Chemistry Reference
In-Depth Information
4.25
The energy released on hydrogenating a carbon-carbon double bond is 26-30
kcal/mol (eq. 4.5). With four double bonds, we can calculate that 104-120 kcal/mol
should be liberated when 1,3,5,7-cyclooctatetraene is hydrogenated. The observed
value (110 kcal/mol) falls within this range and suggests that cyclooctatetraene has
no appreciable resonance energy. One reason is that cyclooctatetraene is not
planar, and its tub-like shape prevents overlap of the
p
orbitals around the ring.
catalyst
+ 110 kcal mol
-1
+
4H
2
4.26
The nitro group has two main contributing resonance structures:
+
+
-
N
O
N
O
O
O
-
Since they are identical and contribute equally, there is only one type of nitrogen-
oxygen bond, intermediate between double and single in length.
4.27
For naphthalene (C
10
H
8
), there are a number of resonance forms:
The C2-C3 bond is more often a single bond in these different resonance forms, so
one would expect that the C2-C3 bond would be longer than the C1-C2 bond.
4.28
The energy released on hydrogenating a carbon-carbon double bond in
cyclohexene is 28.6 kcal/mol. With five double bonds, we can calculate that 143
kcal/mol should be liberated when naphthalene is hydrogenated. The observed value
(80 kcal/mol) is less than this value and suggests that naphthalene has 63 kcal/mol
of resonance energy. For reference, benzene has 36 kcal/mol of resonance energy
so napthalene's resonance energy is less than twice that of benzene. As we can
see from the resonance forms of naphthalene in problem 4.27, both rings do not
have perfect delocalization and in general, only one ring can be perfectly aromatic at
a time; hence, the reduced resonance energy of naphthalene relative to twice that of
benzene.
4.29
NO
2
+
There are 16 valence electrons available (N = 5, and 2
×
O = 2
×
6 = 12 for a
total of 17, but we must subtract 1 electron since the ion is positive).
N
+
O
O
O
NO
The structure with the positive charge on the nitrogen is preferred because each
atom has an octet of electrons. In the structure with the positive charge on the
oxygen, the oxygen atom has only six electrons around it. Note that in aromatic
nitrations, it is the nitrogen atom of NO
2
+
that is attacked and becomes attached to
the aromatic ring.
