Investigations of reactions of selected azaarenes with radicals in water. 2. Chlorine and bromine radicals

Article abstract of DOI:10.1021/jp980655a

The halogen radicals that react with azaarenes are produced by the photooxidation of halogenide anions with hydroxyl and sulfate radicals and exist as complexes of the radical and the respective halogenide anion in the aqueous phase. The main reaction products of the reactions are identified, and in the case of the bromine radicals, the second order rate constants are determined. Oxidation takes place according to the different redox potentials of the two reactants and is especially observed for chlorine radicals. A typical product spectrum comparable with that in reactions with hydroxyl and sulfate radicals has been found. The formation of some oxidation products in reactions of bromine radicals is in contradistinction to the oxidation potentials of the azaarenes and can be understood only by the reaction of their excited states. The halogenation is the main reaction of the azaarenes. Halogenation products of both, the benzene and the pyridine/diazine rings, have been found. The halogenation of the pyridine/diazine ring again requires the reaction of excited states. The majority of derivatives is halogenated in substitution reactions, but in the reaction of benzo[h]quinoline, addition is also observed. The resonance energy per electron is responsible for the change in the halogenation mechanism from substitution to addition.

Full text of DOI:10.1021/jp980655a

6766
J. Phys. Chem. A 1998, 102, 6766-6771
Investigations of Reactions of Selected Azaarenes with Radicals in Water. 2. Chlorine and
Bromine Radicals
Toralf Beitz, Wolfgang Bechmann,* and Rolf Mitzner
Institute of Physical Chemistry and Theoretical Chemistry, UniVersity of Potsdam, Germany
ReceiVed: January 6, 1998; In Final Form: June 3, 1998
The halogen radicals that react with azaarenes are produced by the photooxidation of halogenide anions with
hydroxyl and sulfate radicals and exist as complexes of the radical and the respective halogenide anion in the
aqueous phase. The main reaction products of the reactions are identified, and in the case of the bromine
radicals, the second order rate constants are determined. Oxidation takes place according to the different
redox potentials of the two reactants and is especially observed for chlorine radicals. A typical product
spectrum comparable with that in reactions with hydroxyl and sulfate radicals has been found. The formation
of some oxidation products in reactions of bromine radicals is in contradistinction to the oxidation potentials
of the azaarenes and can be understood only by the reaction of their excited states. The halogenation is the
main reaction of the azaarenes. Halogenation products of both, the benzene and the pyridine/diazine rings,
have been found. The halogenation of the pyridine/diazine ring again requires the reaction of excited states.
The majority of derivatives is halogenated in substitution reactions, but in the reaction of benzo[h]quinoline,
addition is also observed. The resonance energy per electron is responsible for the change in the halogenation
mechanism from substitution to addition.
Introduction
Halogenated aromatics show properties such as high persis-
tence, ecotoxicity, humantoxicity, and bioaccumulability. 1 The
introduction of one or several chlorine atoms into the molecule
Figure 1. Formation of bromine radicals
increases the toxicological and pharmacological effect by several
orders of magnitude.2 Nature used this principle long before
radicals12,13 are likely to be trapped in reactions producing
mankind did. The chemical and metabolic stability of haloge-
bromine radicals. The chlorine radical can be formed by
nated aromatics and the bioaccumulation of the compounds in
hydroxyl radicals only in a strong acidic solution.11 Therefore,
the fatty tissue with the following chronic effects increases with
the formation of chlorine radicals should be possible by hydroxyl
an increasing number of halogen atoms.3
radicals in the acidic aerosol droplets of the troposphere and
Many halogenated compounds detected in the environment
also through sulfate radicals formed as intermediates14 within
can be assigned to anthropogenic sources, such as products of
the SIV oxidation mechanism in acidic and neutral media. A
chemical industry and products formed unintentionally (e.g., in
problem is the stability of the chlorine radicals in aqueous
combustion events or during the disinfection of water). Today,
solution in dependence on the pH. Jacobi et al.18 confirmed
between 700 and 2000 natural chlorine containing compounds
that chlorine radicals are very unstable in alkaline solutions
are known, primitive as well as highly complex ones, synthe-
(k(•Cl2- + OH-) ) (4.0 ( 0.6) × 106 (M s)-1. The decay of
sized by about 270 species of organisms.3,4 The majority of
chlorine radicals results in the formation of OH radicals. But
the more complex compounds represents active substances in
lower concentrations. However, low molecular compounds,5
such as methylene chloride, biogenically produced in about five
million tons annually, are very important for the geochemical
global chlorine cycle.3
-
the authors also show that the rate constant with water (k(•Cl2
+ H2O) < 610 s-1) is very small. We assume that the decay
of chlorine radicals to OH radicals under our reaction conditions
(pH ) 7) is not essential. This assumption is supported by our
observations for the reaction of chlorine radicals with quinoline
discussed later. The bromine radicals are stable until a pH )
11.
