Conversion of anilines into azobenzenes in acetic acid with perborate and Mo(VI): correlation of reactivities

Article abstract of DOI:10.1007/s11696-018-0599-z

Azobenzenes are extensively used to dye textiles and leather and by tuning the substituent in the ring, vivid colours are obtained. Here, we report preparation of a large number of azobenzenes in good yield from commercially available anilines using sodium perborate (SPB) and catalytic amount of Na₂MoO₄ under mild conditions. Glacial acetic acid is the solvent of choice and the aniline to azobenzene conversion is zero, first and first orders with respect to SPB, Na₂MoO₄ and aniline, respectively. Based on the kinetic orders, UV–visible spectra and cyclic voltammograms, the conversion mechanism has been suggested. The reaction rates of about 50 anilines at 20–50?°C and their energy and entropy of activation conform to the isokinetic or Exner relationship and compensation effect, respectively. However, the reaction rates, deduced by the so far adopted method, fail to comply with the Hammett correlation. The specific reaction rates of molecular anilines, obtained through a modified calculation, conform to the Hammett relationship. Thus, this work presents a convenient inexpensive non-hazardous method of preparation of a larger number of azobenzenes, and shows the requirement of modification in obtaining the true reaction rates of anilines in acetic acid and the validity of Hammett relationship in the conversion process, indicating operation of a common mechanism.

Full text of DOI:10.1007/s11696-018-0599-z

Chemical Papers
https://doi.org/10.1007/s11696-018-0599-z
ORIGINAL PAPER
Conversion of anilines into azobenzenes in acetic acid with perborate
and Mo(VI): correlation of reactivities
C. Karunakaran1 · R. Venkataramanan1
Received: 27 March 2018 / Accepted: 15 September 2018
© Institute of Chemistry, Slovak Academy of Sciences 2018
Abstract
Azobenzenes are extensively used to dye textiles and leather and by tuning the substituent in the ring, vivid colours are
obtained. Here, we report preparation of a large number of azobenzenes in good yield from commercially available anilines
using sodium perborate (SPB) and catalytic amount of Na2MoO4 under mild conditions. Glacial acetic acid is the solvent
of choice and the aniline to azobenzene conversion is zero, frst and frst orders with respect to SPB, Na2MoO4 and aniline,
respectively. Based on the kinetic orders, UV–visible spectra and cyclic voltammograms, the conversion mechanism has
been suggested. The reaction rates of about 50 anilines at 20–50 °C and their energy and entropy of activation conform to
the isokinetic or Exner relationship and compensation efect, respectively. However, the reaction rates, deduced by the so
far adopted method, fail to comply with the Hammett correlation. The specifc reaction rates of molecular anilines, obtained
through a modifed calculation, conform to the Hammett relationship. Thus, this work presents a convenient inexpensive
non-hazardous method of preparation of a larger number of azobenzenes, and shows the requirement of modifcation in
obtaining the true reaction rates of anilines in acetic acid and the validity of Hammett relationship in the conversion process,
indicating operation of a common mechanism.
Keywords Homogeneous catalysis · Kinetics · Oxidations · Structure–reactivity relationships · Azo dyes
Introduction
Cu(OAc)2 and Ag2CO3 (Seth et al. 2016), or Ag nanoparti-
cles impregnated on carbon (Cai et al. 2013) or Au–Ag alloy
nanoparticles on P-, N-, F- and S-doped carbon (Gao et al.
2018) as catalyst have been reported recently. Mesoporous
MnO2 (Dutta et al. 2016) and CuBr2 or CuBr in pyridine
(Zhang and Jiao 2010) also catalyze oxidation of anilines
to azobenzenes by air. (Diacetoxyiodo)benzene (Ma et al.
2012) as well as tert-butyl hypoiodite (Takada et al. 2012)
efectively oxidizes anilines to azobenzenes. The oxidation
of anilines to azobenzenes by di-tert-butyldiaziridinone
requires CuBr as catalyst (Zhu and Shi 2013). Aqueous H2O2
oxidizes anilines to azobenzenes but demands perfuorosul-
fonic acid as catalyst (Paris et al. 2018). The requirement
of nanoparticulate precious metals as catalyst (the recovery
of which for reuse is a challenge) or uneconomical oxygen
source hampers commercialization of the reported research
fndings. Sodium perborate (SPB, NaBO3.4H2O) of ani-
onic formula B2(O2)2(OH)24− is a cheap, innocuous, easily
handled large scale industrial chemical used as a source of
H2O2, mainly in detergents (Indi et al. 2017) and mouthwash
(Almohareb 2017; Tran et al. 2017). Azobenzenes are pre-
pared in good yield by SPB-oxidation of anilines in glacial
Azo dyes, a commercially important family of azobenzenes,
are pervasively used to dye textiles and leather articles. As
a consequence of π-delocalization, the azobenzenes display
vivid colors and the color is tuned by the substituent present
at the benzene ring (Yamada et al. 2018); the absorption
wavelength (λmax) is governed by the Hammett relationship.
Merino (2011) has listed various methods of synthesis of
azo compounds in her review. Preparation of azobenzenes
through aerobic oxidative dehydrogenative coupling of ani-
lines using Au nanoparticles supported on TiO2 (Grirrane
et al. 2008) or Cu–Ag nanoclusters, generated in situ using
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-018-0599-z) contains
supplementary material, which is available to authorized users.
* C. Karunakaran
karunakaranc@redifmail.com
1
Department of Chemistry, Annamalai University,
Annamalainagar 608002, India
Vol.:(0123456789)
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Chemical Papers
acetic acid (Karunakaran and Kamalam 2002a). Search of
inexpensive, non-toxic, readily available and easily handled
catalyst for the said conversion is of interest and Na2MoO4
is found to catalyze SPB oxidation of organic sulfdes and
sulfoxides (Karunakaran and Venkataramanan 2006a, b).
