Table 1: Data for benzene, naphthalene, anthracene, and phenanthrene
Name
Resonance energy
(kcal/mol)
Resonance energy
per ring
Dipole (debye)
Benzene
36
36
0.00
Naphthalene
61
30.5
0.00
Anthracene
84
27.7
0.00
Phenanthrene
92
30.3
0.04
Table 2:
Discussion
Electrophilic substitution in polycyclic aromatics occurs at the site of the lowest
local ionization potential. That is, the lower the potential on the aromatic ring, the more
likely it is to give up an electron, and thus, the more susceptible the molecule will be to
electrophilic attack.
While benzene, naphthalene, anthracene, and phenanthrene molecules exhibit
similarities—in that these are all polycyclic aromatic compounds that have a number of
benzene rings fused together, and thus, have aromatic characters—some molecules have
more localization of electrons (i.e., a denser electron area) than others. This can be
confirmed by the dipole value and resonance energy of each molecule as shown in Table
1. According to the polarity and resonance energy, phenanthrene is the only molecule that
has polarity character with a dipole value of 0.004 debye. This suggests that
phenanthrene must have both electron rich areas and electron poor (deficient) areas. In an
electron deficient area, the electron will be given up more easily than in an electron rich
area; therefore, phenanthrene is the most susceptible to an electrophilic attack among the
four molecules. Among the remaining three molecules, anthracene will be the next most
susceptible molecule, followed by naphthalene and benzene.
Comparing resonance energy, the anthracene molecule has the lowest stabilization
energy at 27.7kcal/mol followed by naphthalene with 30.5kcal/mol and benzene with
36kcal/mol. This suggests that anthracene is the least stable molecule among the three.
The varying resonance energies among these molecules are due to different bond lengths;
the electrons for C–C bonding are distributed equally between each of the six carbon
atoms for benzene, which has equal bond lengths. Other molecules, however, do not
share electrons equally, which makes the bond unsymmetrical, and therefore, there will
be an area where electrons are more localized and deficient.
Figure 2 shows that electrons are concentrated to small areas both on quinoline
and isoquinoline. This makes other areas of these compounds relatively electron
deficient. Among the two molecules, the nitrogen atom in isoquinoline is relatively more
distant from the adjacent benzene ring than the nitrogen atom in quinoline. The most
electron deficient site of the isoquinoline is one carbon farther from the nitrogen atom,
which makes isoquinoline more electron deficient than quinoline. Therefore, the
electrophilic substitution on isoquinoline will occur faster than on quinoline. Thus, the
order of the rate of relative electrophilic substitution will be isoquinoline, followed by
quinoline, and finally napthalene (where there is relatively no electron localization). This
result is shown in Table 2. Isoquinoline has the highest dipole moment, suggesting that it
has the most polarity or electron localization; therefore, isoquinoline is the most
susceptible to electrophilic attack with 2.57 debye, followed by quinoline with 2.02
debye, and naphthalene with 0.00 debye.
The inductive effect applies when there are substituents in which nitrogen, oxygen,
and halogen atoms form sigma-bonds to the aromatic ring, and thus, exert an inductive
electron withdrawal, which deactivates the ring. As seen in Figure 3, local ionization
potential maps of adenine, guanine, thymine, cytosine, uracil, and guanine have the most
electron deficient sites (indicated by the blue area). These sites are withdrawn by
electronegative atoms of oxygen and nitrogen, followed by cytosine, uracil, thymine, and
adenine. This is once again confirmed by the dipole moment shown in Table 2. Guanine
has the highest dipole moment of 6.54 debye, illustrating that it has the strongest polarity
and will deactivate the molecule by withdrawing electron density. Cytosine follows with
the value of 6.31 debye, uracil with 4.26 debye, thymine with 4.13 debye, and adenine
with 2.42 debye. This can be attributed to different resonance stabilization energy. As
shown in the ionization potential maps in Figure 1, there is no distinctive difference
among benzene, naphthalene, anthracene, and phenanthrene since the electrons are well
spread out along the conjugated pi system.
END OF SAMPLE
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