While several papers5-7 deal with the mechanism and the
proof of the formation of halogenated organics by biotic and
abiotic sources, in this study the reaction of chlorine and bromine
radicals with aromatics is reported. Both radicals are produced
by the reaction of strong oxidizing agents such as hydroxyl8-10
and sulfate radicals with the halogenide anions. For bromine
the reaction goes via the intermediate [•BrOH-] complex and
the following combination with bromide to form •Br2-11 (Figure
1).
Halogene radicals mainly react with organic compounds in
electron-transfer reactions (ETR). Other reactions, such as the
H-abstraction by the chlorine radical, are of lesser importance.
The reactivity of both reactants depends on the difference of
their redox potentials. For the radicals, the redox potentials
E(•Cl2-/2Cl-) ) 2.0 V and E(•Br2-/2Br-) ) 1.63 V11 are given.
The chlorine radical is the stronger oxidizing agent while the
bromine radical can only react with compounds with high
electron density. The addition of halogen radicals on double
bonds is also reported.
If available in excess, chloride ions can compete with bromide
to form ‚BrCl-. In the oceans, about 80% of the hydroxyl
* Corresponding author.
S1089-5639(98)00655-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/31/1998

Azaarenes with Radicals in Water. 2
J. Phys. Chem. A, Vol. 102, No. 34, 1998 6767
TABLE 1: Composition of the Reaction Mixture
reaction type
azaarene [mM]
radical forming system
identification of products
photoreaction, chlorine radical
photoreaction, bromine radical
kinetic experiments
0.01-1.0
0.01-1.0
m(K2S2O8) ) 1.0 mM + m(NaCl) ) 1 M
m(K2S2O8) ) 1.0 mM + m(KBr) ) 1 M
photoreaction, bromine radical
0.0028
m(H2O2) ) 280 µM - 2.8 mM
m(KBr) ) 84.0 mM
This paper presents studies of the results reactions of binuclear
and trinuclear azaarenes with halogen radicals. Their reactions
in the aqueous phase are of interest because of the 3-4 orders
of magnitude higher solubility of the azaarenes15 compared with
PAH. The other mechanistic reason16 for the use of the
azaarenes are strong gradients of the electron density in the
aromatic ring system, an electron rich benzene and an electron
poor pyridine/diazine ring. From the products formed, conclu-
sions on the reaction mechanism are drawn. In the product
studies both halogen radicals are produced by sulfate radicals
(photolysis of peroxydisulfate). In the kinetic studies, the
bromine radicals are produced by OH radicals (photolysis of
hydrogen peroxide) and the second-order rate constants for the
reactions of the azaarenes with bromine radicals are determined.
µL hexane or acetonitrile for GC injection. The products are
identified by GC/MSD (GC HP 5890 series II and MSD HP
5971, Hewlett-Packard). The products are determined in SCAN
and SIM mode and compared with native samples (see Table
1).
The experimental determination of rate constants is carried
out directly without any special sample preparation by HPLC
(HP 1050 and UV diode array detector HP 1040M series II,
Hewlett-Packard) measuring the disappearance of the azaarenes.
A short column and isocratic mode are sufficient for the
separation of the reactants. Trimethylamine is added to suppress
a possible tailing of the azaarenes.
Kinetic Method. The bromine radicals are produced by the
redox reaction of bromide with hydroxyl radicals, which are
formed by the photolysis of hydrogen peroxide (λ ) 313 nm).