The present study shows efective catalysis of SPB oxida-
tion of anilines to azobenzenes by Na2MoO4 in acetic acid;
molybdenum is an essential trace element for animals and
plants. Quantifcation of the substituent efect on the reaction
rate by the Hammett relationship is well established. As the
para- or meta-substituent infuences the reaction center and,
thus, the reaction rate electronically, Cvijetic et al. (2013)
observed parallel correlation of reaction rate with LUMO
(lowest unoccupied molecular orbital) energy as well as
Mulliken atomic charge. The rate of [4+ 2] cycloaddition
of cyclopentadiene with (E)-2-aryl-1-cyano-1-nitroethenes,
determined by Jasinski et al. (2012), correlates with Ham-
mett constant, σp as well as electrophilicity index, ω and the
reaction constants are similar. These results confrm that the
Hammett substituent constant refects the electrical charac-
teristics of the substituent. Salter-Blanc et al. (2016) correla-
tion analysis on aniline oxidation by MnO2 substantiates the
same; the oxidation conforms to the Hammett relationship
and is governed by the (1) pKa (2) EHOMO (energy of highest
occupied molecular orbital) and (3) EOX (oxidation poten-
tial). Sun et al. (2018) show operation of linear free energy
relationships (LFERs) in the degradation of anilines by
K2FeO4 in the pH range 6–10. Because of the importance of
manufacture of azobenzenes from anilines employing SPB
in glacial acetic acid, here we report molybdenum(VI) catal-
ysis of the conversion and correlate the observed reaction
rates in terms of Hammett relationship. The results show
the routine methodology of obtaining the reaction rates as
incorrect and the logically deduced reaction rates conform to
the LFER. Further, this study gains interest because amino
group on a biomolecule like amino acid, DNA, RNA and
protein is responsible for the reactivity and solubility which
are determined by acidity or basicity of the medium.
of reagent grade. p-Iodoaniline and 2,4,6-tribromoaniline
were prepared following standard procedures. The anilines
purifed by distillation, listed with the experimental and
literature [given in parenthesis (Buckingham 1995)] boil-
ing points (°C), were: aniline 182 (184), m-toluidine 200
(203), m-anisidine 249 (251), m-chloroaniline 227 (230.5),
m-bromoaniline 248 (251), p-phenetidine 252 (253),
p-fuoroaniline 180 (184–186), o,m-dimethylaniline (2, 3)
220 (221–222), o,m-dimethylaniline (2, 5) 212 (213-215),
o,m-dichloroaniline (2, 3) 250 (252), o-toluidine 197 (201),
o-anisidine 222 (225), o-phenetidine 227 (228), o-chlo-
roaniline 207 (208.8), o-bromoaniline 226 (229), methyl
o-aminobenzoate 259 (258-261), o-aminothiophenol 232
(234), N-methylaniline 194 (196) and N,N-dimethylaniline
191 (193). The following anilines were purifed through
recrystallization; the solvent- employed and literature melt-
ing points (°C) (Buckingham 1995) are given in parenthesis
along with the experimental melting points: m-aminophe-
nol 125 (water, 124–126), m-nitroaniline 114 (water, 114),
m-aminobenzoic acid 177 (water, 178), m-aminoacetophe-
none 98 (ethanol, 97-99), m,p-dimethylaniline (3, 4) 49
(petroleum ether 40–60, 51), m,p-dichloroaniline (3, 4) 71
(ligroin, 72), p-toluidine 42 (water, 42–44), p-anisidine 56
(petroleum ether 40–60, 57), p-aminophenol 184 (water,
186), p-chloroaniline 70 (petroleum ether 40–60, 70–71),
p-bromoaniline 64 (aqueous ethanol, 66), p-iodoaniline 67
(petroleum ether 60–80, 67–68), p-nitroaniline 147 (water,
148), p-aminobenzoic acid 188 (water, 188–188.5), ethyl
p-aminobenzoate 90 (ethanol, 92), p-aminoacetophenone
105 (water, 106), p-aminoacetanilide 166 (water, 165–167),
sulfanilic acid (water, 25), sulfanilamide 165 (aqueous etha-
nol, 165–166), p-phenylenediamine 145 (ether, 147), o,m-
dimethoxyaniline (2, 5) 80 (petroleum ether 40–60, 81), o,m-
dichloroaniline (2, 5) 50 (ligroin, 50), o,p-dichloroaniline
(2, 4) 63 (ethanol, 63), o,o,p-trichloroaniline 78 (ligroin,
78.5), o,o,p-tribromoaniline 121 (benzene/ethanol, 122),
o-phenoxyaniline 45 (ligroin, 45–46), o-aminophenol 173
(water, 174), o-nitroaniline 71 (water, 72), o-aminobenzoic
acid 145 (water, 144–148), o-phenylenediamine 102 (water,
104), o,o-dinitroaniline 141 (ethanol, 141–142) and diphe-
nylamine 53 (ligroin, 52–54). Acetic acid, methanol (analar)
and ethanol were purifed following standard procedures and
distilled for use.
Experimental
NaBO3.4H2O, Na2MoO4.4H2O, Mn(OCOCH3)2.4H2O,
Fe(OCOH)2.2H2O, NiCl2.6H2O, Cu(OCOCH3)2.
H 2 O , Z r O C l 2 . 8 H 2 O , C d ( O C O C H 3 ) 2 . 2 H 2 O ,
SnCl2.2H2O, Hg(OCOCH3)2, Pb(OCOCH3)2.3H2O,
SbCl3, Bi(NO3)3.5H2O, K2Cr2O7, Co(NO3)2.6H2O,
Pd(OCOCH3)2.3H2O, K2TiO(C2O4)2.2H2O, NH4VO3,
(NH4)6Mo7O24.4H2O, Li2MoO4, Na2WO4.2H2O,
FeSO4.7H2O, Fe(NH4)2(SO4)2.6H2O, Fe(NH4)
(SO4)2.12H2O, Ce(SO4)2.4H2O, Ce(NH4)4(SO4)4.2H2O,
Ce2(C2O4)3.9H2O, H3VO4, H2MoO4, H2WO4, H3PMo12O40,
H3PW12O40, NaOH and trichloroacetic acid, all reagent
grade, were used as received. All the anilines used were
SPB (0.011 mol) and Na2MoO4 (0.001 mol) were
dissolved in glacial acetic acid (5–7 mL) and aniline
(0.015 mol) was added drop-wise or in portions at 50 °C.
