Great enhancement in the oxidation ability of dilute nitric acid in nanoscale water-droplets of reverse micelle systems

Article abstract of DOI:10.1246/bcsj.79.1215

In reverse micelle systems, a large enhancement of the oxidation ability of dilute nitric acid was discovered, and its oxidation mechanism was explored. The Br^- ion in the surfactant, CTAB, was oxidized to Br₂ (or Br₃^-) in the CHCl₃/CTAB/H₂O reverse micelle system with W 1/4 1:0{4:0 by diluted nitric acid (0.25-2.5 mol dm^-3 in 1.0 vol% H₂O portion) at 15-40°C where CTAB stands for cetyltrimethylammonium bromide, and the W value is the ratio of [H₂O]/ [surfactant]. At higher concentrations of nitric acid and temperatures, faster reactions occurred. Otherwise, long reaction times were needed, e.g., 10 h for 1.0 mol dm^-3 HNO₃ at 25°C. Light or ambient oxygen did not appear to affect the reaction. The ratio of produced Br₂ or (Br₃^-) to the initial amount of HNO₃ indicated the following reaction scheme: 2HNO₃ + 2Br^- → Br₂ + NO₂^- + NO ₃^- + H₂O. The nitroyl ion (or nitronium ion), NO2⁺, was proposed as the intermediate active species. The addition of HClO₄ as a proton source caused the complete reduction of N(V) as follows: NO₃^- + 6H⁺ + 5e^- → 1/2N₂ + 3H₂O. The hydrogen-bonding structure of H2O in the CTAB or CTAC (cetyltrimethylammonium chloride) micelle system was found to be distorted, compared to that in bulk water, by the ¹H NMR chemical shift of H₂O. The change in ¹H NMR chemical shift also demonstrated the consumption of protons during the oxidation of Br^-, but not of Cl^-, by dilute HNO₃.

Full text of DOI:10.1246/bcsj.79.1215

Ó 2006 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 79, No. 8, 1215–1222 (2006) 1215
Great Enhancement in the Oxidation Ability of Dilute Nitric Acid
in Nanoscale Water-Droplets of Reverse Micelle Systems
ꢀ
Yumi Furuya, Kiyomi Miyamoto, Akio Narutaki, and Ryosuke Kato
Masashi Hojo, Tadaharu Ueda, Chihiro Daike, Fumiko Takezaki,
Department of Chemistry, Faculty of Science, Kochi University, 2-5-1 Akebono-cho, Kochi 780-8520
Received December 12, 2005; E-mail: mhojo@cc.kochi-u.ac.jp
In reverse micelle systems, a large enhancement of the oxidation ability of dilute nitric acid was discovered, and its
ꢁ
ꢁ
oxidation mechanism was explored. The Br ion in the surfactant, CTAB, was oxidized to Br2 (or Br3 ) in the CHCl3/
CTAB/H2O reverse micelle system with W ¼ 1:0{4:0 by diluted nitric acid (0.25–2.5 mol dm in 1.0 vol % H2O por-
ꢁ3
ꢂ
tion) at 15–40 C where CTAB stands for cetyltrimethylammonium bromide, and the W value is the ratio of [H2O]/
[
times were needed, e.g., 10 h for 1.0 mol dm HNO3 at 25 C. Light or ambient oxygen did not appear to affect the
surfactant]. At higher concentrations of nitric acid and temperatures, faster reactions occurred. Otherwise, long reaction
ꢂ
ꢁ3
ꢁ
reaction. The ratio of produced Br2 or (Br3 ) to the initial amount of HNO3 indicated the following reaction scheme:
ꢁ
ꢁ
ꢁ
þ
2
HNO3 þ 2Br ! Br2 þ NO2 þ NO3 þ H2O. The nitroyl ion (or nitronium ion), NO2 , was proposed as the inter-
mediate active species. The addition of HClO4 as a proton source caused the complete reduction of N(V) as follows:
ꢁ
þ
ꢁ
NO3 þ 6H þ 5e ! 1/2N2 þ 3H2O. The hydrogen-bonding structure of H2O in the CTAB or CTAC (cetyltrimeth-
1
ylammonium chloride) micelle system was found to be distorted, compared to that in bulk water, by the H NMR chemi-
1
cal shift of H2O. The change in H NMR chemical shift also demonstrated the consumption of protons during the
ꢁ
ꢁ
oxidation of Br , but not of Cl , by dilute HNO3.
The nitrate ion has high chemical stability, especially at
low concentrations. Standard redox potentials indicate that it
should serve as an excellent oxidizing agent, but in order to
react with suitable reactants to form elemental nitrogen or
ammonia, special conditions are required, such as the presence
sively smaller as the amount of water in the micelle system
increases.10
Many studies have focused on the properties of water or sol-
vation in reverse micelle systems.11–14 Some researchers have
also explored the possibility of using reverse micelle water
droplets as ‘‘nanoreactors,’’ because their sizes can be control-
1
of catalysts and high temperature and pressure. Concentrated
led,1 or as hosts for proteins and small molecules. Iwamoto
et al.17 have studied the change in the polarization of water
in AOT [sodium bis(2-ethylhexyl) sulfosuccinate] reverse
micelle spectrophotometrically.