The hydroxyl radicals are trapped quantitatively by the surplus
of bromide. The bromine radicals formed react with the
azaarenes by ETR. Continuous irradiation leads to a steady-
Experimental Section
Materials. Naphthalene, quinoline, isoquinoline, cinnoline,
quinoxaline, quinazoline, and phthalazine as well as the tri-
nuclear azaarenes phenazine, acridine, benzo[c]cinnoline (BcC),
phenanthridine, and benzo[h]quinoline (BhQ) were purchased
from Aldrich. The identification of most of the reaction
products was proved using native samples, which were com-
mercial products of Aldrich and Merck (quinoline-N-oxide; 2-,
4-, 5-, 6-, 8-hydroxyquinoline; isoquinoline-N-oxide; 1-, 3-,
5-hydroxyisoquinoline; 1-, 2-hydroxynaphthalene; 4-hydroxy-
quinazoline; 2-hydroxyquinoxaline; 1(2H)-phthalazinone; 1H-,
3H-quinazoline-2,4-dione; 2,3-dihydro-1,4-phthalazinedione; 2,3-
dihydroxyquinoxaline; 2,4-, 2,6-dihydroxyquinoline; 8-hydroxy-
quinoline-N-oxide; phthalimide; 2-cumaranone; 1,2-phthaldial-
dehyde; benzene-1,2-diacide; cumarine; dihydrocumarine; 1,2-
dicyanobenzene; 2-cyanobenzaldehyde; pyridine-2-aldehyde;
pyridine-3-aldehyde; pyridine-4-aldehyde; anisole; 1-, 2-bromo-
naphthalene; 1-chloronaphthalene; 3-, 8-bromoquinoline; 4-bromo-
isoquinoline; 2-, 4-, 6-chloroquinoline; 9(10H)-acridinone;
6(5H)-phenanthridinone). The extractants ethyl acetate and
methylene chloride as well as the solvents hexane (GC grade),
acetonitrile and methanol (HPLC grade) were obtained from
Baker. Sodium peroxodisulfate, hydrogen peroxide, sodium
bromide, and trimethylamine were obtained from Aldrich.
Procedure. A high-pressure mercury lamp of 150 W (TQ
150, Heraeus) is used for irradiation as an immersion lamp in
a photoreactor thermostated at 25 °C. For λ ) 313 nm the
respective emission line of the complex line spectrum is isolated
by a filter solution (aqueous sodium dichromate solution). The
reaction vessel has a volume of 400 mL. The reaction mixtures
are air saturated.
-
state concentration of bromine radicals [•Br2 ]ss. Sufficiently
small concentrations of organic substances only have a small
influence on the half-life of the radicals. Then the kinetics can
be described as first-order reaction
-
k ) [•Br2 ]ssk(•Br2- + RH)
with k(•Br2- + RH) as the second-order rate constant (correla-
tion coefficients > 0.99). The first-order rate constants are
determined as a function of the hydrogen peroxide concentration.
The slope of the k/c(H2O2) plot is proportional to the second-
order rate constant. Its direct determination is not possible as
the steady-state concentration of the bromine radicals is
unknown in this method. A calibration with hydroquinone for
-
which the reaction data are known (k(hydroquinone + •Br2
)
) 1 × 108 L(mol s)-1) gives the absolute values of the second-
order rate constants.
Results
Photoreactions of Naphthalene, Quinoline and Isoquino-
line with Halogen Radicals. The addition of peroxodisulfate
to a solution containing bromide and naphthalene leads to the
selective formation of 1-bromonaphthalene. The yield is
increased from 30% to 80% by irradiation. The oxidation
potentials of both reactants have similar values E(•Br2-) ) 1.63
V and E(Naph) ) 1.67 V. Although an oxidation cannot be
expected, traces of five oxidation products are observed.
The large difference between the oxidation potentials of both
reactants, isoquinoline (1.98 V) or quinoline (2.06 V) and the
bromine radical (1.63 V) should not allow the oxidation of the
aromatic system but in the reactions oxidation products are
observed. With isoquinoline a relatively high concentration of
isoindole-1,3-dione (Figure 2) and with quinoline traces of
indole-2,3-dione are identified. In both cases only the pyridine
ring is oxidized.
For extraction experiments several solvents of different
polarity are tested: cyclohexane, toluene, methylene chloride,
and ethyl acetate. Additionally, the SPME (solid-phase micro-
extraction) as a new extraction technique is tested with two
different polar phases (polyacrylate and polydimethylsiloxane).
The highest analytical sensitivity is achieved with the classical
liquid-liquid extraction with methylene chloride and ethyl
acetate as solvents. Therefore, the aqueous samples (pH ) 7)
are extracted with ethyl acetate (+NaCl). An aliquot of the
solvent extract is evaporated to dryness and dissolved in 500
Halogenated derivatives of azaarenes are of higher importance
for the product spectrum than the oxidation products. Five and
six brominated derivatives of isoquinoline and quinoline are

6768 J. Phys. Chem. A, Vol. 102, No. 34, 1998
Beitz et al.