The temperature was maintained at 50 °C for an hour. The
resultant was poured into water (500 mL) and stirred. The
solid was fltered, dried and recrystallized from petroleum
ether (40–60 °C).
SPB in glacial acetic acid, with catalytic amount of
Na2MoO4, efectively converts many anilines into azoben-
zenes, identifed by their IR (Fig. S1–39 in Supplementary
1 3

Chemical Papers
material) and UV–visible (not presented) spectra and melt-
ing points with literature values in parenthesis (Buckingham
1995); the yields are between 70 and 85%. o-Aminothio-
phenol is oxidized, by SPB in acetic acid with Na2MoO4,
to the corresponding disulfde, identifed by its IR spectrum
(Fig. S40 in Supplementary material) and melting point.
The products of N-methylaniline and diphenylamine are
likely to be the respective hydroxylamines; the products
dissolve in aqueous NaOH and regenerate on acidifcation.
Further, the IR spectra (Fig. S41 and 42 in Supplementary
material) show strong broad absorbance around 3425 and
3443 cm−1, respectively, indicating the presence of hydroxyl
function. N,N-Dimethylaniline oxide is the likely prod-
uct of N,N-dimethylaniline, as seen from its IR spectrum
(1650 cm−1; Fig. S43 in Supplementary material). Use of
aniline in excess over SPB ensures the absence of overoxi-
dation. The absence of>N→O frequency (ca. 1335 cm−1)
in the recorded IR spectra supports the absence of overoxi-
dation and the agreement of the determined melting points
with those of the literature values shows the purity of the
products.
128 (119) °C; o,m-(OCH3)2 (2, 5): 1508, 1465 cm−1, 436,
312 nm, 75 °C, 83%; o,m-Cl2 (2, 3): 1451 cm−1, 328 nm,
89%; o,m-Cl2 (2, 5): 1459 cm−1, 312 nm, 90%; o,p-Cl2:
1460 cm−1, 323 nm, 90%; o,o,p-Cl3: 1473 cm−1, 344 nm,
95 °C, 90%; o,o,p-Br3: 1454 cm−1, 323 nm, 81%; o-CH3:
1583, 1502 (1580, 1483) cm−1, 390, 314 nm, 54 (54) °C,
87%; o-OCH3: 1459 (1489, 1460) cm−1, 315 nm, 90%;
o-OC2H5: 1585, 1470 (1583, 1476) cm−1, 426 nm; o-OC6H5:
1478 cm−1, 327 nm, 215 °C, 83%; o-OH: 1463 cm−1, 421,
316 nm, 93%; o-Cl: 1464 cm−1, 419, 330 nm, 138 (138) °C,
76%; o-NO2: 1428 cm−1, 406, 301 nm, 82%; o-COOH:
1443 cm−1, 342, 321 nm, 76%; o-COOCH3: 1487, 1434
(1495, 1440) cm−1, 322 nm, 151 (150) °C, 46%; o-NH2:
1529, 1487 cm−1, 455, 304 nm, 83%; o-SH: 1472 (1474)
cm−1 (–S–S–), 334 nm, 91 (93) °C, 76%; o,o-(NO2)2:
1435 cm−1, 422, 318 nm, 138 °C, 92%; –NHCH3: 3425 cm−1
(> NOH), 333 nm, 135 °C, 71%; –N(CH3)2: 1650 (1635)
cm−1 (N → O), 306, 256 nm, > 230 °C, 40%; –NHC6H5:
3443 cm−1 (>NOH), 80%.
The kinetic runs were made under pseudo-frst-order
conditions with [aniline] ≫ [SPB] at 20–50 °C. SPB was
dissolved in glacial acetic acid, standardized and used
immediately. Required weight of Na2MoO4 was dissolved
in glacial acetic acid in a SMF, diluted stepwise and used
without allowing it to stand. Desired concentration of aniline
in acetic acid was prepared. Required volumes of the solu-
tions were mixed and the progress of the conversion was
followed by iodometric estimation of the unconsumed oxi-
dant. The ΔH# and ΔS# were obtained through least square
analysis of log k versus 1/T plots adopting the logarithmic
form of Eyring relationship log10 (k/T)=10.318+(ΔS#/19.
147)−(ΔH#/19.147T).
Substituent, ῡ (IR), λmax (UV–vis), mp, yield: H: 1477,
1435 (1480, 1450) cm−1, 439 (443) nm, 68 (71) °C, 80%;
m-CH3: 1600, 1485 (1606, 1483) cm−1, 412, 323 nm, 54
(54) °C, 81%; m-OCH3: 1600, 1491 (1594, 1489) cm−1,
312 nm, 80 (74) °C, 83%; m-OH: 1515 (1474) cm−1
,
323 nm,71%; m-Cl: 1481 cm−1, 417, 321 nm, 103 (103) °C,
82%; m-Br: 1458 cm−1, 427, 325 nm, 125 (126) °C, 83%;
m-NO2: 1525, 1480 cm−1, 321 nm, 138 (144) °C, 93%;
m-COOH: 1523, 1459 cm−1, 322 nm,>280 (340) °C, 84%;
m,p-Cl2: 1463 cm−1, 441, 332 nm, 156 (158) °C, 89%;
p-CH3: 1598, 1498 (1580, 1483) cm−1, 437, 335 nm, 144
(144) °C, 69%; p-OCH3: 1599, 1506 (1594) cm−1, 421,
313 nm, 87%; p-OC2H5: 1505, 1469 (1505, 1476) cm−1,
426 nm, 85%; p-F: 1497 cm−1, 422, 321 nm, 103 (103) °C,
83%; p-Cl: 1477 cm−1, 438, 332 nm, 187 (188) °C, 84%;
p-Br: 1471 cm−1, 442, 333 nm, 204 (205) °C, 88%; p-I:
1467 cm−1, 340 nm, 240 (242) °C, 89%; p-NO2: 1474,
1445 cm−1, 373 nm, 134 (128) °C, 90%; p-COOH:
1422 cm−1, 328 nm, > 280 (330) °C, 89%; p-COCH3:
1457 cm−1, 462 nm, 206 °C, 70%; p-SO2NH2: 1458 cm−1,
450, 328 nm, 25 (25) °C, 80%; p-NH2: 92%; o,m-(CH3)2 (2,
3), 1472 cm−1, 402, 320 nm, 91%; o,m-(CH3)2 (2, 5): 313 nm,
Results and discussion
Aniline into azobenzene
SPB in glacial acetic acid, with catalytic amount of
molybdenum(VI), efectively converts most of the anilines
into azobenzenes, identifed by their IR (Fig. S1–S43 in Sup-
plementary material) and UV–visible spectra and melting
points; the yields are good (see “Experimental” section).
