Sodium perborate/NaNO2/KHSO4-triggered synthesis and kinetics of nitration of aromatic compounds

Article abstract of DOI:10.1007/s11164-018-3473-2

Sodium perborate (SPB) was used as efficient green catalyst for NaNO₂/KHSO₄-mediated nitration of aromatic compounds in aqueous acetonitrile medium. Synthesis of nitroaromatic compounds was achieved under both conventional and solvent-free microwave conditions. Reaction times were comparatively shorter in the microwave-assisted than conventional reaction. The reaction kinetics for nitration of phenols in aqueous bisulfate and acetonitrile medium indicated first-order dependence on [Phenol], [NaNO₂], and [SPB]. Reaction rates accelerated with introduction of electron-donating groups but retarded with electron-withdrawing groups. Kinetic results did not fit well quantitatively with Hammett’s equation. Observed deviations from linearity were addressed in terms of exalted Hammett’s constants (σˉ or σ_eff), para resonance interaction energy (ΔΔG_p) parameter, and Yukawa–Tsuno parameter (r). This term provides a measure of the extent of resonance stabilization for a reactive structure that builds up charge (positive) in its transition state. The observed negative entropy of activation (?ΔS^#) suggests greater solvation and/or cyclic transition state before yielding products.

Full text of DOI:10.1007/s11164-018-3473-2

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3473-2
Sodium perborate/NaNO2/KHSO4-triggered synthesis
and kinetics of nitration of aromatic compounds
K. C. Rajanna1
P. K. Saiprakash1
Suresh Muppidi1 Srinivas Pasnoori1
•
•
•
Received: 25 January 2018 / Accepted: 11 May 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Sodium perborate (SPB) was used as efficient green catalyst for NaNO2/
KHSO4-mediated nitration of aromatic compounds in aqueous acetonitrile medium.
Synthesis of nitroaromatic compounds was achieved under both conventional and
solvent-free microwave conditions. Reaction times were comparatively shorter in
the microwave-assisted than conventional reaction. The reaction kinetics for
nitration of phenols in aqueous bisulfate and acetonitrile medium indicated first-
order dependence on [Phenol], [NaNO2], and [SPB]. Reaction rates accelerated with
introduction of electron-donating groups but retarded with electron-withdrawing
groups. Kinetic results did not fit well quantitatively with Hammett’s equation.
Observed deviations from linearity were addressed in terms of exalted Hammett’s
constants (rꢀ or reff), para resonance interaction energy (DDGp) parameter, and
Yukawa–Tsuno parameter (r). This term provides a measure of the extent of res-
onance stabilization for a reactive structure that builds up charge (positive) in its
transition state. The observed negative entropy of activation (-DS#) suggests
greater solvation and/or cyclic transition state before yielding products.
Keywords Kinetics Á Mechanism Á Sodium perborate/sodium nitrite/bisulfate Á
Nitration Á Aromatic compounds Á Hammett’s equation Á para resonance
interaction energy (DDGp) parameter
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11164-
018-3473-2) contains supplementary material, which is available to authorized users.
&
K. C. Rajanna
kcrajannaou@yahoo.com
1
Department of Chemistry, Osmania University, Hyderabad, TS 500 007, India
123

