Kinetics of aromatic iodination reactions using iodine, diiodine pentoxide and sulfuric acid in acetic acid

Article abstract of DOI:10.1039/a706735f

Kinetic studies of an aromatic iodination reaction using benzene or acetanilide, iodine, diiodine pentoxide and sulfuric acid in glacial acetic acid have been carried out. The reaction, followed using FT-Raman spectroscopy to monitor the disappearance of the aromatic over time, exhibits fractional order dependence in benzene but first order dependence in acetanilide. The difference in order between the two aromatics indicates that benzene participates in an equilibrium reaction before the rate limiting step of the reaction. This reversible step is proposed to be the reversible formation of a π-complex between adsorbed I₂ and benzene as it adsorbs onto I₂O₅. Based on the calculated order, acetanilide most likely does not form a π-complex with adsorbed I₂, rather it reacts from solution to form a σ-complex with activated I₂ on the I₂O₅ surface. In both reactions, it is proposed that the rate limiting step is the formation of the σ-complex.

Full text of DOI:10.1039/a706735f

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Kinetics of aromatic iodination reactions using iodine, diiodine
pentoxide and sulfuric acid in acetic acid
Linda C. Brazdil,* Jill L. Fitch, Carlo J. Cutler, Denise M. Haynik and
Eryn R. Ace
Department of Chemistry, John Carroll University, University Heights, OH 44118, USA
Kinetic studies of an aromatic iodination reaction using benzene or acetanilide, iodine, diiodine
pentoxide and sulfuric acid in glacial acetic acid have been carried out. The reaction, followed using
FT-Raman spectroscopy to monitor the disappearance of the aromatic over time, exhibits fractional
order dependence in benzene but first order dependence in acetanilide. The difference in order between
the two aromatics indicates that benzene participates in an equilibrium reaction before the rate limiting
step of the reaction. This reversible step is proposed to be the reversible formation of a ð-complex
between adsorbed I2 and benzene as it adsorbs onto I2O5. Based on the calculated order, acetanilide
most likely does not form a ð-complex with adsorbed I2, rather it reacts from solution to form a
ó-complex with activated I2 on the I2O5 surface. In both reactions, it is proposed that the rate limiting
step is the formation of the ó-complex.
σ-complex formation, the formation of π-complexes has been
Introduction
observed when aromatics react with I2, Br2 or HCl. In some
instances π-complexes can be isolated. However, the π-complex
is less stable than the σ-complex and it is hard to prove that it is
present in a reaction.
Iodination of aromatic rings is an electrophilic substitution
reaction with wide application in organic synthesis, particularly
in the synthesis of pharmaceuticals.1–6 Like bromine and chlor-
ine, iodine can be polarized to make it more electrophilic.
However, elemental iodine is the least electronegative of the
three and usually requires the presence of an oxidizing agent
before it takes part in aromatic substitution reactions. Soulen
and co-workers reported a method for iodinating both activated
and deactivated aromatic compounds7 that eliminates many of
the problems associated with current methods of iodination,
particularly problems with heavy metal iodide salt waste.1,8–14
This method is shown in Scheme 1.
I2O5 has been used previously to activate I2 for electrophilic
aromatic substitution. I2O, Iϩ and I1ϩϩ2n where n = 1 or 2 were
proposed as intermediates in concentrated H2SO4 solutions.16
The latter intermediate was formed by reaction of Iϩ with n
molecules of I2. These intermediates formed triiodochloro-
benzene from chlorobenzene. Iodine that was not used
re-formed I2. However, scarcely any reaction was observed in
dilute H2SO4. In other cases,17–19 iodination using I2 and I2O5
in methanol occurred on alkyne substituent groups rather than
on the aromatic ring itself. In contrast, the current method of
electrophilic aromatic substitution occurs selectively with only
catalytic amounts of H2SO4.
Because of the importance of aromatic iodination reactions
and the ability of the current method to carry out this reaction
without the heavy metal iodide wastes often associated with this
process, a study was undertaken to understand the mechanism
by which this reaction occurs. The kinetics of two aromatics, 1a
and 1b, were investigated in this study.