5,16
10
nitric acid has strong oxidation ability, although dilute nitric
acid in aqueous solution at room temperature exhibits no appa-
rent oxidation trend. Cotton and Wilkinson have described that
ꢁ3
nitric acid with a concentration of less than 2 mol dm has
2
almost no oxidation ability. Mixtures of concentrated nitric
acid and sulfuric acid (‘‘mixed acid’’) are used for nitration
of organic compounds, and the nitroyl ion (or nitronium ion)
We have proposed that the properties of bulk water should
disappear in the conditions that a huge network of water
cannot be constructed, since water may gain the properties of
bulk water upon the formation of the huge network molecules.
þ
3
NO2 is regarded as the active intermediate. Raman spectra
þ
4
18
of NO2 have been observed in solution as well as in the solid
In the following cases, water may lose the normal prop-
5
14
6
state. Using N NMR, Ross et al. have examined the nitric
acid/nitronium ion equilibrium in aqueous sulfuric acid.
Dimeric nitric acid, (HNO3)2 or (DNO3)2, was detected by
FTIR in HNO3 or DNO3:H2O:Ar matrices.7
erties, more or less, of the bulk water: (1) the residual water
(ꢃmmol dmꢁ ) in organic solvents; (2) aqueous solutions, or
organic solvent–water mixtures, containing highly concentrat-
ed salts; (3) nanoscale water droplets in nano-tube or reverse
3
7
We have been studying the structure of water in nanoscale
water-droplets of reverse micelles, stabilized by surfactants in
micelle systems (the number of water, nw < ꢃ10 ); (4) waters
on metal electrodes, ion-exchange resins, proteins, the inter-
face of organic solvents, and glass vessels; (5) the water in
supercritical conditions. Such waters may be ‘‘reduced’’ to iso-
lated H2O molecules (as the ‘‘dihydrogen ether,’’ [R](H)–O–
(H)[R]1 with poor solvating ability). Concentrated salt effects
on the solvolysis rate on typical SN1 substrates, t-butyl and
adamantyl halides (RX), were elucidated in relation to the in-
8
organic solvents. The size of the ‘‘water-pool’’ or ‘‘water-drop-
let’’ may vary from 1 to ꢃ100 nm, as a linear function of the W
9
value, where W ¼ [H2O]=[surfactant]. A decrease in the W
9c
value, as well as the addition of metal salts at a constant W val-
ue caused drastic configuration changes between the octahedral
2
þ
2ꢁ
[
(
Co(H2O)6] (the pale red color) and the tetrahedral [CoX4]
ꢁ
X ¼ Cl, Br: the blue color) in CHCl3/[CTAC or CTAB]/H2O
teraction between the X ion of (or from) the substrate (in the
activated state) and added metal ions in the ‘‘dihydrogen
8
reverse micelle system. It is well known that the properties
of water localized in the interior of reverse micelles differ
from those of bulk water, and the difference becomes progres-
ether’’ solution.19 Based on the Raman spectra,
demonstrated the distortion of the bulk water structure and
19c,d
we have
Published on the web August 7, 2006; doi:10.1246/bcsj.79.1215

1
216 Bull. Chem. Soc. Jpn. Vol. 79, No. 8 (2006)
Oxidation Ability of Dilute Nitric Acid
the alteration of properties of water into those of a ‘‘non-aque-
ous’’ solvent with the addition of the organic solvents and con-
centrated salts (LiClO4 and Et4NBr), as well as with increasing
temperature.
In the present paper, we would like to report that dilute
nitric acid in a small water droplet of reverse micelle systems
has an extraordinarily strong oxidation ability, which is strong
temperature. All of the absorbance values were converted and are
reported as 1.0 cm path length.
1
H NMR measurements of the water in reverse micelle systems
ꢂ
were carried out at 25 ꢅ 0:1 C with JOEL FT-NMR spectrometer
(
Model JNM-LA400). D2O in an inner tube was used as the lock-
ing solvent, and the spectra were referenced to the D2O (HOD)
signal (ꢀ ¼ 4:65 ppm). An alternate locking solvent (CDCl3, con-
taining TMS) in the inner tube gave minor differences in chemical
shift values. The usage of CDCl3 (containing 0.05% TMS) as the
reverse micelle system in a single NMR tube gave a tendency sim-
ilar to the results of double-tube measurements in the region of
concern.
ꢁ
ꢁ
1
enough to oxidize Br ions to Br2 (or Br3 ). H NMR was uti-
lized to observe not only changes in hydrogen-bonding struc-
ture of H2O with various W values but also proton consump-
tion during the oxidation reaction in reverse micelles. While
studying the configuration changes between octahedral [Co-
ꢀ
The Detection of Br2 or the Br3 Ion. In acidic solutions,
Rosaniline reacts with free bromine (Br2) to form red-colored
2
þ
2ꢁ
(H2O)6]
and tetrahedral [CoBr4] forms in the CHCl3/
CTAB/H2O reverse micelle system, we observed that a rather
strong yellow-color developed only when (dilute) nitric acid
was present in Br ion rich conditions; the possibility of for-
mation of yellow-colored cobalt(II) and cobalt(III) complexes
8
compounds described as either pentabromorosaniline or tetrabro-
morosaniline.22 In the present study, the Rosaniline method was
ꢁ
slightly adjusted to be used with the chloroform reverse micelle
system: An appropriate amount of acetonitrile was added to the
reverse micelle system in order to make a (miscible) single-phase
solution for CHCl3 and a H2SO4 aqueous solution. The appear-
ance of the red color around 570 nm confirmed the formation of
was denied by our careful examination of many cobalt com-
0,21
plexes. Kimura et al.2
have reported the reactions of nitrous
and phosphinic acids with the iodide ion in the presence of
molecular oxygen in acidic solutions.