The photochemical reaction of chlorine radicals with qui-
noxaline leads to the formation of five chlorinated products
(three monochlorine and two dichlorine derivatives) in high
yield. They are produced in the ratio 5-Cl-:6-Cl-:2-Cl-:
dichlorine isomers ) 8:2:1:0.25. Calculations of frontier orbitals
also favor a substitution on the atom C5. There is a relatively
high halogenation of the pyrazine ring with its low electron
density. In the reaction with bromine radicals, the formation
of only two brominated derivatives on the C5 and C6 positions
is observed in a ratio of 4:1.
Two (ratio of 40:1) and four chlorinated derivatives are found
in the photoreaction of phthalazine and quinazoline with chlorine
radicals, all substituted at the benzene ring. Brominated
products of phthalazine are not proved. Oxidation and chlorina-
tion reactions of phthalazine have similar yields. In the reaction
of quinazoline with bromine radicals, two brominated isomers
in significant concentrations are formed.
Figure 2. Photoreaction of isoquinoline with bromine radicals (MP,
main product; BP, byproduct; T, trace) (extent of reaction of isoquino-
line is 0.2).
Photoreaction of Selected Trinuclear Azaarenes with
Halogen Radicals. The experimentally determined oxidation
potentials are given for the linear anellated azaarenes phenazine
(1.98 V), acridine (1.66 V), and for the angular anellated
azaarenes BcC (1.86 V), phenanthridine (1.92 V), and BhQ (1.79
V). Therefore redox reactions of phenazine are not observed.
The analysis of the reaction of acridine with chlorine radicals
shows three competitive reactions. Four chlorinated derivatives
were produced at all possible positions of the terminal benzene
ring. The oxidation leads to the formation of acridone as the
main reaction product but also to the oxidative degradation of
the terminal benzene rings, producing the quinoline-2-aldehyde.
For phenazine the selective formation of 1-Cl-phenazine as main
product, 2-Cl-phenazine as a trace and a dichlorinated compound
is observed. The selectivity of the bromination of phenazine
is even higher (1- and 2-bromophenazine with a ratio of about
40:1). In the analysis of the bromination of acridine 12
brominated products were found, 3 monobromine (ratio 1:2:3),
and 9 dibromine derivatives. The concentration of monobro-
minated compounds is significantly higher than that of the
dibrominated ones. The trend of a significantly higher product
yield of bromination compared with the chlorination was also
confirmed for phenazine and acridine.
Figure 3. Photoreaction of quinoline with chlorine radicals (extent of
reaction of quinoline is 0.3).
found, among them the main products; products with one
(isoquinoline) or two (quinoline) substitutions at the pyridine
ring with its low electron density attract attention. Consider-
ations of the frontier orbitals show a preferred formation of the
brominated isomers on positions 5 and 8.
The oxidation potentials of the azaarenes and chlorine radicals
have similar values. The analysis of the reaction mixture shows
a parallel formation of oxidized and chlorinated compounds with
a different extent of chlorination. It was observed that both
the naphthalene reaction paths produced similar yields. The
analysis of the product mixture of isoquinoline shows a more
complex pattern. Oxidation products of the pyridine ring (higher
concentration) are found but also traces of 5-isoquinolinol.
Because the oxidation potential of quinoline is somewhat higher,
the chlorination reaction dominates and only oxidation products
of the pyridine ring can be observed (Figure 3). This supports
our assumption that the decay of chlorine radicals to OH radicals
under the given reaction conditions is not essential. The
presence of OH radicals (formed in the decay of chlorine
radicals) should lead to oxidation products of the benzene ring.
The chlorinated derivatives are the main products. With
isoquinoline six monochlorine and six dichlorine compounds
and with quinoline seven monochlorine and five dichlorine
compounds are observed. The proof of all seven possible mono
chlorine isomeres gives evidence for the high reactivity of
chlorine radicals. On the other hand, the strong differentiation
of the isomer concentrations corresponds to the variation of the
electron density in the molecule.
The BcC should be oxidized as well as chlorinated by chlorine
radicals and will only be brominated by bromine radicals. This
forecast has been experimentally confirmed. The yield of
chlorination is higher than that of the oxidation. Three
chlorinated derivatives in similar concentrations have been found
and also traces of oxidation products. Only traces of four
monobromine isomers were formed in the reaction of BcC with
the bromine radicals.