K. C. Rajanna et al.
Introduction
Sodium perborate (SPB) is a crystalline, less toxic, inexpensive, and highly
stable boron hydrate with formula NaBO3ÁnH2O (n = 1–6). It is a peroxo salt, being
a highly useful reagent in organic synthesis and quite often used as a substitute for
highly unstable hydrogen peroxide solutions. It is soluble in water and releases
hydrogen peroxide, but is not merely a mixture of hydrogen peroxide and sodium
borate. It is a stable, ecofriendly, and effective reagent in organic synthesis [1–20].
Kabalka noted that H2O2 sometimes requires quite harsh conditions that are
incompatible with many functional groups, while SPB offers an interesting mild
alternative [4–8]. In another publication, Kabalka’s research group [9] revealed that
SPB/boron trifluoride etherate is an effective reagent in dry tetrahydrofuran under
chilled conditions (0 °C) for smooth hydroperoxide rearrangement of electron-rich
and highly substituted benzylic tertiary alcohols to phenols. McKillop and
coworkers observed that mixing SPB with acetic acid generates even more
powerful oxidation species [10]. SPB oxidizes organic sulfides [11], anilines
[12, 13], and indole [14] and enables uncatalyzed and Ru(III)-catalyzed oxidation of
5-oxoacids [15, 16] and oxidation of L-ascorbic acid [17]. SPB oxidizes organic
compounds such as aromatic aldehydes, iodoarenes, and various sulfur heterocycles
[18, 19]. Nitriles undergo smooth oxidative hydration to amides when aqueous
methanol is employed as solvent. Firouzabadi et al. accomplished [20] conversion
of several primary aromatic amines to corresponding nitrocompounds in good yield
with SPB in micellar media using catalytic amount of tungstophosphoric acid
(H3PWÁnH2O). Pilarski and coworkers observed that SPB is a principal component
of washing powders, and used it as an ecofriendly oxidant in palladium-catalyzed
C–H acyloxylation of alkenes with excellent regio- and stereoselectivity [21]. This
method enabled conversion of both terminal and internal alkenes, and even allowed
benzylic C–H oxidation. Over the years, numerous useful methods have been
developed for preparation of nitroaromatic compounds [22–26]. In a recent review,
Guobing and Minghua [27] elaborately discussed several conventional and
nonconventional synthetic protocols for nitration of aromatic compounds, including
nitration of aromatic hydrocarbons, arylboronic acids, aryl halides and pseudo-
halides, and aryl carboxylic acids, and oxidation of aryl primary amines and azides.
Our continued interest in development of ecofriendly synthetic protocols for
nitration reactions, and their kinetic and mechanistic aspects [28–37], prompted us
to pursue the present study into nitration of certain aromatic and heteroaromatic
compounds under conventional and microwave-assisted conditions, using sodium
perborate (SPB)/NaNO2, in presence of KHSO4. To gain insight into the mechanism
of nitration, we also carried out kinetic study in aqueous acetonitrile medium.
123

Sodium perborate/NaNO2/KHSO4-triggered synthesis and…
Experimental
General
Reagent-grade sodium perborate (SPB), potassium bisulfate (KHSO4), sodium
nitrite (NaNO2), and phenols were obtained from Avra, Aldrich, Merck, or SD Fine
Chemicals. High-performance liquid chromatography (HPLC)-grade acetonitrile
was used throughout for kinetic studies. Laboratory distilled water was further
purified over permanganate and distilled twice, while other organic solvents were
fractionally distilled before use. Isolated products of reactions were characterized by
spectroscopic methods and physical data such as melting/boiling points. Infrared
(IR) spectra were recorded on a PerkinElmer FT-IR spectrometer. 1H nuclear
magnetic resonance (NMR) spectra were recorded on a Varian VNMRS-300 MHz
spectrometer. Chemical shifts are reported in ppm relative to CHCl3 (7.26) with
tetramethylsilane (TMS) as internal standard. Mass spectra were recorded on a
ZAB-HS mass spectrometer using electrospray ionization (ESI). Melting points
were recorded using a Bu¨chi B-545 capillary melting point apparatus.
General procedure for synthesis of nitrocompounds
For nitration, constituent reactants such as sodium nitrite (12 mmol) were mixed
with aromatic or heteroaromatic compound (10 mmol), KHSO4 (1 mmol), and
catalytic amount of SPB (1 mmol). The mixture was refluxed at 60 °C for
appropriate time then treated with NaHCO3 solution and extracted with ethyl acetate
(30 mL). The organic layer was separated, dried over Na2SO4, and evaporated
under vacuum. Crude compound was purified by column chromatography using
binary mixture of ethylacetate and hexane as eluent. This procedure afforded
2-nitrophenols as major product (80 %) and 4-nitrophenol in smaller proportion
(20 %). These products were characterized by physical data and spectroscopic
methods: 1H NMR (300 MHz, CDCl3): d 9.56 (s, 1H, OH), 8.19 (dd, 1H,
J = 9.1 Hz, J = 7 Hz), 7.59 (s, 1H, J = 8 Hz J = 7.5 Hz) 6.99 (d, 1H,
J = 8.5 Hz) 9.18 (d, 1H, J = 8 Hz); m/z = 139; melting point = 45–49 °C). The
same regioselective change was observed when aniline underwent nitration,
affording para 4-nitroaniline as major component (80 %), whereas 2-nitroaniline
was observed in only smaller proportion (20 %).
Microwave (MW)-assisted procedure for synthesis of nitrocompounds
Reactants such as sodium nitrite (12 mmol), with aromatic or heteroaromatic
compound (10 mmol), KHSO4 (1 mmol), and SPB (1 mmol), were thoroughly
mixed with silica gel and placed in MW oven. Reaction progress was monitored by
thin-layer chromatography (TLC) until completion. The reaction mixture was then
treated with NaHCO3 solution and extracted with ethyl acetate (30 mL). Final
product was obtained by using the same workup steps as detailed in the preceding
section. Sodium perborate/NaNO2/KHSO4-initiated nitration reactions with several
123