ꢁ
Br3 in the reverse micelle system.
The Formation Constant and Absorptivity of Br3 in
ꢀ
Experimental
ꢁ
CHCl3. The interaction between Br2 and Br was determined
4
Reagents. Commercially obtained surfactants were used as
received: cetyltrimethylammonium bromide (CTAB, >95% by
titration) and chloride (CTAC, 95% by titration) from Wako;
sodium bis(2-ethylhexyl) sulfosuccinate (AOT, >99:0%) from
to have a large formation constant, K = ca. 3 ꢆ 10 (Br2 þ
ꢁ
ꢁ
ꢁ
ꢁ
Br ꢀ Br3 ) in CHCl3 at room temperature. A Br3 salt, n-
þ
Bu4N Br3 , as well as the free bromine (Br2) in the presence
of various concentrations of the bromide ion (n-Bu4NBr) were
used to evaluate the formation constant. The absorptivity ("/
þ
Sigma. The amounts of acidic impurities (e.g., R3NH ) in CTAB
and CTAC were evaluated to be ꢃ0:03 and ꢃ0:05 mol %, respec-
ꢁ
1
3
ꢁ1
ꢁ
mol dm cm ) at ꢁmax (273 nm) of Br3 was estimated to be
8
4
tively. Chloroform was obtained from Wako and Aldrich as GR
5:1 ꢆ 10 . Free Br2 gave an absorption peak at 407 nm (log " =
23
grade and ACS reagent grade, containing 0.3–1.0% ethanol and
amylenes as the stabilizer, respectively. The chloroform contain-
ing ethanol as the stabilizer was used in the experiments, unless
otherwise stated. Acetonitrile and dichloromethane (CH2Cl2) of
GR grade from Wako were used as received. Nitric acid (60 and
70%, GR grade) and perchloric acid (70%, GR grade) from Wako
were used as purchased.
GR grade bromine (Br2) from Wako, the tribromide salt (n-
Bu4NBr3) from Aldrich, NaNO3 and NaNO2 from Wako, sodium
ca. 2.3) in CHCl3, cf., ꢁmax ¼ 417 nm (log " ¼ 2:32) in CCl4.
ꢁ
We assumed that all of the Br2 was converted into Br3 in the
ꢁ
CTAB reverse micelle at lower W values (e.g., [Br ] ¼ 0:28
ꢁ3
ꢁ
mol dm at W ¼ 2:0). The amount of Br2 (or Br3 ) produced
ꢁ
by the oxidation of Br from CTAB was estimated by the absorb-
ꢁ
ance of Br3 at 400 nm (" ꢃ 600). In the dichloromethane/CTAB
reverse micelle and in acetonitrile solution, absorptivity values at
400 nm were evaluated to be similar to that in the chloroform/
2
4
CTAB reverse micelle. Gabes and Gerding reported the IR
ꢁ
ꢁ
ꢁ
hyponitrite hydrate (Na2N2O2 xH2O), nitronium (NO2BF4,
and Raman spectra of Br3 , I3 , IBr2 , etc. in the solid state.
ꢄ
>
95%) and nitrosyl tetrafluoroborate (NOBF4) from Aldrich were
Results and Discussion
used as purchased. Rosaniline (Fuchsin) [4-((4-aminophenyl)(4-
imino-2,5-cyclohexadine-1-ylidene)methyl)-2-methylbenzamine
monohydrochloride], o-tolidine, and DPD (N,N-diethyl-p-phenyl-
enediamine) were purchased from Wako. The following amylenes
and related compounds were purchased: 2-metyl-1-butene (>98%),
ꢀ
The Formation of Br3 in CHCl /CTAB Reverse
3
Micelle System. Effects of the Addition of Salts, the Tem-
perature, and the W Value on Reaction Rates: A colorless
solution of the reverse micelle at W ¼ 2:0 ([H2O]/[CTAB])
containing nitric acid with a relatively low concentration,
2
(
-methyl-2-butene (>95%), 2-pentene (>95%), and 1-hexene
>95%) from TCI; 3,3-dimethyl-1-butene (>95%) and 2,3-di-
ꢁ3
e.g. 1.0 mol dm HNO3 in a 0.1 mL H2O portion (or 0.010
methyl-2-butene (>95%) from Aldrich. Other commercially ob-
tained salts or reagents of GR and ACS reagent grades were used.
Procedure and Equipment. Reverse micelle solutions were
prepared by adding 0.10 mL of diluted nitric acid of various con-
centrations to an organic solvent (e.g., CHCl3, total 10 mL finally)
containing a surfactant (e.g., CTAB), which was sonicated in an
ultrasonic bath for two minutes to make uniform the reverse mi-
ꢁ3
mol dm HNO3 in a whole 10 mL H2O/CHCl3) turned yel-
ꢂ
low after ca. 8 h at 25 C (cf., Fig. 1). Increasing the concentra-
tion of HNO3, 2.0 and 3.0 mol dm , caused an increase in
absorbance and an acceleration in the reaction rate. The yel-
ꢁ3
ꢁ
low product was identified to be Br2 (or Br3 ) by using the
Rosaniline method22 as mentioned in the Experimental section.
The effects of added salts to the reverse micelle system were
celle. The solutions were kept at constant temperatures (e.g., 25 ꢅ
ꢁ
2
ꢁ3
ꢂ
examined; the addition of 1:0 ꢆ 10 mol dm LiClO (or 1.0
4
0
:02 C) in a Yamato constant-temperature water bath (Model
ꢁ3
mol dm in the 1.0 vol % amount of H2O) to the reverse
BK-15), equipped with a cooling dip (Model BE 200F).