X
X
Mo(VI)
HOAc
2
+ 2 H2O + 2 NaBO2
NH
N
N
2 + 2 NaBO3
X
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Chemical Papers
Choice of solvent and catalyst
Kinetic orders
SPB dissolves in acetic acid (HOAc) readily and less so in
ethylene glycol (EG) but insoluble in other organic solvents
like 2-methoxypentan-2,3-diol, glycerol, ethanol, 2-ethox-
yethanol, 2-butoxyethanol, methanol (MeOH), 2-propanol,
t-butanol, dioxane, acetonitrile and dimethylformamide.
While Mn(II), Fe(II), Ni(II), Cu(II), Cd(II), Sn(II), Pb(II),
Sb(II) and Bi(III) fail to catalyze the reaction in acetic acid,
Cr(VI), Co(II) and Pd(II) catalyze decomposition of SPB.
Although Zr(IV) catalyzes the reaction, the kinetic results
lack reproducibility. Many commercially available salts or
oxoacids of transition or inner transition metals are insoluble
in glacial acetic acid (see “Experimental” section).
The uncatalyzed conversion of anilines into azobenzenes
in glacial acetic acid by SPB is sluggish at room temper-
ature, although it occurs at elevated temperatures (McK-
illop and Sanderson 1995; Muzart 1995). SPB in glacial
acetic acid produces peracetic acid on aging and peracetic
acid oxidizes aniline to azobenzene very much faster than
SPB (Karunakaran and Kamalam 2002a). The Mo(VI)-
catalyzed conversion of anilines to azobenzenes by SPB in
glacial acetic acid was studied under the conditions: [ani-
line]≫[SPB]≫[Mo(VI)] with fresh solution of SPB as well
as Na2MoO4 in acetic acid; the catalyst was thrown out from
Na2MoO4 solution in acetic acid on standing. The reaction
was initiated through the addition of aniline to SPB and
Na2MoO4, all dissolved in acetic acid. The UV–Vis spectra
of the reaction solution, recorded at diferent reaction times,
show the formation of azobenzene (Fig. 1). The kinetics of
the reaction was studied by iodometric estimation of the
unconsumed oxidant at diferent time intervals. The reaction
is zero order with respect to the oxidant; the oxidant con-
centration decreases linearly with reaction time, as shown by
Fig. 2a. Also, the reaction rate does not difer signifcantly
on increasing [SPB]0 from 2 to 10 mM [[aniline]0 =0.10 M,
[Mo(VI)]=0.50 mM, 35 °C, rate=0.64 0.01 μM s−1]. Not
only the consumption of the oxidant but also the product
formation is zero order; plot of absorbance (440 nm) vs.
reaction time is linear almost through the origin (Fig. 2a).
The conversion is frst order with respect to aniline; the reac-
tion rate increases linearly with [aniline]0 (Fig. 2b). The
decomposition of the oxidant is insignifcant. Also, the reac-
tion is frst order with respect to Mo(VI), as evident from
the linear plot of reaction rate vs. [Mo(VI)] (Fig. 2b); the
Fig. 1 UV–Vis spectral time scan of the reaction in acetic acid.
[SPB]0 =5.0 mM, [aniline]0 =0.10 M, [Mo(VI)]=0.50 mM, 35 °C
[Mo(IV)]/mM
Time/s
0
0.3
0.6
0.9
1.2
12000
0
4000
8000
6
4
2
3
2
1
0
(b)
(a)
0.3
0.6
0.9
2
1
1.2
1.5
0
0
4000
Time/s
0
2000
6000
8000
0.2
[Aniline]/M
0.4
0.6
Fig. 2 a Linear decrease of [SPB] or increase of [PhN=NPh]
with reaction time. [SPB]0 =5.0 mM, [aniline]0 =0.10 M,
[Mo(VI)]=0.50 mM, solvent=HOAc, 35 °C. b Rate dependence
on [aniline]0 or [Mo(VI)]. [SPB]0 =5.0 mM, solvent=HOAc, 35 °C,
[Mo(VI)]=0.50 mM or [aniline]0 =0.10 M
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Chemical Papers
uncatalyzed reaction is sluggish. Trichloroacetic acid (TCA)
suppresses the reaction rate. Addition of TCA [0.10 M in
methanolic (60% v/v) acetic acid (40% v/v)] decreases the
reaction rate from 0.16 to 0.071 μM s−1 [[SPB]0 =5.0 mM,
[aniline]0 =0.050 M, [Mo(VI)]=0.50 mM, 35 °C]. Further
addition of TCA (0.20, 0.30, 0.40 and 0.50 M) does not
have any signifcant efect. Acetic acid is the most suitable
solvent for the reaction. The reaction is very slow in EG
(Table S1). Addition of methanol or EG or water slows down
the product formation. Moreover, dissolution of SPB in ace-
tic acid or EG or water shows little diference in the kinetic
results. Boric acid and borate do not infuence the reaction
rate in acetic acid (Table S2). The reaction in acetic acid
does not proceed through a radical mechanism. Addition of
vinyl monomer acrylamide or acrylonitrile neither leads to
polymerization nor suppresses the reaction rate (Table S1).
