b. (5 pts) Draw the structure of 2z and label it
O
O
H
H
O
HH
H
H
H
O
H
H
H
H
H
2y
H
2z
c. (5 pts) Explain your answer, why is one site easier to deprotonate?
The proton comes off the oxygen atom first because the O atom is
electronegative. Both anions (conjugate bases) are resonancestabilized.
CHE 230-001 Organic Chemistry Exam 2 (9 total pages including periodic table)
-2-
3. (10 pts)
Provide IUPAC names for the following compounds. Use E,Z nomenclature when
necessary.
O
Br
a
c
b
(a. 3 pts.)
(b. 4 pts.)
(c. 3 pts.)
a. _____1-propyloxypentane
b. _____1-ethyl-4-methylcycloheptane
c. _____1-bromobutane
4a. (10 pts.)
Use an
energy diagram and very few
words to explain why
cyclohexane does not attained
the boat conformation when it
ring flips
4b. (05 pts.)
What structure does cyclohexane pass through during ring
flipping?
4a. The boat conformation slips easily into the lower energy twist boat
conformations to avoid prow/prow interactions.
4b. Half-chair and twist boat structures.
CHE 230-001 Organic Chemistry Exam 2 (9 total pages including periodic table)
-3-
5a (5 pts.) A model might come in handy for this problem. Consider
methylcyclopentane. Draw the conformer of lowest energy.
5b (10 pts.) Explain your answer in 5a. Why did you choose the conformation
that you drew? What is (are) the major factor(s) affecting the conformation of
methylcyclopentane? A very short answer could be enough.
This conformer places the methyl substituent (the largest substituent
in the molecule) in a pseudoequatorial position. There is more room in
the eq positions than in the ax positions.
6. (15 pts.) Answer the question based on the following facts. The pKa of
cyclohexylamine is 42. Find the pKa of toluene if a 1:1 mixture of the conjugate
base of cyclohexylamine and toluene equilibrate with an
equilibrium constant K = 10.0
NH2
Work space below. Show your work for partial credit. The
hydrogen atoms shown are lost in the deprotonation reactions to which the pKa values refer.
Keq=Ka(toluene)/Ka(CHA) = 10 = 10-pKa(toluene)/10-42,
pKa(toluene) = 41.
CH3
CHE 230-001 Organic Chemistry Exam 2 (9 total pages including periodic table)
-4-
7. Consider the two transformations below.
7a. (5pts.) Which tranformation has the most favorable G as written?
ANSWER: __7z______ (which reaction has the least positive G)
Rxn 7y
Rxn 7z
7b. (5 pts) Explain your answer above. What is the major factor making these
two transformations different energetically? Give a very short answer!
Unfavorable 1,3-diaxial interactions between the substituent and the
hydrogen atoms render the conformer with the larger substituent in
the axial position less stable.
8. (10 pts) Calculate the ring strain in cyclooctane given the following
information. Remember that there is no ring strain in cyclohexane.
9 O2 +
6 CO2 + 6 H2O
E = 942.0 kcal/mol
12 O2 +
8 CO2 + 8 H2O
E = 1268.8 kcal/mol
(answer to above)
Ring strain = (1268.8/8 – 942.0/6)*8 = 12.8 kcal/mol
(Book values from Table 6.1 on page 200)
Ring strain = (1268.8/8 – 944.4/6)*8 = 9.6 kcal/mol