UV–visible absorption spectra were measured using a Hitachi
double-beam spectrophotometer (Model U-2000) and a Shimadzu
ꢁ3
micelle containing 0.9 mol dm HNO3 caused the solution
color to change immediately to yellow. The higher the LiClO4
concentration, the quicker the yellow color developed. Light
(
Model UV-2550) in 0.1, 0.2, and 1.0 cm quartz cuvettes at room

M. Hojo et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 8 (2006) 1217
Fig. 1. The change in absorbance at 400 nm with time for
various concentrations of HNO3 in the reverse micelle
ꢂ
(
(
CHCl3/CTAB) of W ¼ 2:0 at 25 C: ( ) 1.0; ( ) 2.0;
ꢁ
3
) 3.0 mol dm HNO3 (in the 1.0 vol % H2O phase).
Fig. 3. The effects of temperature on the reaction rates for
)
various HNO3 concentrations at W ¼ 2:0: ( ) 1.0; (
ꢁ3
1
.6; ( ) 2.0; ( ) 2.5 mol dm HNO3 (in the H2O phase).
time of the CTAB reverse micelle system (W ¼ 2:0) in the
ꢁ
3
presence of 1.0 mol dm HNO3 at various temperatures. At
ꢂ
ꢁ
2
5 C, the formation reaction of Br2 (or Br3 ) did not complete
ꢂ
even after 15 h while, at 40 C, the reaction already completed
with a yield greater than 90% (the yield of Br2 with respect to
the HNO3 amount) after 2 h. The reaction rate was evaluated at
the ‘‘half-reaction’’ time, i.e., the time at which the yield reach-
es 50% completion for each reaction: ca. 11, 7, 3, and 1 h for
25, 30, 35, and 40 C for 1.0 mol dm HNO3. The ‘‘half-reac-
tion’’ times for various HNO3 concentrations are shown in
Fig. 3; the reaction proceeded very quickly at 40 C for every
ꢂ ꢁ3
ꢂ
ꢁ3
sample of 1.0–2.5 mol dm HNO3 concentration. The yields
of Br2 with respect to the amount of HNO3 were ca. 90% at
W ¼ 2:0.
However, the reaction was slower at lower W values for
ꢁ3
every HNO3 concentration (1.0–2.5 mol dm ) at a constant
temperature. For example, the ‘‘half-reaction’’ time of 4.2, 2.7,
and 1.3 h was observed for W ¼ 1:0, 2.0, and 4.0, respectively,
Fig. 2. The change of absorbance at 400 nm with time
ꢁ
3
for 1.0 mol dm HNO3 in the reverse micelle (CHCl3/
CTAB) of W ¼ 2:0 at various temperatures: ( ) 25; (
)
ꢁ3
ꢂ
ꢂ
with 2.0 mol dm HNO3 at 25 C (Fig. 4). Similar decelera-
tions in the reaction rates with decreasing W value were also
observed in dichloromethane/CTAB reverse micelle system.
It has been noted that the presence of small amounts of water
in organic media causes a promotion of the reaction with nitro-
3
0; ( ) 35; ( ) 40 C.
over the reaction vessel and ambient oxygen had no apparent
ꢁ
effects on the Br oxidation; whereas, it has been reported that
ꢁ
ꢁ
20,21
air, or oxygen gas, affected the oxidation of I into I3 .
Dilute nitric acid with concentration in the 0.25–2.5 mol dm
ꢁ
3
3
nium salts. At smaller W values, in the present reverse micelle
(
in 1.0 vol % H2O) range produced almost the half-amount Br2
systems, the water-droplets may lose the properties of bulk
water to a greater extent, which might cause the decelerated
reaction rates.
molecules to each HNO3 molecules. Considering the amount
ꢁ
of the product, the formation of Br2 (or Br3 ) seems to follow
the reaction in Eq. 1.
ꢀ þ þ
The Oxidation of Br by NO2 and NO Ions in
ꢁ
þ
Acetonitrile: The oxidation ability of NO2 towards Br
was examined in acetonitrile solution, becuase the solubility
2
HNO3 þ 2Br ! Br2 þ NO2 þ NO3ꢁ þ H2O:
ꢁ
ꢁ
ð1Þ
Other metallic salts, such as Mg(ClO4)2 and LiBr, also caused
the prompt oxidation of Br with HNO3 in the reverse micelle
of NO2BF4 in CHCl3 was too low, cf., Ref. 3. Bontempelli
et al. reported electrochemical reduction of NO2 and NO
in acetonitrile. Figure 5 shows that the observed concentrations
ꢁ
26
þ
þ
at W ¼ 1:0 and 2.0. However, no distinct increase in the oxida-
tion rate was observed for non-metal salts, Me4NBr, Et4NBr,
n-Bu4NBr, and n-Bu4NClO4. Cabos and Delord’s neutron
scattering data for the (water + NaCl/AOT/heptane) reverse
micelle appear to show that water-droplet size is increased
by the presence of the electrolyte.25
ꢁ
ꢁ3
of Br2 (or Br3 ) from 0.1 mol dm Et4NBr are proportional to
the concentrations of NO2BF4 added; the Br2 concentrations
ꢁ3
ꢁ3
were evaluated to be 1.6, 4.3, and 9:1 ꢆ 10 mol dm for
ꢁ3
ꢁ3
2.0, 5.0, and 10 ꢆ 10 mol dm NO2BF4, using a absorptivity
ꢁ
(") of 600 at 400 nm for Br3 in acetonitrile. The stoichiometry
þ
Figure 2 shows the changes in absorbance at 400 nm with
between the amounts of Br2 and NO2 verified that a two-elec-

1
218 Bull. Chem. Soc. Jpn. Vol. 79, No. 8 (2006)
Oxidation Ability of Dilute Nitric Acid
ꢁ
Fig. 6. Observed amounts of Br2 (or Br3 ) from various
Fig. 4. The change in absorbance at 400 nm with time at
various W values of the reverse micelle (CHCl3/CTAB)
concentration of HNO3 in the presence of excess amounts
of HClO4 in the reverse micelle (CHCl3/CTAB) of W ¼
ꢁ
3
ꢂ
containing 2.0 mol dm HNO3 at 25 C: ( ) 1.0; (
)
ꢁ
3
2
:0: ( ) 0.25; ( ) 0.50; ( ) 1.0 mol dm HNO3. The
1
.4; ( ) 2.0; ( ) 4.0 of the W values. The run of W ¼
dotted lines show the theoretical values (see the text).
