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CHE538 Exam 2, Principles of Organic Chemistry Fall 2005
Warning: write/print neatly! If I can’t read it you don’t get credit.
H
C
1. Think about the cyclopentadieyl anion in this problem.
a. (10 pts.) What are the energies in terms of resonance stabilization (beta) of
the five Huckel MOs of this molecule? Go with Frost.
b.
(10 pts.) The pKa of cyclopentadiene is 15 whereas the pKa of 1,4pentadiene is 30. Please explain this observation. How is pKa related to
energy?
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c.
(15 pts.) Use reflection plane symmetry to simplify the 5x5 secular
determinant of C5H5(-) by writing out a symmetric and a dissymmetric
secular determinant. Redraw the structure and number the atoms so I can
follow your work. You don’t need to solve the determinants.
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d. (10 pts.) Guess at what the HOMO(s) and the lowest-energy pi MO of C5H5(-)
look like. I would like you to draw 3 MOs in the space below. Let’s look at
them from above; just draw circles on a pentagon instead of 2-lobed p orbitals.
2. Consider cyclohexanone below. In solution phase the molecule undergoes chair to
chair conformational change. The energy barrier between the two chair conformers is
O
O
O
about 10.5 Kcal/ mol.
a. (10 pts.) What is the point group of the chair cyclohexanone? What is the
point group of C5H5(-)?
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b. (10 pts.) The absorbance of the carbonyl is at 1716 cm-1. How many times
does the carbonyl vibrate before the molecule changes conformation?
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c. (10 pts.) Even though the structure is dynamically flat, is it
reasonable to think that the transition state of the conformational
change is flat. Why or why not? Hint: the twisted boat structure at
right is a stationary state on the path of conformational change.
O
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3. (10 pts.) Explain why carbocations and carbenes rearrange and anions do not. Your
explanation needs to include an orbital energy diagram.
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4.
(15 pts.) What partial charges are predicted for the butadienyl radical cation at C1
and C2 by Huckel theory? Work the problem below. Step one is to find the
coefficients. The symmetry method or the Frost mnemonic should help. If you don’t
have time let the coefficient at C1 be n and the one at C2 be m. Work the problem
algebraically.
=
H
H
C
H
C C
H
H
H
C
H
H
C
H
C C
H
H
H
C
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Constants:
•
c: 299,792,458 m/s; speed of light in vacuum
•
h: 6.626 075 5 x 10^-34 J*s; Planck's constant
•
NA: 6.022 136 7 x 10^23 mol^-1; Avogadro's number[1]
•
R: 8.314 510 J/(mol*K); molar gas constant[1]
•
k: 1.380 658 x 10^-23 J/K; Boltzmann's constant R/NA
A useful conversion factor:
•
one calorie (cal) is 4.184 J
A few useful equations:
•
E= hv
•
C= lambda x v
•
K = kT/h e^(-DG/RT).
More work space. Refer me to this area if you have to.