R
R
R
R
I
I
I
I2, I2O5, H2SO4
CH3CO2H
+
+
I
I
1a R = H
b R = NHCOCH3
2
3
4
Scheme 1
In spite of its selectivity toward monoiodination of aromatics
other than 1a,7 it appears that both iodine atoms from I2 are
incorporated into the desired products. Based on the yields
reported, this must be the case, assuming that no iodine from
I2O5 is incorporated into the products, an assumption that has
yet to be verified. Previous work in our laboratory has shown
that when 1a is iodinated in this manner, 3a and small amounts
of 4a are produced in addition to 2a.15 The amounts of 3a and
4a versus 2a can be regulated by the amount of I2 added to the
reaction mixture.
Since the mechanism of this iodination reaction is not
known, we undertook an investigation of its kinetics. Generally,
electrophilic aromatic substitution takes place by an attacking
electrophile forming a carbocation, or σ-complex, with the
aromatic in a rate limiting step. The leaving group is removed
and the aromaticity of the ring is restored in a subsequent
fast step. The formation of a π-complex prior to σ-complex
formation has been proposed for several electrophilic aromatic
substitution reactions.8,9 Although less common than direct
Results
The iodination of 1 was followed under various conditions
using Raman spectroscopy. A factorial design bracketing the
conditions previously described15 was used to ensure that the
kinetics observed were consistent with those under typical syn-
thetic reaction conditions. Table 1 gives the concentrations of
reactants used in these experiments. During the course of each
reaction, the Raman band at 1000 cmϪ1 for 1 decreased with
time in comparison to the acetic acid band at 900 cmϪ1. Bands
associated with 2a (1017, 996, 657 and 266 cmϪ1), 2b (1072 and
1594 cmϪ1), 3a (1051 and 684 cmϪ1) and/or 4a (1045 and 317
cmϪ1) increased in intensity.
As shown in Table 2, the initial rates for this reaction increase
with concentration of 1. The orders, calculated by comparing
the increase in observed rate with increasing concentration of
1 while other concentrations are held constant, are 0.55, 0.70
and 0.85 in 1a at 323, 333 and 343 K, respectively; the reaction
is first order in 1b at all temperatures studied. Orders in I2 and
J. Chem. Soc., Perkin Trans. 2, 1998
933

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Table 3 Initial rates and rate coefficients for 1a iodination (1.0 ) with
changes in I2 and H2SO4 concentrations
Table 1 Experimental design parameters for iodination reactions with
60 µl H2SO4 in 2 ml glacial acetic acid
[1a]/
( 0.05)
[1b]/
( 0.05)
Amt I2/
g ( 0.02)
Amt I2O5/
g ( 0.02)
Amt I2/g
Amt I2O5/g
Amt H2SO4/µl
Initial rate/ hϪ1
Condition
0.0510
0.1000
0.0275
0.0500
0.1013
0.0275
0.0275
0.0275
0.0800
0.0800
0.1600
0.1600
0.1600
0.0800
0.0800
0.0800
60.0
60.0
60.0
60.0
60.0
0.0
0.19
0.35
0.11
0.19
0.36
0.01
0.18
0.31
1
2
3
4
5
6
7
8
0.50
1.50
0.50
1.50
0.50
1.50
0.50
1.50
0.55
1.65
0.55
1.65
0.55
1.65
0.55
1.65
0.11
0.11
0.11
0.11
0.33
0.33
0.33
0.33
0.15
0.15
0.44
0.44
0.15
0.15
0.44
0.44
30.0
60.0
ever, their formation occurred at a lower rate than when the
reactions were carried out in acetic acid solvent, as indicated by
the smaller product yields observed after 24 h.
Table 2 Initial rates and 1 rate coefficients for aromatic iodination
reactions
Because the orders in 1a are less than one in all of the reac-
tions, the possibility of aromatic adsorption onto I2O5 was
explored. The straight line dependence observed for the natural
log of 1a adsorbed per gram of I2O5 vs. the natural log of 1a
concentration (lines with correlation coefficients >0.99) indi-
cate that adsorption data obey the Freundlich isotherm, an iso-
therm which describes many systems involving solutes from a
liquid solution adsorbing onto a solid adsorbent. Under the
same conditions, 1b does not adsorb onto I2O5.