2:0 in Fig. 4 is different from that in Fig. 1.
ꢀ
The Oxidation of Br with HNOx (x ¼ 1, 2, and 3) in the
Presence of Excess Amounts of Protons. The Addition of
HClO4 to HNO3: As mentioned above, the amount of Br2
ꢁ
produced from the oxidation of Br with diluted nitric acid
was found to be proportional to the concentration of HNO3
in the water-droplets (of 1.0 vol % H2O) of the reverse micelle
system. The addition of HClO4, as the proton source, to the
HNO3 system caused a remarkable increase not only in the
reaction rate but also in the amount of Br2. For example
ꢁ3
with HNO3 concentrations of 0.50 mol dm , the presence of
þ
ꢁ
the 2 and 4 equivalents of H to a unit of NO3 ([HClO4]=
HNO3] ¼ 1 and 3 in Fig. 6) gave the 1 and 2 equivalents of
Br2, respectively. Similar results were observed for 0.25 and
[
ꢁ3
1.0 mol dm HNO3 systems. The following oxidation mecha-
nisms are proposed:
NO3 þ 2H ! NO2 þ H2O ! NO2ꢁ ꢁ 2e ;
ꢁ
þ
þ
ꢁ
ð4Þ
ð5Þ
ꢁ
þ
þ
ꢁ
ꢁ
NO2 þ 2H ! NO þ H2O ! NO ꢁ 2e :
ꢁ
Fig. 5. Observed amounts of Br2 (or Br3 ) from 0.1
ꢁ
þ
3
þ
mol dm Et4NBr with increasing concentration of NO2
The oxidation steps expressed by Eqs. 4 and 5 are both two-
electron processes. The next step is thought to be Eq. 6. How-
ever, the amount of Br2 was more or less than the amount
theoretically predicted.
and NO in acetonitrile: ( ) NO2BF4; ( ) NOBF4. The
dotted line shows the theoretical value (see the text).
tron oxidation process was occurring, Eq. 2.
ꢁ
þ
þ
ꢁ
NO þ 2H ! N þ H2O ! 1/2N2 ꢁ e :
Note that the ratio of the number of electrons to the added pro-
ð6Þ
þ
ꢁ
ꢁ
NO2 þ 2Br ! Br2 þ NO2 :
ð2Þ
ꢁ
þ
Similarly, the yield of Br2 from oxidation of Br by NOBF4
was 73–85%, and the oxidation process with NO can be ex-
pressed by Eq. 3.
ꢁ
þ
ton concentration (1e /[2H ]) in Eq. 6 is the half, compared
ꢁ
þ
with that (2e /[2H ]) in Eq. 4 or 5, and the difference resulted
in the differet slope in Fig. 6. The overall reaction should be
Eq. 7.
þ
ꢁ
ꢁ
NO þ 2Br ! Br2 þ NO :
In subsequent experiments, dilute nitric acid in acetonitrile
ð3Þ
ꢁ
þ
ꢁ
NO3 þ 6H þ 5e ! 1/2N2 þ 3H2O:
ð7Þ
The standard redox potential of Eq. 7 is reported to be very
ꢁ
ꢂ
ꢁ3
was found to oxidize Br at 25 C: nitric acid–MeCN solu-
tions of 0:84{6:7 ꢆ 10 mol dm in the presence of 0.28
ꢁ2
o
27,28
high, i.e., E ¼ 1:246 V.
Considering the excess amount
ꢁ
ꢁ3
ꢁ
mol dm Et4NBr gave Br2 (or Br3 ) of ca. 70–75% yields
after 24 h (Eq. 1). Based on the above experimental results,
of the Br2 production, the final product from NO3 can be
þ
27,28
NH3OH as well as N2. The standard potentials of Br2
and Br in aqueous solution are 1.087 and 1.050 V, respec-
þ
ꢁ
NO2 species is proposed as a reaction intermediate in aceto-
nitrile and also in the reverse micelle.