The chlorine radicals and phenanthridine form seven monochlo-
rine and fourteen dichlorine isomers. Their concentrations vary
by a factor of 6 for the monochlorinated (ratio 1:1.5:1.5:2:4:4:
6) and 8 for the dichlorinated compounds. The oxidation leads
to the gradual degradation of the terminal ring A and the
pyridine ring but the yield is significantly lower than in the
chlorination reaction. Highly oxidized products, such as the
2H-isoindole-1,3-dione and phthalic acid, are found. For
phenanthridine the yield of bromination again exceeds that of
the chlorination. Seven monobromine and eight dibromine
isomers are found. The higher specificity of the electrophilic
attack on the individual C atoms results in a variation of the
monobromine and dibromine isomers by a factor of about 900
(1:1:10:10:40:300:900) and 140 (1:1:5:6:7:16:140), respectively.
Photoreaction of Benzodiazines with Halogen Radicals.
The oxidation potentials of quinazoline (2.7 V) and quinoxaline
(2.3 V) are significantly higher than that of the chlorine radical
(2.0 V). Therefore an oxidation of both compounds is not
observed. The oxidative degradation of phthalazine is possible
(1.95 V). Four oxidation products are found corresponding to
the pattern of the reaction with hydroxyl and sulfate radicals.
For BhQ with its lower oxidation potential, three reaction
ways exist in the reaction with chlorine radicals. Beside the

Azaarenes with Radicals in Water. 2
J. Phys. Chem. A, Vol. 102, No. 34, 1998 6769
incompatible as reference compound. Thus the constants should
not be taken as absolute values but are very suitable for an
illustration of the reactivity gradation of the azaarenes. For three
compounds (quinoline, isoquinoline, and quinoxaline) next to
the azaarene concentration of 2.8 µM two further concentrations
are taken (10 and 30 µM). For a given azaarene in these
experiments identical rate constants are found.
Discussion
The results show a different reaction behavior of halogen
radicals with aromatics. The competition of two reactions will
be observed (i.e., the oxidative degradation and the formation
of halogenated derivatives). The oxidation of aromatics occurs
in dependence on the relation of redox potentials of halogen
radicals and azaarenes. High oxidation potentials shifts the
relation of both parallel reactions in favor of the halogenation.
The reactivity of the different C atoms with halogen radicals
follows the charge gradient resulting from substitution of a CH
group by a nitrogen atom. While 50% oxidation products were
observed for the reaction of chlorine radicals with naphthalene,
the oxidation decreases according to the difference of redox
potentials for quinoline, isoquinoline, and phthalazine, respec-
tively, and does not occur for quinoxaline and quinazoline. In
reactions with bromine radicals the oxidation has a minor
importance and oxidation products can only be observed in
traces for naphthalene and for isoquinoline. The trinuclear
azaarenes follow the same trend. The oxidation of phenazine
is proved only by traces of products. For BcC and phenan-
thridine with redox potentials close to that of the chlorine
radicals, the chlorinated products dominate and the oxidation
products are of small yield. The oxidative degradation is
important for acridine and BhQ with their significantly lower
oxidation potentials. The oxidation of trinuclear azaarenes
follows the π sextett concept of Clar. According to the concept,
the maximum number of sextetts is formed. Therefore the
oxidation of the central ring will be preferred, resulting in the
selective formation of acridone (acridine) and the specific
oxidation of BhQ on the positions 5 and 6. The good correlation
with the oxidation potentials indicates that as a first step in the
mechanism only an electron transfer from the aromatics to the
halogen radical and the following nucleophilic attachment of
water is possible.
In a former study the oxidation of quinoline and isoquinoline
by hydroxyl and sulfate radicals is investigated in dark reactions
and photoreactions. In the dark reactions, the molecules are
oxidized on the benzene ring, while in the photoreactions the
transition to the oxidative attack on the pyridine ring occurs.