Initial rate
k(1a)/
Initial rate
for 1b/ hϪ1 hϪ1
( 0.05) ( 0.05) ( 0.1)
k(1b)/
for 1a/ hϪ1 hϪ1
Conditiona
T/K
( 0.01)
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
323
323
323
323
323
323
323
323
333
333
333
333
333
333
333
333
343
343
343
343
343
343
343
343
0.15
0.25
0.19
0.35
0.19
0.35
0.25
0.44
0.19
0.40
0.23
0.49
0.22
0.46
0.28
0.60
0.20
0.50
0.33
0.75
0.28
0.71
0.35
0.90
0.21
0.21
0.28
0.28
0.28
0.28
0.34
0.33
0.27
0.30
0.37
0.37
0.35
0.35
0.45
0.45
0.37
0.37
0.52
0.53
0.50
0.50
0.63
0.64
1.44
4.41
1.47
4.40
1.40
4.02
1.60
4.80
2.03
7.07
2.20
5.90
2.04
6.12
1.99
6.12
2.60
8.20
2.54
8.02
2.60
7.40
2.97
9.06
2.67
2.67
2.67
2.65
2.64
2.42
2.91
2.91
3.69
3.72
3.63
3.60
3.71
3.71
3.69
3.71
4.91
5.00
4.70
4.15
4.89
4.48
5.50
5.46
Discussion
All of the results can be explained according to generally
accepted mechanisms of electrophilic aromatic substitution.
From the calculated orders, the rate law of the reaction is
rate = kobs[1]x[I2]0.90[H2SO4]0.80 where x = 0.55 at 323 K, 0.70
at 333 K and 0.85 at 343 K. The fractional orders in 1a, I2 and
H2SO4 indicate that these are probably adsorbed on the surface
of I2O5 prior to or during the rate limiting step in the reaction,
as shown in Scheme 2.
I2O5 + H+
HI2O5
(1)
(2)
+
HI2O5+ + I2
H+I2O5 Iδ– Iδ+
a Conditions as given in the experimental design described in Table 1.
Iδ– Iδ+
H+I2O5
Iδ– Iδ+
+
H+I2O5
(3)
I2O5 could not be calculated from these data since I2 is not
completely dissolved under these conditions and I2O5 is insol-
uble in glacial acetic acid. For this reason reactions were carried
out using lower amounts of I2 that were completely soluble in
the reaction mixture. The iodination rates of 1a under these
conditions indicate that the order in I2 is 0.90 0.05, as calcu-
lated from Table 3. It also was noted that a Raman band at
188 cmϪ1 appeared either in place of or along with the Raman
band for iodine at 216 cmϪ1 while the reaction progressed. By
changing the amount of H2SO4 added to the reaction mixture,
the order in H2SO4 is calculated to be 0.80 0.05 from the rate
data in Table 3.
H+I2O5–Iδ––I
H
Iδ– Iδ+
H+I2O5
δ+
(4)
(5)
H+I2O5-Iδ––I
I
HI3O5 + H
δ+
H
+
I
I
B: +
H
(6)
+
+ BH
Linear regressions of ln k1 vs. 1/T for series in which the
concentration of 1 changed while the amounts of I2 and I2O5
were held constant in the temperature range 323–343 K were
performed. The calculated activation energies from these
regressions are thus 27.8 1.6 kJ molϪ1 for 1a and 28.3 1.0 kJ
molϪ1 for 1b.
Initial rates for the disappearance of 1b vs. time for three
reactions under condition 6 (see Table 1) at 333 K, one in glacial
acetic acid, another in [2H]acetic acid, and a third in [2H4]acetic
acid, are all 6.12  hϪ1. Therefore, there is no kinetic isotope
effect for the deuterated solvent in the iodination of 1b.
When iodination reactions were performed using chloroform
in place of acetic acid, no iodinated aromatic products were
observed. When small amounts of acetic acid were included in
the reaction mixture, iodinated products were formed. How-
Scheme 2
Using a Langmuir–Hinshelwood type mechanism,20 the rate
of reaction between adsorbed species is proportional to the
concentration of the adsorbed species, not to the concentration
of species in solution. This concentration of adsorbed species,
θ, is given by θ = K[A]/(1 ϩ K[A]), where [A] is the concen-
tration of species A in solution and K is the equilibrium con-
stant for its adsorption onto a substrate. The presence of [A] in
both the numerator and the denominator leads to observed
fractional orders. These equilibria, (1)–(3) are shown in Scheme
2 for acid, I2 and 1a adsorption on I2O5, respectively. Freundlich
isotherms20 also predict fractional orders for adsorbed species,
but do not provide a physical basis for this observation.