3
tively.28

M. Hojo et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 8 (2006) 1219
formation of Br2 was detected in the CHCl3 reverse micelle
ꢁ
system. The fact that Cl was not oxidized by dilute HNO3
1
was confirmed also by H NMR chemical shift of H2O in the
CHCl3/CTAC reverse micelle system (vide infra). The oxida-
ꢁ
ꢇ
tion of Cl to Cl2 by the nitrate radical, NO3 , in aqueous
solution has been reported, though.32
Effects of Alcohols and Amylenes in CHCl3: Commer-
cially obtained chloroform usually contains small amounts of
stabilizers, such as ethanol or amylenes, and the effects of
ꢁ
these compounds on the oxidation Br were examined in
the CHCl3/CTAB reverse micelle at W ¼ 2:0 containing 2.0
ꢁ3
ꢂ
mol dm HNO3 at 25 C. The addition of 1.0, 2.0, and 5.0
vol % ethanol to the reverse micelle system caused observable
decreases in the reaction rate: ca. 1, 2, and 6 h delay, respec-
tively, compared with the system of original chloroform con-
taining 0.3–1.0% EtOH. The effects of 1.0, 2.0, and 5.0 vol %
octanol were slightly smaller than those of ethanol. However,
the addition of acetonitrile slightly accelerated the oxidation
reaction rates. Acetonitrile, present not only in the organic
phase but also in the water-droplets, may distort the water
structure further, whereas, octanol may encapsulate water-
droplets and stabilize the water structure through the OH of
the alcohol. The addition of alcohols and acetonitrile did not
ꢁ
ꢁ
Fig. 7. Observed amounts of Br2 (or Br3 ) from NO3 ,
ꢁ
ꢁ
NO2 , or NO with increasing concentration of HClO4
in the reverse micelle (CHCl3/CTAB) of W ¼ 2:0: (
)
ꢁ
ꢁ
3
ꢁ3
0
0
.5 mol dm
NaNO3; ( ) 0.50 mol dm
NaNO2; ( )
3
.25 mol dm Na2N2O2 (in 1.0 vol % H2O phase). The
dotted lines show the theoretical values (see the text).
ꢁ
influence the final yield of Br2 (or Br3 ).
In the course of the present study, however, we noticed that
ꢁ
ꢁ
o
Br2 þ 2e ꢀ 2Br
ðE ¼ 1:087 VÞ:
ðE ¼ 1:050 VÞ:
Oxidation potential of nitric acid is high enough to oxidize Br
into Br2 or Br3 in solution; however, it seems not to be strong
ð8Þ
ð9Þ
ꢁ
ꢁ
ꢁ
ꢁ
o
Br3 þ 2e ꢀ 3Br
ꢁ
the yield of Br2 (or Br3 ) produced using the CTAB reversed
micelle system were too small, especially with lower concen-
trations of HNO3, whenever chloroform containing amylenes
was used as the solvent. Amylenes include several alkenes,
i.e., 1-pentene, 2-pentene, 2-methyl-1-butene, and 2-methyl-
2-butene, and the effects of amylene and related compounds
ꢁ
ꢁ
o
28
enough to oxidize Cl to Cl2 (E ¼ 1:396 V).
The Addition of HClO4 to NaNO3, NaNO2, and
Na2N2O2: Ogawa et al.29 examined the electrochemical re-
ꢁ
on the production of Br2 (or Br3 ) were examined. Figure 8
duction of the nitrate ion at mercury electrodes in the presence
of metal ions to explore why the nitrate ion (nitric acid) is a
powerful oxidizing agent in acidic solution and not an oxidiz-
ing agent in neutral or alkaline solution. Figure 7 shows the
ꢁ
shows the amount of Br3 produced using the CTAB reverse
micelle system containing 2.0 mol dm HNO3 in the presence
ꢁ3
ꢂ
of various amounts of 2-methyl-2-butene at 35 C. Without the
alkene, more than 95% yield of Br2 was obtained; however,
ꢁ
ꢁ
ꢁ
2ꢁ
oxidation abilities of NO3 , NO2 , and NO (N2O2 ) in
the absence and in the presence of protons using the CTAB
ꢁ
3
reverse micelle at W ¼ 2:0. A 0.50 mol dm NaNO3 (in the
þ
1
.0 vol % H2O phase) reversed micelle without H did not
ꢁ
ꢁ
oxidize Br ; however, in the presence of HClO4, the NO3
ꢁ
ꢁ
ion oxidized Br to Br2 (or Br3 ). The amounts of Br2 for
ꢁ
[
HClO4]=[NO3 ] ¼ 0{6 seemed to be accordance with the
ꢁ
reaction mechanism in Eqs. 4–6. The oxidation of Br did
not occur in solutions containing either 0.5 mol dm NaNO2
ꢁ3
ꢁ3
or 0.25 mol dm Na2N2O2 without the addition of protons.
Although, in the presence of HClO4, the oxidation of Br into
ꢁ
Br2 occurred quite well, it was rather difficult to fit precisely
the experimental and theoretical values to each other. At any
rate, these results suggested the reduction of N atom from a
þ5 oxidation state to a 0 oxidation state. The overall reaction
scheme should follow the scheme in Eq. 7.
ꢁ
The quantitative formation of Br3 was observed in the
CH2Cl2 reverse micelle, as well as the CHCl3 reverse micelle
system. The reverse micelle system with an anionic surfactant,
AOT (W ¼ 2:0{10:0), and heptane as the solvent, also, oxi-
Fig. 8. The change in absorbance at 400 nm with time for
ꢁ3
2
.0 mol dm HNO3 (in a 1.0 vol % H2O phase) in the
presence of various concentrations of 2-methyl-2-butene
ꢁ
ꢁ
ꢁ3
dized Br to Br3 in the presence of 0.1 mol dm n-Bu4NBr.