Furthermore, anionic surfactants like aerosol OT (AOT) and
sodium lauryl sulfate (SLS) do not remarkably infuence the
conversion.
oxodiperoxomolybdenum(VI) [MoO(O2)2] (Lydon et al.
1987). As the reaction is rapid and the formation constant
is very large, under the condition [H2O2] ≫ [Mo(VI)],
the catalyst practically exists as MoO(O2)2 (Lydon et al.
1987). The zero order of the reaction with respect to the
oxidant is in conformity with the same. The first-order
dependence of the reaction rate on [Mo(VI)] as well as
[aniline] points to oxidation of aniline by the reactive per-
oxo molybdenum(VI) species as slow and rate-limiting.
UV–Vis spectroscopic study provides evidence of forma-
tion of oxodiperoxomolybdenum(VI). Figure 3 displays
the UV–Vis spectra of (1) Na2MoO4, (2) SPB and (3) SPB
with Na2MoO4, all dissolved in glacial acetic acid. While
SPB does not absorb signifcantly in the UV–Vis region,
Na2MoO4 absorbs at 292 nm. The UV–Vis spectrum of
SPB dissolved in glacial acetic acid along with Na2MoO4
shows absorption at 309–323 nm. This is in agreement with
the wavelength of absorption of MoO(O2)2, reported by
Thompson and coworkers (328 nm in aqueous perchloric
acid) (Lydon et al. 1987). Cyclic voltammetric study sup-
ports the basis of the reaction mechanism. While the sol-
vent glacial acetic acid does not undergo reduction in the
working potential range, fresh solution of SPB in acetic acid
shows a cathodic peak at − 31 mV (Figs. S44 and S45 in
Supplementary material). Aging of SPB in glacial acetic
acid is refected in the cyclic voltammogram (CV) also; the
cathodic peak potential is shifted from −31 to +77 mV on
aging of SPB (Fig. S46 in Supplementary material). Further,
aging of the catalyst molybdenum(VI) in glacial acetic acid,
probably resulting in oligomerization of molybdenum(VI)
species, is also refected in the CV of fresh and aged solution
of Na2MoO4; while the fresh solution does not show any sig-
nifcant reduction, the aged catalyst exhibits a cathodic peak
at + 928 mV (Fig. S47 and S48 in Supplementary mate-
rial). Cyclic voltammetric results also support the formation
of molybdenum(VI) peroxo species; the CV of SPB with
Na2MoO4 exhibits a cathodic peak potential at +128 mV
(Fig. S49 in Supplementary material). That is, addition of
Na2MoO4 shifts the reduction potential of SPB by 159 mV
(from −31 mV). Scheme 1 is the suggested detailed reac-
tion sequence. Phenylhydroxyl amine has been suggested
as an intermediate in some of the peroxo oxidation of ani-
lines to azobenzenes (Karunakaran and Kamalam 2002a).
The reported mechanism is the slow and rate-determining
Mechanism
SPB on dissolution in glacial acetic acid affords H2O2
quantitatively and on aging, peracetic acid is formed at
the expense of H2O2 (Karunakaran and Kamalam 2000,
2002a). In acetic acid, Na2MoO4 exists as MoO3, the coor-
dinated water molecule omitted (Karunakaran and Muthu-
kumaran 1995). With H2O2 in acidic medium, MoO3 forms
0.8
Na2MoO4
0.6
0.4
SPB + Na2MoO4
0.2
SPB
0
350
300
400
450
Wavelength/nm
Fig. 3 UV–Vis spectral evidence of MoO(O2)2
Scheme 1 .
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Chemical Papers
oxidation of aniline to phenylhydroxyl amine followed by
the fast oxidation of the latter to nitrosobenzene which cou-
ples with aniline to yield azobenzene. But in the title reac-
tion, phenylhydroxyl amine is not an intermediate; oxidation
of phenylhydroxyl amine by SPB in the presence of Mo(VI)
is not fast. Experiments show that the rates of Mo(VI)-cat-
alyzed SPB oxidation of phenylhydroxyl amine and aniline,
under identical conditions, do not difer signifcantly (Fig.
S50).
relationship between ΔH# and ΔS# (Fig. 4a). The com-
pensation efect occurs when the free energy of activation
(ΔG#) within the reaction series does not vary signifcantly,
whereas the ΔH# and ΔS# do. Large experimental error also
leads to false compensation (Liu and Guo 2001). The com-
pensation efect and isokinetic efect are not synonymous,
and the occurrence of one does not imply the occurrence of
the other. The isokinetic relationship demands the existence
of common intersection point of Arrhenius lines of a series
of similar reactions. A simple method to evaluate the isoki-
netic relationship, suggested by Exner, is to plot the loga-
rithms of rate constants at two diferent temperatures against
each other. A linear plot implies a valid isokinetic relation-
ship and conversion of anilines into azobenzenes by SPB in
acetic acid with Mo(VI) as catalyst conforms to such a linear
relationship (Fig. 4b). The validity of the isokinetic relation-
ship implies operation of a common mechanism throughout
the reaction series. The observation that N-methylaniline,
N,N,-dimethylaniline and diphenylaniline conform to the
compensation law (Fig. 4a) and to the isokinetic relation-
ship (Fig. 4b) supports the suggested rate-limiting step, the
slow reaction of oxodiperoxomolybdenum(VI) with molecu-
lar anilines. As the fast reactions following the rate-limiting
one do not fall under the requisites of the theories of reaction
rates and linear free energy relationships, formation of dif-
ferent products does not preclude common mechanism up
to the rate-determining step.
Solvent specifcity
Well proved is the validity of linear salvation energy rela-
tionships (LSER) (Kamlet et al. 1983; Swain et al. 1983)
and numerous reactions were found to conform to LSER in
a variety of organic solvents (Vyas and Sharma 2002; Sar-
aswat et al. 2003). The Taft LSER (Kamlet et al. 1983) is:
log rate=A0 +pπ*+bβ+aα, where π* is the solvent polarity
for solvent–solute interaction of non-specifc type, β refers
to the solvent hydrogen-bond acceptor basicity and α repre-
sents the solvent hydrogen-bond donor acidity. Swain et al.