The shift of the reaction center to the pyridine ring correlates
with the shift of the electron density. The shift can be described
by HOMO-LUMO calculations and experimentally measured
as higher pKa values in the excited states S1 and T1. Oxidation
products of the benzene ring are also proved. The cause is the
parallel reaction from the ground and the excited states, because
the hydroxyl as well as the sulfate radicals are able to oxidize
the molecule in the ground state as well (EOx > 2.5 V). The
oxidation by the chlorine radical should only occur according
to the redox potentials for a few azaarenes in the ground state,
and the oxidation by the bromine radical should not occur. The
proof of oxidation products in the reaction of isoquinoline with
bromine radicals and quinoline with chlorine radicals contradicts
this forecast and points to a participation of excited states in
the oxidation. The proof of oxidation products of the pyridine
ring instead of the benzene ring is an additional indication. The
observed formation of 1-bromonaphthalene in a dark reaction
Figure 4. Photoreaction of BhQ with chlorine radicals (extent of
reaction of BhQ is 0.3).
TABLE 2: Second Order Rate Constants of the
Photoreaction of Bi- and Trinuclear Azaarenes with
Bromine Radicals
rate constant
absolute (relative)
substance
cinnoline
phthalazine
isoquinoline
quinoxaline
quinazoline
quinoline
benzo[c]cinnoline
phenazine
acridine
[105 L(mol s)-1
]
error [104 L(mol s)-1
]
0.48
5.69
( 0.032 (6.68%)
( 0.34 (7.19%)
( 0.40 (2.33%)
( 0.87 (2.04%)
( 1.11 (2.20%)
( 3.31 (3.01%)
14.39
43.95
48.93
105.34
reaction too slow
0.167
1.01
28.32
70.91
( 0.016 (10.60%)
( 0.084 (8.81%)
( 1.41 (5.11%)
( 6.75 (6.75%)
benzo[h]quinoline
phenanthridine
expected oxidation and substitution reactions addition reactions,
until now not observed, have led to the formation of chlorine
adducts and chlorine-hydroxy adducts. Due to the electrophilic
substitution five chlorinated isomers of BhQ are found. Their
concentrations vary by a factor of 5 (ratio: 1:1:1.5:2:5). The
addition products, two monochlorine adducts and five chlorine-
hydroxy-adducts, are observed in a yield similar to the substitu-
tion products. The oxidation is the main reaction and leads to
the formation of degradation products of the central benzene
ring (Figure 4).
In the reaction of BhQ with bromine radicals, oxidation and
bromination products are found in similar content; although on
the basis of the redox potential of BhQ, an oxidation was not
expected. Compared with the addition, the substitution is the
main reaction. Three monobromine (ratio: 1:2:7.5) and two
dibromine products are found but only one bromine adduct and
one bromine-hydroxy adduct.
Kinetics. The rate constants show a significantly lower
reactivity and higher selectivity of the bromine radicals in
comparison to the hydroxyl and sulfate radicals and can be used
to study the gradation in the reactivity of azaarenes. The rate
constants of the reactions of bromine radicals with the azaarenes
vary in a factor of 200. Because of its low value it is not
determined for BcC. Thus the bromine radicals have an even
higher selectivity against the azaarenes than the carbonate
radicals.17 The absolute errors were determined close to 2-3%
and increases for substances with very low rate constants (Table
2).
In the literature, the number of substances studied in reactions
with the bromine radical is very small. From the reported
substances, only hydroquinone could be used as reference
compound (k ) 1 × 108 L(mol s)-1).10 This compound reacts
very fast under the used measuring conditions and is nearly

6770 J. Phys. Chem. A, Vol. 102, No. 34, 1998
Beitz et al.
and the missing bromination products in the dark reaction with
azaarenes point to a thermal formation of bromine radicals. The
kinetic experiments confirm a further reaction with strongly
different reaction rates. The reactions with azaarenes start only
after the activation of azaarenes by light. This is another piece
of evidence for the participation of excited states in the
bromination of azaarenes.
reactive oxygen species and the following formation of oxidation
products by ETR. The concentration of chlorinated substitution
and addition products are explained by similar reaction rates of
consecutive reactions. The addition reactions seem to prefer
the central C5-C6 bond and the terminal benzene ring. In
phenanthrene this bond shows pure double bond character
decreasing in direction to BhQ. Compared with benzene or
naphthalene, the terminal ring has a lower REPE (resonance
energy per electron). The absence of the addition reactions for
BcC, phenanthridine, the linear anellated trinuclear, and the
binuclear azaarenes supports this assumption.
It seems that a threshold value of REPE exists for a change
in the halogenation mechanism. The Dewar resonance energy
becomes too low for the restoration of aromaticity after
formation of the σ complex. Addition and addition-oxidation
products are formed.