In the CHCl3/CTAC reverse micelle system, however, no oxi-
in the reverse micelle (CHCl /CTAB) of W ¼ 2:0 at
3
ꢁ
30
ꢂ
dation of Cl to Cl2 was detected using o-tolidine and DPD
N,N-diethyl-p-phenylenediamine),31 as indicators; while the
35 C: ( ) 0; ( ) 0.002; ( ) 0.005; ( ) 0.0075; ( )
ꢁ3
(
0.01 mol dm 2-methyl-2-butene.

1
220 Bull. Chem. Soc. Jpn. Vol. 79, No. 8 (2006)
Oxidation Ability of Dilute Nitric Acid
ꢁ
ꢁ3
Table 1. The Yields of Br2 (or Br3 ) from 2.0 mol dm
aÞ
ꢁ3
HNO3 (or 0.020 mol dm HNO3 as the Whole Solution)
in the Presence of Various Concentrations of Alkene Com-
pounds in CHCl3/CTAB of W ¼ 1:4 at 25 C
ꢂ
Concn of alkenes
mol dm 3
ꢁ
0
0.002 0.005 0.0075 0.01
/
3
2
2
2
1
2
,3-Dimethyl-1-butene 100% 89% 50%
16% 1%
0% 0%
5% 1%
11% 0%
8% 1%
7% 1%
,3-Dimethyl-2-butene
-Methyl-1-butene
-Methyl-2-butene
-Hexene
99% 72%
0%
98% 77% 39%
97% 69% 37%
100% 73% 40%
100% 75% 37%
-Pentene
bÞ
Theoretical value
100% 80% 50%
25% 0%
Fig. 9. Changes in the 1H NMR chemical shift value of
water with the W value in the reverse micelle, CHCl /
a) In the water-droplets (1.0 vol %) in the micelle system.
ꢁ3
ꢁ3
b) Nitric acid of 0.02 mol dm can form 0.01 mol dm Br2,
which reacts an alkene to form the dibromoalkane.
3
ꢂ
CTAB or CTAC/H O (of 1.0 vol %) at 25 C: ( ) CTAB;
2
(
) CTAC.
addition of 2-methyl-2-butene caused decreases in the yield:
ca. 75, 40, 10, and 0% yield with 0.002, 0.005, 0.0075, and
ꢁ
3
0
.01 mol dm
2-methyl-2-butene, respectively. The above
results indicate that Br2, produced in the oxidation reaction,
reacts with the butene stoichiometrically to form, probably,
the dibromobutane through common reaction pathways.33
Another alkene, 1-hexene, caused a distinct decrease in the
reaction rate and in the final yield of Br2 using similar exper-
ꢂ
imental conditions at 25 C. However, this system gave an
interesting time change in the Br2 concentration for the cases
ꢁ3
especially with 0.0075 or 0.01 mol dm of 1-hexene. With
ꢁ
3
0
small amount of Br3 was observed after 3.5 h but it decreased
.01 mol dm of 1-hexene, for instance, the production of a
ꢁ
gradually to disappear completely after 6 h. The tentative for-
ꢁ
mation of the Br3 species was confirmed by the appearance
and the disappearance of the absorption peak at around
1
Fig. 10. Changes in the H NMR chemical shift value
of water in the reverse micelle, CHCl3/CTAB/H2O (of
1
2
73 nm as well as tailing at 400 nm. A similar but more distinct
ꢁ
.0 vol %) versus increasing concentrations of strong acids
ꢂ
ꢁ3
change in Br3 production was observed with 0.01 mol dm
-methyl-1-butene or 3,3-dimethyl-1-butene. On the other
at 25 C: ( ) HCl; ( ) HNO3, measured just after prepa-
ration; ( ) HClO4.
3
hand, 2-methyl-2-butene (as described above) and 2-pentene
did not alter the reaction behavior. The different behavior
between 1-alkenes and 2-alkenes can be attributed to the reac-
tion rates of Br2 toward the alkenes; the bromination of 2-
alkenes is much faster than that of 1-alkenes because 2-alkenes
ꢂ
ous W values at 25 C. The chemical shift (ꢀ) decreased re-
markably with decreasing W value for CTAB (W ¼ 1:0{4:0).
The decrease in the chemical shift value of H2O indicates that
the hydrogen-bonding structure of water has been distorted.
However, the CTAC system showed smaller gradual decrease
in ꢀ value than the CTAB system. The difference between
CTAB and CTAC may be caused by the water-distortion abil-
ity of ions due to sizes of Br and Cl . At any rate, the ꢀ val-
ues of water in the reverse micelle systems are much smaller
than that of bulk water (ꢀ ꢃ 4:7), which indicates the
hydrogen-bonding structure of water in the reverse micelle
systems is indeed distorted, compared with that of bulk water.
Higuchi and Misra demonstrated that the activity of water in
AOT-hydrocarbon reverse micelles decreased as the W value
decreased. For the alternate changes in the O–H stretching re-
37
gion of H2O/AOT/heptane with W values, D’Aprano et al.
assigned ‘‘bonded’’ and ‘‘bulk’’ waters.
Figure 10 shows the effects of strong acids, such as HCl,
HNO3, and HClO4, on the ꢀ value of H2O in the CHCl3/
34
have more electron-donating groups than 1-alkenes. Table 1
ꢁ
shows the final yields of Br3 in the reverse micelle of W ¼
ꢂ
ꢁ3
1
dimethyl-2-butene was added, no Br3 was produced; the
:4 at 25 C. Interestingly, when 0.005–0.01 mol dm of 2,3-
ꢁ
ꢁ
ꢁ
þ
intermediate species NO2 may have attacked directly the
18,35
butene due to its greater electron density. The nitration of phe-
nol compounds by dilute nitric acid in reverse micelle systems
will be reported later.
