Page 784 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

b. Figure 8.P14(a, b) gives the computed minimum-energy structure at 767
2
the MP4(SDQ)/6-31G(d) level. Diagram (c) is the − (r)inthe
PROBLEMS
C(1)−C(2)−C(9) plane. Does this structure correspond to any of those you
have suggested in Part (a)? What structural representation would be most
consistent with the calculated minimum-energy structure?

8.15. a. The heats of combustion ( H and heats of hydrogenation ( H for
c
H 2
addition of 1 mol of H and the estimated stabilization energy (SE) for
2
benzene and cyclooctatetraene (in kcal/mol) are given below. The H and
c
H are also given for [16]annulene. Compare the stabilization energy of
H 2
[16]annulene with benzene and cyclooctatetraene on a per CH basis.

Benzene Cyclooctatetraene [16]Annulene
781 1086 2182
H c
−5 16 25.6 28.0
H H 2
SE 36 4 ?

b. The enthalpies of the reaction of the cyclooctatetraene and [16]annulene
dianions with water have been measured.

2Na C H 2− +2H O → C H n+2 +2NaOH
+
n
n
l
2
n
H =−33 33kcal/mol for cyclooctatetraene
H =−10 9kcal/mol for [16]annulene.
Using these data and the enthalpy of the reaction of sodium with water:
2Na +2H O → 2NaOH aq +H H =−88 2kcal/mol
2
2
l
s
calculate H for the reaction:
+
2Na +C H → 2Na + C H 2−
n
n
n
n
s
Why might the reaction of [C H 2− with water be less exothermic than
16
16
2−
for [C H ? How do you interpret the difference in the heat of reaction
8
8
of the two hydrocarbons to form the respective dianions?
8.16. Consider the two structures shown for kekulene, one suggesting inner and outer
1
annulenes and the other a series of phenanthrene-like units. H-NMR and bond
length data are given. The NICS values are calculated as −4 3 for the angularly
fused rings and −10 8 for the linearly fused rings. Indicate properties that
you would expect to be associated with each structure. Do you consider the
properties to be more consistent with the double-annulene or the phenanthrene-
like structures?