The selectivity of bromine radicals is significantly high with
a variation of rate constants by a factor of 200 compared with
a factor of 4 for the hydroxyl radicals and a factor of 40 for the
carbonate radicals. This is due to the lower oxidation potential
(1.63 V) of the bromine radicals. The observed oxidation
products could only be explained by the interaction of azaarenes
with the radiation and the following reaction out of the excited
state. The analysis documents the formation of brominated
substitution and oxidation products. The bromination on the
pyridine and the diazine ring will also be facilitated by the
reaction out of the excited states. The observed rate constants
contain the sum of all parallel reactions, that means the
halogenation out of the ground and excited states and the
oxidation out of the excited states.
The rate constants of the bromine radicals are not suitable
for a discussion of structure-reactivity relations because of the
parallel reactions and because of the changing ratio of bromi-
nation to oxidation in dependence on the photophysical data of
the azaarenes. In the experiment an excellent agreement with
the gradation of reactivities of carbonate radicals (which reacts
only by ETR) was found. The gradation of rate constants for
the carbonate radicals follows the ionization energies and
considers corrections according to the photophysical data
(absorbance and quantuum yield of ISC). The similar gradation
of rate constants of carbonate and bromine radicals (i.e., the
parallel reactions of bromination and ETR points to a common
rate determining reaction). This can be the formation of the σ
complex. The kinetic experiments support the thesis of the
formation of this intermediate, which can be formed from the
ground and the excited states resulting in the same gradation
of rate constants.
The represented results document the significance of the
halogenation in the reactions. Many halogenated isomeric
products are observed. The reaction with bromine radicals leads
to several monobromine isomers and with chlorine radicals to
a higher number of monochlorine and dichlorine isomers. The
absence of the dibrominated derivatives for binuclear azaarenes
is due to the lower reactivity of bromine radicals. The
differences in the concentrations of brominated isomers are
larger than those of the chlorinated isomers, which points to a
higher selectivity of bromine radicals. The stronger electrophilic
chlorine radicals have a higher reactivity. This leads to a
chlorination of individual C atoms of the pyridine and diazine
rings and the formation of dichlorinated aromatics. Despite their
high reactivity, strong differences in concentrations of chlori-
nated isomers can be observed. Altogether, a higher yield of
brominated than chlorinated products is found. This can be
explained by the 1 order of magnitude higher rate constant for
the formation of bromine radicals at pH ) 7 (k(•Br2z- + •SO4
)
-
) 3.5 × 109 (M s)-1; k(•Cl2- + •SO4-) ) 3.1 × 108 (M s)-1).10
An exception are azaarenes with low electron densities that only
form brominated products in a very low yield.
The halogenation of the pyridine/diazine ring, which is
nonreactive against SEAr reactions, is observed for quinoline,
isoquinoline and quinoxaline, respectively. Analogous to the
explanation of the oxidation, this halogenation is explained by
the participation of excited states.
An interpretation of the halogenation mechanism only on the
basis of the described results is difficult. We prefer an
explanation that includes the mechanism of the electrophilic
substitution. While the electrophilic substitution occurs in
nonpolar solvents by a positive polarization of the molecular
halogen by a metal catalyst, the halogen radicals are negatively
charged complexes. An electrophilic attack will only be
probable if the halogen complex decays starting with the
interaction of the halogen species with the aromatics [Ar ... ‚Br
Br-]. We believe that the halogenide leaves the complex at
the beginning of the interaction with the aromatics. An
intermediate σ complex is formed by the halogen radical and
the aromatics. The high energy of the aromatic resonance
stabilization enables the restoration of the π sextett under release
of hydrogen. Another possibility, the radicalic H abstraction
and the following addition of halogen radicals, is energetically
not possible for aromatic compounds in the ground state (C6H5-
H, HD ) 456 kJ/mol; H-Br, HD ) 363 kJ/mol). In the excited
state S1, the C-H bonding energy seems even to increase as
the experimentally observed decrease of the C-H and the
increase of the C-C bond lengths show.
References and Notes
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The halogenation of all binuclear and the most trinuclear
azaarenes leads to halogenated substitution products. Chlori-
nated addition products and chlorine-hydroxyl products were
found in the reaction of BhQ with chlorine radicals. Substitution
and addition products are formed in similar concentrations.
These products prove the formation of an intermediate σ
complex. That is the starting point for the parallel formation
of aromatic substitution products under abstraction of hydrogen
(rebuild of aromaticity), the chlorinated addition products, and
the chlorine-hydroxyl-products by addition of water or
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