Observation by 1H NMR of the Water Structure
Changes in Reverse Micelle Systems. The states of the
water in AOT reverse micelles in heptane were investigated
36
1
23
11
by H and Na NMR spectroscopy over a large W value
1
range (1:6 < W < 50): in the H NMR spectrum, the chemical
shift of H2O shifted downfield with an increase in the amount
of water and gradually approached that of ordinary water.
Figure 9 shows the change in the chemical shift of H2O in
CHCl3/CTAB and CTAC reverse micelle systems with vari-
ꢂ
CTAB reverse system of W ¼ 2:0 at 25 C. The sample of
HNO3 was used just after preparation because the oxidation

M. Hojo et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 8 (2006) 1221
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8 H. Nagayama, M. Hojo, T. Ueda, Y. Nishimori, M.
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1
0
1
1
1
1
2
1
3
Fig. 11. Changes in the 1H NMR chemical shift value of
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H2O (1.0 vol %), at W ¼ 2:0 versus HNO3 concentration
14
15
16
H. Shirota, K. Horie, J. Phys. Chem. B 1999, 103, 1437.
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ꢁ3
at 25 C: ( ) 1.0; ( ) 2.0; ( ) 3.0 mol dm HNO3 (in
the 1.0 vol % H2O phase).
1
996, 100, 12638.
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1
7
reaction proceeded as a function of time. In increasing the con-
centration of the strong acids caused an increase in the ꢀ value.
Hydrochloric acid gave the largest effect among all the acids
1
8
Kharkov Univ. Bull. 2004, 626, Chemical Series 11(34), 47.
19 a) L. C. Manege, T. Ueda, M. Hojo, Bull. Chem. Soc. Jpn.
ꢁ3
while perchloric acid gave the least effect, up to 2 mol dm salt
concentrations. These results are probably based on the differ-
ence of anion effects; we have observed that anions with the
larger crystal ionic radius cause the larger effects on the chem-
1
998, 71, 589. b) L. C. Manege, T. Ueda, M. Hojo, M. Fujio,
J. Chem. Soc., Perkin Trans. 2 1998, 1961. c) M. Hojo, T. Ueda,
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Chem. Soc. Jpn., in press.
ꢁ
ꢁ
ical shifts of water towards upper-field, as Cl < NO3
<
ꢁ
ꢁ
ꢁ
38
Br < I ꢃ ClO4 , in 2.0% and 20 vol % EtOH–H2O solu-
2
0
M. Kimura, M. Sato, T. Murase, K. Tsukahara, Bull. Chem.
Soc. Jpn. 1993, 66, 2900.
M. Kimura, N. Ieyama, M. Matsumoto, K. Shimada, K.
Tsukahara, Bull. Chem. Soc. Jpn. 2001, 74, 1871.
E. R. Wright, R. A. Smith, B. G. Messick, In Colorimetric
3
9
1
tions. The H NMR chemical shift can identify the presence
of strong acids in reverse micelle systems, as well as in bulk
water or bulk H2O–EtOH mixed solution.
2
1
Figure 11 shows the change in ꢀ value with time of H2O in
the CHCl3/CTAB reverse micelle system of W ¼ 2:0 contain-
2
2
Determination of Nonmetals, ed. by D. F. Bolltz, J. A. Howell,
John Wiley & Sons, New York, 1978, Chap. 2, p. 47.
23 Kagaku Binran, Kiso-Hen, 5th ed., Chemical Society of
Japan, Maruzen, Tokyo, 2004, p. II-748.
24 W. Gabes, H. Gerding, J. Mol. Struct. 1972, 14, 267.
25 C. Cabos, P. Delord, J. Phys. Lett. 1980, 41, L-455.
ꢁ3
ing 1.0, 2.0, and 3.0 mol dm HNO3 (in the 1.0 vol % water)
ꢂ
at 25 C; all of the solutions became yellow in NMR tubes,
ꢁ
indicating the formation of Br3 . The ꢀ values decreased with
time in accordance with the formation of Br3 (cf. Fig. 1) and
ꢁ
the decreases in ꢀ values correspond to the decreases in the
HNO3 concentrations in the CTAB reverse micelle systems.
Now, the consumption of protons during the oxidation reaction
2
6
G. Bontempelli, G.-A. Mozzocchin, F. Magno, J. Electro-
anal. Chem. 1974, 55, 91.
Encyclopedia of Electrochemistry of the Elements, ed. by
A. J. Bard, Mercel Dekker, New York, 1978, Vol. 8, Chap. VIII-5.
Standard Potentials in Aqueous Solution, ed. by A. J. Bard,
2
7
(
Eq. 1) was verified. The decrease in the ꢀ value was observed
ꢂ
for CTAB reverse micelle of W ¼ 4:0 as well as 2.0 at 25 C.
2
8
As for the CHCl3/CTAC system of W ¼ 4:0, however, the
ꢁ3
R. Parsons, J. Jordan, Mercel Dekker, New York, 1985.
29 N. Ogawa, R. Kikuchi, A. Nakamura, H. Aizawa, S.
Ikeda, Anal. Sci. 1993, 9, 847; N. Ogawa, H. Kodaiku, S. Ikeda,
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original ꢀ value (2 mol dm HNO3) of ca. 4.22 ppm remained
constant even after six days, meaning that Cl is not oxidized
ꢁ
by HNO3.
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