K. C. Rajanna et al.
other compounds also afforded corresponding nitroderivatives (Scheme 1) via both
conventional and MW-assisted reactions.
Method of following reaction kinetics
Stock solutions required for each set of experiments were always prepared fresh and
stored in amber-colored bottles in a refrigerator. Requisite amounts of each reagent
were pipetted out into reaction flasks, and the contents thermally equilibrated at
desired temperature for about 20 min in a constant-temperature bath with accuracy
of 0.1 °C. Reaction flask A contained requisite amounts of aqueous SPB and
bisulfate, while flask B contained required amounts of phenol and nitrite. The
reaction was initiated by rapid addition of thermally equilibrated flask A (perborate,
bisulfate) to flask B solution. Progress of the reaction kinetics was followed
spectrophotometrically at 405 nm, because most of the nitrophenols absorb at
around 400 nm. Aliquots of reaction mixture were withdrawn into matched cuvette
of 1 cm length and placed in the thermostatic cell compartment. Absorbance (or
optical density, OD) of nitroaromatic compound (product) at given time t is defined
as (At) while A0 is the absorbance (if any) before reaction onset and the absorbance
at infinite time (at the end of reaction) is A?, thus (A? - At) is proportional to
(a - x), the concentration of reactant at any given time, while (A? - A0) is
proportional to the initial concentration of the reactant (a).
Salient kinetic features of reaction
The reaction kinetics was followed under isothermal conditions, using requisite
amounts of phenol, SPB, and NaNO2 in aqueous acetonitrile media. In all
experiments, [NaNO2] was taken in large excess over [SPB], i.e., [NaNO2]0 [[ [-
SPB]0, in aqueous acetonitrile medium, so that in situ-generated [NO?2 ] was equal
to [SPB]0. Under pseudoconditions ([Phenol] = [NaNO2]0 [[ [SPB]0), and kinetic
plots of ln[(A? - A0)/(A? - At)] versus time passing through the origin indicated
first order in [SPB] (Figs. 1, 2). The observed first-order rate constant, k0, was
evaluated from the slope of these plots. The plot of k0 versus [Substrate] was linear,
passing through the origin (under otherwise similar conditions), indicating first-
order dependence on [Substrate]. Results for phenol, p-cresol, and p-bromophenol
as typical examples are shown in Fig. 3. On the other hand, plots of 1/(A? - At)
versus time were found to be linear with positive gradient and definite intercept on
Scheme 1 Nitration of aromatic compounds under different conditions
123