Adsorption data indicate that 1a is adsorbed on I2O5 under
934
J. Chem. Soc., Perkin Trans. 2, 1998

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reaction conditions. As would be expected, less adsorption
occurs at higher temperatures, leading to an increase in the
observed order of the reaction in 1a.
librium (3) in Scheme 2] is formed irreversibly with 1b; this
would necessitate a single arrow replacing the equilibrium
arrows for this step when 1b is involved. Once formed the π-
complex would react rapidly to form the σ-complex. The reac-
tion still would have a first order dependence in 1b. As with 1a
above, the σ-complex may be formed prior to or simultaneously
with the breaking of the I᎐I bond.
Additionally, reaction of acids with I2O5 is known. Specific
16
reactions of H2SO4 with I2O5 to produce I2O3ؒSO3 or [IO]2-
21
SO4 during this reaction are unlikely since H2SO4 can be
replaced with phosphotungstic acid in the reaction mixture.22
Evidence for I2–I2O5 interaction, as depicted in equilibrium
(2) of Scheme 2, is seen in FT-Raman spectra. The shift to
lower energy of the I2 peak during reaction indicates that the
I᎐I bond is still intact, but weakened, as would be expected in
an activated molecule. The order in iodine is further support
that the I᎐I bond is intact. If I2 adsorbed dissociatively, the I᎐I
bond would be broken and the expected order in I2 would be
equal to or less than 0.5 unless this bond breaking is rate limit-
ing. The order in I2 also would be 0.5 or less if I2 was involved in
a pre-equilibrium in which it dissociated. While reaction with
I2O5 resulting in a soluble species containing an intact, activated
I2 moiety is possible, it is judged to be less likely since it must
react with an adsorbed molecule of 1.
When acetic acid was replaced with chloroform in these reac-
tions, no iodinated aromatic products were observed. There-
fore, acetic acid must play a role in the reaction. However, there
is no change in the rate of reaction when acetic acid is replaced
with [2H]acetic acid or [2H4]acetic acid. This indicates that the
proton in acetic acid does not activate I2 in this system. If,
however, I2 is interacting with the oxygen in the acetic acid
molecule, no effect of deuterium would be expected. During
experiments conducted using deuterated solvents, the Raman
band corresponding to 1b at 1005 cmϪ1 was shifted to 1011
cmϪ1. This indicates exchange of H and D between the solvent
and 1b. This type of exchange is not surprising under condi-
tions conducive to electrophilic aromatic substitution.27 How-
ever, there is an excess of deuterated solvent in the reaction
when compared to 1b (0.035 mol deuterated solvent versus
0.0033 mol 1b) so the number of protons replacing deuterium
on acetic acid is not enough to account for the lack of an
observed kinetic isotope effect.
While it is possible that I2 reacts with the oxygen in acetic
acid to form an intermediate, it appears that I2O5 activates I2
in this system, particularly since no reaction is seen without
I2O5. It is unclear from the present study what the exact role
of acetic acid is. It probably stabilizes the transition state of the
π-complex or facilitates desorption of 2 from the I2O5 surface.
Diacetate compounds of 2 are known21 and, while their role
here is unclear, it appears that they may play a vital role in the
formation of iodinated aromatics.
With the formation of the iodinated products, I2O5 is left
with an iodine atom adsorbed, and Hϩ is regenerated. The
I2O5 with Hϩ and iodine atoms attached probably reacts with
another equivalent of 1 or with an aromatic which is already
monoiodinated to produce diiodinated products. This would
account for formation of 3a and 4a. It is consistent with Soulen
and co-workers’ report of incorporation of both iodine atoms
from I2 7 and the lack of iodide waste in the present study. The
acid catalyst that is regenerated then can react with I2O5 and
catalyze further reaction. No detailed information about these
steps is given from the current study.