(1983) LSER equation is: log rate =aA+bB+C, where A
and B are anion and cation solvating power of the solvent,
respectively. As SPB is insoluble in most of the organic
solvents, the reaction was conducted by adding diferent
organic solvents to the reactants and catalyst dissolved in
acetic acid; the composition of acetic acid in the reaction
solution was 80% (v/v) and the added organic solvent was
20% (v/v). The stated solvent parameters of the reaction
medium, like the relative permittivity (dielectric constant),
were deduced adopting the principle of additivity (Table S3
in supplementary material). Analysis of the measured reac-
tion rates in binary solvents (presented in Table S4) in terms
of the stated LSERs shows no such correlation pointing out
the solvent-specifc nature of the title reaction.
Correlation of reactivities
Table 1 shows that the variation of the reaction rate fails
to commensurate with the very wide range of substituents
employed. That is, the electron-releasing or demanding
power of the substituents does not sufciently refect on the
spread of the reaction rate. Further, the specifc reaction rate
(k2) fails to conform to the usual Hammett relationship at
any of the temperatures studied. Figure 5a is the scattergram
resulting from Hammett’s correlation of rate at 35 °C. The
reaction center is likely to cross-conjugate with the para-
substituents in aniline and correlation of reaction rates of (1)
para-substituted and (2) meta-substituted anilines separately
using Hammett (σp or σm) or Brown-Okamoto (σ+p or σ+m) or
modifed Hammett (σp) relationship is unsuccessful (at any
of the temperatures measured). Analysis of the reaction rates
(at 20 or 35 or 50 °C) in terms of the dual substituent param-
eter (DSP) equations (para: σI, σR; σI, σ+R; σI, σ−R; F, R; meta:
σI, σR; σI, σR+; σI, σR−; F, R) (Shorter 1991) fails to provide
satisfactory correlation. The values of the substituent con-
stants used are listed in Tables S5 and S6 in supplementary
material. The Hammett relationship and its diferent modi-
fed forms are applicable to para- and meta-substituted ben-
zene derivatives but not to the ortho-compounds; besides the
well-known inductive and resonance efects associated with
Common mechanism
The structure of aniline, the substituent attached to the
ring, determines the reactivity and the structure–reactivity
relationships are well established (Hansch et al. 1991). The
efect of substituent on the rate of aniline to azobenzene con-
version by SPB in the presence of Mo(VI) has been studied
with about 50 para-, meta- and ortho-substituted anilines.
The activation parameters were calculated from the second-
order rate constants [k2 =rate/[aniline]0[Mo(VI)]] at 20, 35
and 50 °C using the Eyring relationship by the method of
least squares (Table 1). The conversion is not isoentropic; in
an isoentropic reaction series, only the enthalpy of activa-
tion (ΔH#) determines the reactivity. Neither the reaction is
isoenthalpic. The reactivity in an isoenthalpic series is solely
governed by the activation entropy (ΔS#). On the other hand,
compensation law exists (Liu and Guo 2001). It is the linear
1 3

Chemical Papers
Table 1 Second-order rate constants (k2), activation parameters and specifc reaction rates of molecular anilines (k†2) in acetic acid
Entry
Substituent
102 k2/M−1 s−1
ΔH#/kJ mol−1
−ΔS#/J K−1 mol−1
k†2/M−1 s−1
20
35
50 °C
20
35
50 °C
1
2
3
4
5
6
7
8
H
m-CH3
0.225
0.174
0.361
2.21
1.17
1.07
1.55
13.1
1.42
1.44
1.28
2.95
2.83
0.717
2.51
1.94
0.910
0.789
4.60
4.28
5.49
72.1#
4.74
4.12
5.21
14.7
6.19
3.23
7.41
10.4
5.57
3.25
76.7
81.5
68.8
88.8
69.7
52.5
71.3
88.7
58.2
81.5
69.9
78.0
84.9
72.4
33.5
19.0
56.6
−26.5
55.0
110
49.7
−13.0
87.7
21.8
50.2
23.4
7.54
50.2
0.279
0.277
0.182
1.43
0.0365
0.0621
0.00774
0.0246
0.0803
0.596
0.0215
1.77
1.45
1.70
0.784
8.48
0.172
0.177
0.0317
0.160
0.369
3.26
0.115
7.19
6.40
5.70
6.80
2.77
46.5
0.572
0.507
0.129
0.794
0.807
14.7
0.339
38.5
39.2
m-OCH3
m-OHa
m-Cl
m-Br
m-NO2
m-COOH
m-COCH3
m,p-(CH3)2
m,p-Cl2
0.303
0.505
0.312
0.454
0.616
0.131
0.471
0.478
0.197
0.186
insoluble
22.2
0.581
0.351
0.766
0.111
0.277
0.458
0.275
0.280
0.265
0.241
0.491
2.26
0.153
0.100
0.222
0.047
0.235
0.290
0.175
0.356
0.711
13.0
0.223
0.241
0.107
0.140
0.158
1.32
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
p-CH3
p-OCH3
p-OC2H5
p-SO3H
p-NH2
p-Cl
p-Br
p-I
p-NO2
p-COOH
p-COOC2H5
p-COCH3
p-NHCOCH3
p-SO2NH2
o,m-(CH3)2 (2, 3)
o,m-(CH3)2 (2, 5)
o,m-(OCH3)2 (2, 5)b
o,m-Cl2 (2, 3)