In iodination reactions previously studied under similar con-
ditions, a first order dependence on I2 was observed. Intermedi-
ates which have been proposed in these systems are HIO, HIO3
and CH3CO2I.23–26 The authors propose that the formation of
the activated iodine species is the rate limiting step in these
reactions. Indeed, they observed a zero-order dependence in 1
for the reactions, indicating the attack by the activated iodine
species on 1 is fast compared with the activation step. Although
conditions in the present study are similar to those reported
previously, iodine activation clearly is not the rate limiting step
since a dependence on 1 is observed. The difference in the oxid-
izing agent, I2O5 vs. peracetic acid, and the heterogeneous
nature of the present system are sufficient to change the mech-
anism of the iodination reaction. A possibility consistent
with all of these observations is that I2O5 polarizes the I2 as it
adsorbs, making it more electrophilic.
As depicted in Scheme 2, the first step in the reaction is prob-
ably the reaction of acid with I2O5. I2 then is adsorbed onto I2O5
associatively, generating an electrophilic iodine species. There
are two explanations, compatible with known mechanisms for
electrophilic aromatic substitution and consistent with the data,
that can describe the mechanism beyond this point. One is that
the activated I2 binds reversibly with 1a through its delocalized
electron cloud to form a π-complex, as shown in equilibrium (3)
of Scheme 2. Through further interactions, a σ-complex is
formed from the π-complex. The rate limiting step could be
σ-complex formation [reaction (4) in Scheme 2] or C᎐H
bond cleavage [reaction (6) in Scheme 2]. If formation of the
σ-complex is rate limiting, it may be formed prior to or simul-
taneously with the breaking of the I᎐I bond. In the latter case,
reactions (4) and (5) of Scheme 2 would be combined. The
other explanation is that reversible adsorption of 1a occurs on a
surface site on I2O5 unoccupied by I2 prior to an irreversible
step to form a σ-complex. This mechanism does not necessitate
the formation of a π-complex. However, the electrophilic nature
of the iodine supports the idea of 1a adsorption on sites con-
taining I2. No conclusive evidence for the site of 1a adsorption
has been obtained in this study, although 1a has been shown to
adsorb onto I2O5 at 333 K both in the presence and absence
of I2.
Experimental
Materials
All chemicals were A.C.S. reagent grade unless otherwise
noted. 1a, I2O5 (98ϩ%), [2H]acetic acid and [2H4]acetic acid
(both 98% D) were obtained from Aldrich. I2O5 was stored in a
desiccator due to its hygroscopic nature. 1b (A.C.S. reagent
grade) was obtained from J. T. Baker. H2SO4 and glacial acetic
acid were obtained from Fisher Scientific, and U.S.P. I2 crystals
were obtained from Malinckrodt. All chemicals were used
without further purification.
Iodination reactions
These possibilities are also consistent with the first order
dependence in 1b. Since the activated aromatic system forms a
more stable complex than does 1a on initial attack of the acti-
vated I2, the reverse reaction for 1b would be negligible. Add-
itionally, there is no evidence that 1b adsorbs on the surface of
I2O5 prior to reaction with activated I2. Rather, the observed
first order dependence in 1b indicates reaction from solution
with adsorbed I2 to form a σ-complex in a rate limiting step.
Thus reactions (3) and (4) in Scheme 2 would be combined if 1b
replaced 1a in this scheme. It is possible that a π-complex [equi-
Generally, I2, I2O5 and H2SO4 were added to glacial acetic acid.
The mixture was equilibrated to reaction temperature. After
approximately 5 min, 1 was also added and the reaction
proceeded.
For kinetic experiments, the disappearance of 1 was followed
directly using FT-Raman spectroscopy. Spectra were collected
using 180Њ collection optics, 500 mW power, 4 cmϪ1 resolution,
a scan range of 100–4000 cmϪ1, and 10 scans per spectrum.
Low wattage was needed in order to avoid localized heating of
the dark colored solution. A heated cell was designed for per-
J. Chem. Soc., Perkin Trans. 2, 1998
935

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forming reactions in the Raman spectrometer. The cell holds a
quartz cuvette with a 1.0 cm path length and a Teflon cap.
Heating is achieved with a heating block positioned behind the
cuvette and regulated electronically. Heat was dispersed by the
use of a water jacket. This system maintains temperature of
0.5 K.
For 1a reactions, spectra were obtained every 1 to 3 min for
the first 10–15 min and every 5 min thereafter to 45 min. For 1b
reactions, spectra were obtained every 1 to 3 min for a total of
20 min. The reaction mixture was stirred after each spectrum
was obtained to ensure good contact between the I2O5 and the
rest of the reactants. The mixture was not exposed to the laser
between scans.
of 1, glacial acetic acid and H2SO4 to 333 K in a water bath.