o,m-Cl2 (2, 5)
o,p-Cl2
o,o,p-Cl3
o,o,p-Br3
o-CH3
o-OCH3
o-OC2H5
o-OC6H5
o-OHc
1.39
1.02
4.31
17.7
–
3.18
1.75
2.69
0.415
0.938
1.17
0.761
1.61
1.28
1.02
1.55
8.68
0.579
0.376
0.785
0.249
0.758
1.14
0.987
1.02
1.93
–
–
–
0.965
0.185
0.0884
0.170
0.00132
0.00655
0.0119
0.00943
0.211
0.0353
0.375
0.519
–
1.02
–
4.11
1.96
2.14
0.232
0.0850
0.106
0.0820
4.90
0.474
6.75
6.66
12.9
7.78
9.64
1.94
3.59
4.08
2.39
6.49
3.56
4.34
6.29
31.0#
1.47
1.22
2.68
0.779
2.92
4.83
2.62
3.24
5.63
244
78.8
78.7
63.9
72.4
64.5
54.6
54.0
80.0
65.8
73.2
64.2
66.2
57.0
63.0
62.7
71.3
63.4
71.1
68.7
55.2
51.6
74.0
56.4
56.7
38.0
71.1
63.5
86.0
18.6
23.3
67.6
54.8
73.9
104
110
20.4
68.9
45.2
70.3
50.5
104
87.1
81.7
64.6
79.1
50.9
62.3
103
110
10.1
103
101
173
57.0
81.3
12.7
0.440
0.598
0.00495
0.0222
0.0304
0.0261
1.22
0.171
1.59
1.64
0.633
2.43
8.70
0.00232
0.00137
0.00383
0.000496
0.0027
0.256
0.185
0.329
0.0298
18500
0.00669
0.00642
0.00111
0.00256
0.00357
0.0338
0.00878
0.00516
0.0136
0.00263
0.00871
1.01
0.0223
0.0167
0.0463
0.00822
0.0336
4.27
1.04
2.77
2.99
0.942
0.0808
62800
0.0227
0.0239
0.00196
0.0101
0.0150
0.195
0.236
347000
0.0633
0.0610
0.00528
0.0427
0.0441
0.987
44.2
o-Cl
o-Br
o-NO2
o-COOH
o-COOCH3
o-NHd2
0.757
0.899
0.188
0.549
0.664
7.60
2.11
2.29
0.507
2.33
1.95
38.5#
o-SH
fast
–
4.73
0.444
0.263
o,o-(NO2)2
–NHCHe3
–N(CH3)2
–NHC6H5
–
0.144
49.1#
5.31
–
–
–
–
0.00144
107
23.1
15.9
1.63
0.857
58.8
62.5
60.0
69.5
76.5
89.9
10.3
1.93
0.00307
34.8
7.10
0.0100
2.85
0.0333
[SPB]0 =10 mM, [aniline]0 =0.10 M, [Mo(VI)]=1.0 mM
#Extrapolated values: a5.31×10−3 M−1 s−1 at 10 °C, b7.87×10−3 M−1 s−1 at 10 °C, d3.52×10−3 M−1 s−1 at 10 °C, e1.88×10−2 M−1 s−1 at 10 °C
c[aniline]0 =0.02 M
1 3

Chemical Papers
1
100
80
60
40
20
(a)
(b)
0
-1
-2
-3
k
-50
0
50
100
150
200
-4
-3
-2
-1
0
-ΔS#/J K-1 mol-1
log k2 (20° C)
Fig. 4 a The compensation efect. b The Exner plot
-1.4
1.5
0.5
(b)
(a)
-1.6
-1.8
k
k
-0.5
-1.5
-2.5
-2.0
-2.2
-2.4
-0.4
0
0.4
0.8
1.2
1.6
-0.4
0
0.4
0.8
1.2
1.6
σ
σ
Fig. 5 a The Hammett plot. b The Hammett plot employing rate constants of molecular anilines
the para- and meta-substituents, the ortho-substituents cause
steric efect. The determined ΔH# and ΔS# of the reactions of
all the para-, meta- and ortho-substituted anilines conform
to the isokinetic relationship (Fig. 4a). So are the measured
reaction rates at two extreme temperatures (Fig. 4b). This
indicates that the reaction mechanism of ortho-substituted
anilines is not diferent from that of para- and meta-substi-
tuted anilines and justifes the analysis of reaction rates of
ortho-substituted anilines in terms of the structure–reactivity
relationship. The reaction rates of ortho-substituted anilines
also fail to conform to Charton’s LDS (localized, delocal-
ized, steric) equation (Aslam et al. 1981); the carboxy, nitro
and methoxycarbonyl substituents individually take either
planar or orthogonal orientation, requiring appropriate steric
substituent constants (ν).
structure–reactivity relationship. In non-aqueous, basic
and neutral media, anilines stay as free bases (molecular
anilines), whereas in acidic medium, they exist in dual
form—free bases and conjugate acids. The acidity of the
medium and the pKa (of aniline) determine the relative abun-
+
dance of the two forms ([PhNH2]/[PhNH3 ]). So far, the
substrate-independent specifc reaction rates (=k0/[aniline]0
or k′/[aniline]0, where k0 and k′ are pseudo-zero-order and
pseudo-frst-order rate constants under the condition ani-
line concentration is in large excess over other reactants)
were deduced employing the total concentration of aniline
+
([PhNH2]+[PhNH3 ]=[aniline]T or [aniline]0) in the reac-
tion solution. Since the pKa varies from 5.36 (p-OCH3) to
−5.23 [o,o-(NO2)2] and molecular anilines are vulnerable
to oxidation than the anilinium ions, the reported k0/[ani-
line]0 and k′/[aniline]0 are not the real specifc reaction rates.
Hence, the so far reported structure–reactivity correlation
employing k0/[aniline]0 or k′/[aniline]0 is erroneous. The
Reactions of anilines in non-aqueous, basic, neutral
and acidic media (26 in number), as listed by Karuna-
karan and Kamalam (2002b), are reported to conform to
1 3

Chemical Papers
real specifc reaction rates are to be deduced using molecu-
lar aniline concentration ([PhNH2]) and not [aniline]0. The
concentration of free bases may be deduced from the acid
strength of the medium and the pKa values of the anilines
and acetic acid (Perrin 1965; Dean 1987; Buckingham et al.
1996). Although the pKa values are associated with aque-
ous solutions, an in depth examination shows that they may
be employed to deduce the concentrations of free bases in
glacial acetic acid. Coupling acetic acid-ionization equi-
librium with that of anilinium ion eliminates [H2O] and
[H3O+]. The ratio of ionization constants of acetic acid
and anilinium ion provides the equilibrium constant of the
reaction PhNH2 +HOAc↔PhH3+ +−OAc. In the absence
of water, self-ionization of HOAc is unlikely, and hence
p-H2NC6H4NH2 are at rapid equilibrium which supplies the
free base at a steady concentration. As the protonated amino
+
group (–NH3 ) is highly electron withdrawing, the ΔG# of
the reaction of p-+H3NC6H4NH2 is likely to be large leading
to slow conversion.
The specifc reaction rates of ortho-substituted molec-
ular anilines (free bases) have been analyzed in terms of
Charton’s LDS equation. The reaction of o-aminophenol
is fast; it is likely to enter into intra-molecular H-bonding.
Like m-OH, o-OH may also bind with MoO(O2)2 prior to
oxygen transfer to the amino group. In glacial acetic acid,
o-phenylenediamine, like p-phenylenediamine, may exist in
three forms, viz., diprotonated, monoprotonated and unpro-
+
tonated species. Calculation shows o-+H3NC6H4NH3 as
+
[PhNH3 ] = [−OAc]. Solving the said equations provides
predominant species (59.6%) and o-H2NC6H4NH2 in trace
(1.1%) with o-+H3NC6H4NH2 at 39.3%. The reaction rate of
o-+H3NC6H4NH2 has been deduced using its concentration
in glacial acetic acid. The specifc reaction rates of ortho-
substituted molecular anilines, excluding aminophenol and
phenylenediamine, satisfactorily conform to Charton’s LDS
equation (R2 =0.97–0.99); each dataset involves ten substitu-
ents, viz., methyl, methoxy, ethoxy, phenoxy, chloro, bromo,
nitro, carboxy, methoxycarbonyl and hydrogen. The con-
formity of the deduced reaction rates of molecular anilines
to LFER shows operation of a common mechanism in the
Mo(VI)-catalyzed oxidative dehydrogenative coupling of
anilines yielding azobenzenes using SPB as oxygen source
in acetic acid.
[PhNH2]. The specific reaction rates (k2† = k0/[PhNH2]
[Mo(VI)]), thus deduced and presented in Table 1, conform
to the usual Hammett relationship at all the temperatures
studied. Figure 5b is the Hammett plot at 35 °C (correla-
tion coefficient, r = 0.94, standard deviation, sd = 0.32,
number of data points, n=23, reaction constant, ρ=−2.22
at 20–50 °C). The plots involve the ordinary benzoic acid-
based σ values and include p-nitro, p-ethoxycarbonyl, and
p-carboxy substituents. In anilines, these substituents enter
into cross-conjugation with the reaction center and withdraw
electron strongly through resonance, necessitating the use
of σ−p (Fig. 5b).
m-Aminophenol exhibits very high reactivity and does
not conform to the structure–reactivity relationship. A pos-
sible reason is assistance of hydroxyl group in the reaction
process. It may be, initial binding of MoO(O2)2 with m-OH
function followed by the oxygen transfer. Attack on amino
nitrogen by MoO(O2)2 with neighbouring hydroxyl group
participation may also enhance the reactivity of aminophe-
nol. p-Phenylenediamine in glacial acetic acid may exist in
Conclusion
A large number of anilines are converted into azobenzenes
under mild conditions in good yield using cheap, stable,
innocuous, commercially available and industrial manu-
factured SPB with non-toxic Na2MoO4 in trace as catalyst.
Glacial acetic acid is the solvent of choice. This method is
commercially viable as it requires neither precious metal
nanoparticles as catalyst (the recovery of which is a chal-
lenge) nor uneconomical oxygen source for the reaction.
Furthermore, boric acid and molybdenum(VI), the reac-
tion wastes, are micronutrients for plants which address the
environmental concern. The determined kinetic orders of
the conversion along with the UV–visible spectroscopic and
cyclic voltammometric results suggest reaction of MoO(O2)2
with molecular aniline as slow and rate limiting. The enthal-
pies and entropies of activation of the oxidation of anilines,
about 50 in number, conform to the compensation efect
and the reaction rates to Exner’s relationship. While oxida-
tions of anilines yielding mixture of products are reported to
conform to LFER, the titled reaction rates, deduced adopting
the conventional method, fail to comply with the Hammett
+
three forms, viz., p-+H3NC6H4NH3 , p-+H3NC6H4NH2 and
p-H2NC6H4NH2. Calculation using the pKa (+1) value of
p-phenylenediamine, pKa of acetic acid and the acidity of
the medium shows that the concentration of the free base is
negligible (0.03%). Further, calculation using the pKa (+2)
value of the amine reveals that it exists largely in diproto-
nated form (77%) and signifcantly in monoprotonated form
(23%). The specifc reaction rate of p-+H3NC6H4NH2 was
obtained using its concentration in the reaction medium;
the method adopted for other anilines is not applicable to
the diamine as it exists in three forms. p-+H3NC6H4NH2
as the reactive substrate results in protonated amino group
+
(-NH3 ) as the substituent which is highly electron demand-
ing (σp = 0.60). The deduced specifc rate does not refect
the same, and fails to fit the Hammett line. A possible
reason is complication due to the parallel reaction of the
highly reactive free base (p-H2NC6H4NH2) present in trace
(0.03%) in the reaction medium. p-+H3NC6H4NH2 and
1 3

Chemical Papers
solvatochromic parameters, π*, α, and β and some methods for
simplifying the generalized solvatochromic equation. J Org Chem
48:2877–2887. https://doi.org/10.1021/jo00165a018
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lines (free aniline base), obtained by a modifed method,
conform to the usual Hammett linear free energy relation-
ship, indicating the faw in the method of calculation adopted
so far. Thus, the determined reaction constant allows predic-
tion of the true rate of conversion of any aniline into cor-
responding azobenzene using Na2MoO4/SPB in acetic acid
and enables to arrive at the observable experimental reaction
rate through reverse calculation.
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