Raman spectra were collected to determine the initial con-
centrations of 1 present. Concentrations were calculated as
described above. I2O5 then was added to each solution. The
solutions were kept at 333 K and shaken every 5 min. After 1 h,
the liquid was separated from the I2O5 and Raman spectra were
collected. The amount of 1 adsorbed onto I2O5 was calculated
from the difference in concentrations between the solutions
before and after contact with I2O5.
Acknowledgements
This material is based upon work supported by the National
Science Foundation under Grant Number CHE-9520439. The
authors are grateful for the support provided by this organiz-
ation. Two of the authors (J. L. F. and C. J. C.) also are grateful
for BP America Summer Fellowships.
Both aromatics, 1, have bands in the Raman region which are
well separated from the solvent and product peaks and are
strong enough to follow throughout the reaction. The strongest
and most defined band for 1 is at approximately 1000 cmϪ1
(1000 for 1a and 1005 cmϪ1 for 1b). This band corresponds to a
C᎐C ring vibration in both aromatics. The height of this peak
was normalized against an acetic acid peak at 900 cmϪ1 and
compared to calibration curves in order to calculate the concen-
tration of 1 present at any time in the reaction. The Raman
peaks associated with 1 and acetic acid were shifted during
reactions using deuterated solvent. Therefore, the aromatic
peak at 1011 cmϪ1 was normalized against an acetic acid peak
at 866 cmϪ1 for [2H]acetic acid, and at 805 cmϪ1 for [2H4]acetic
acid.
Reactions of 1a were carried out on a larger scale in round
bottom flasks as described previously.15 Mixtures of 1a (4.3 g,
1.1 ) in 50 ml solvent (glacial acetic acid, chloroform or a
mixture of 45 ml acetic acid and 5 ml chloroform), 14.0 g I2, 7.3
g I2O5 and 1.5 ml H2SO4 were allowed to react for 24 h. After
the mixtures were cooled to room temperature, 100 ml distilled
water was added to dissolve I2O5. The resulting mixtures were
filtered. This separated 3a (in acetic acid: 8.35 g, 46%; in acetic
acid–chloroform mix: 1.56 g, 8.6%; in chloroform: none
observed) from the mixture. 4a (in acetic acid: 8.0%; in acetic
acid–chloroform mix, 1.5%; in chloroform: none observed) and
2a (in acetic acid: 45%; in acetic acid–chloroform mix, 8.0%; in
chloroform: none observed) formed an oily layer which was
separated and washed with an aqueous solution of KI and
Na2S2O3 and dried over CaCl2.15,25 The products were weighed
and then dissolved in acetic acid and analyzed by Raman
and 1H NMR spectroscopy to determine the amount of
each present. All NMR spectra matched those of authentic
samples: δH(300 MHz; CD3CO2D) 3a 7.46 (4 H, s, I-C6H4-I); 4a
7.05 [2 H, d, C(4)H], 7.89 [2 H, d, C(3)H]; 2a 7.08 [2 H, t,
C(2)H], 7.31 [1 H, t, C(4)H], 7.73 [2 H, d, C(3)H].
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In order to determine whether nitrogen blanketing was neces-
sary, comparative studies were performed in which some 1b
iodination experiments were conducted under flowing nitrogen
with analogous reactions performed using only an initial blan-
ket of nitrogen or without any nitrogen blanket. Analysis of
crude products indicated approximately 65% yield of 2b in all
cases; only trace amounts of 2-iodoacetanilide were observed.
NMR analysis confirmed product identification: δH(300 MHz;
CD3CO2D) 2b 7.37 [2 H, d, C(2)H], 7.63 [2 H, d, C(3)H]. This
implied that the reactions performed under all three conditions
were comparable. Therefore, further reactions were not blan-
keted in nitrogen.
Paper 7/06735F
Received 8th September 1997
Accepted 17th December 1997
Adsorption studies
Adsorption studies were carried out by equilibrating mixtures
936
J. Chem. Soc., Perkin Trans. 2, 1998