Sodium perborate/NaNO2/KHSO4-triggered synthesis and…
0.45
(c) Catechol;k' = 0.0196
R² = 0.9992
0.4
0.35
0.3
(b) Phenol;k' = 0.012
R² = 0.9956
0.25
0.2
0.15
0.1
(a) p-Cresol; k' = 0.008
R² = 0.9978
0.05
0
0
5
10
15
20
25
Time(min)
Fig. 1 Pseudo-first-order plot for nitration of some phenols. 103[Substrate] = 5.0 mol/dm3;
104[SPB] = 5.0 mol/dm3; 103[NaNO2] = 5.0 mol/dm3; 103[KHSO4] = 0.010 mol/dm3; MeCN (%
V/V) = 5.0; temperature = 308 K. Substrate = (a) p-cresol, (b) phenol, (c) p-catechol
10
p-Cresol
8
6
4
2
0
R² = 0.997
Phenol
R² = 0.999
0
0.01
0.02
0.03
0.04
0.05
0.06
[Substrate] (mol/dm3)
Fig. 2 (k0) versus [Sub] in SPB/NaNO2-mediated nitration reaction of phenols. 104[SPB] = 5.0 mol/
dm3; 103[NaNO2] = 5.0 mol/dm3; 103[KHSO4] = 0.010 mol/dm3; MeCN (% V/V) = 5.0;
temperature = 303 K
0.25
p-Br phenol;
R² = 0.996
0.2
0.15
0.1
0.05
0
0
0.01
0.02
0.03
0.04
0.05
0.06
[Substrate] (mol/dm3)
Fig. 3 (k0) versus [Sub] in SPB/NaNO2-mediated nitration reaction of phenols. 104[SPB] = 5.0 mol/
dm3; 103[NaNO2] = 5.0 mol/dm3; 103[KHSO4] = 0.010 mol/dm3; MeCN (% V/V) = 5.0;
temperature = 303
123

K. C. Rajanna et al.
ordinate (vertical axis) under pseudo-second-order conditions ([NaNO2]0 [[ [-
SPB]0 = [S]0). These observations suggest that the reaction follows overall second-
order kinetics (Fig. 4). Second-order rate constants were obtained from (k0/[S]),
because k0 = k[S] under pseudoconditions. The rate constants measured were found
to be reproducible to 3 to 5 %. Correlation coefficients (R2) of the linear
regressions were usually higher than 0.990.
Effect of [Additives]
To provide insight into the mechanism and rate law, the rate dependences on [SPB],
[NaNO2], and [KHSO4] were also investigated. Rate data (Table 1) revealed that
variation in [KHSO4] (0.005–0.050 mol/dm3), [NaNO2] (0.001–0.010 mol/dm3),
and [SPB] (0.001–0.002 mol/dm3) under otherwise similar conditions did not affect
the rate to any significant extent. Together, these observations probably indicate that
SPB is a stoichiometric reagent that generates nitronium ion in situ in presence of
mild acid obtained from dissociation of bisulfate anion (HSO-4 ). Furthermore, the
constancy in the k0 values with increase in [SPB] confirms the unity order with
respect to [SPB].
Test for free radicals
To detect whether free radical intermediates were formed in situ during the course
of the reaction, olefin monomers such as freshly prepared acrylamide or deareated
acrylonitrile were added to the reaction mixture under nitrogen atmosphere. Sodium
perborate (SPB) did not initiate/induce polymerization of added olefinic monomers
even after 24 h under reflux conditions, indicating absence of free radical
12
(b)Phenol
y = 0.0144x + 6.5238
R² = 0.9883
10
8
6
4
(a) p-Chlorophenol
y = 0.0034x + 3.7465
R² = 0.9885
2
0
0
50
100
150
200
250
300
Tim(min)
Fig. 4 Second-order plots for some phenols. 104[Substrate] = 5.0 mol/dm3; 104[SPB] = 5.0 mol/dm3;
103[NaNO2] = 5.0 mol/dm3; 103[KHSO4] = 5.0 mol/dm3;
MeCN (% V/V) = 5.0;
temperature = 303 K: (a) p-chlorophenol, (b) phenol
123

Sodium perborate/NaNO2/KHSO4-triggered synthesis and…
Table 1 Effect of variation of [Additives] on pseudo-first-order rate constant (k0) in SPB/NaNO2-me-
diated nitration reaction of phenol. 103[Phenol] = 5.0 mol/dm3; MeCN (% V/V) = 5.0;
temperature = 303 K
104[SPB] (mol/ dm3)
103[NaNO2] (mol/ dm3)
[KHSO4] (mol/ dm3)
102k0 (min)
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
10.0
15.0
20.0
5.00
5.00
5.00
5.00
5.00
5.00
1.00
2.00
4.00
8.00
10.0
20.0
20.0
20.0
0.005
0.010
0.015
0.020
0.025
0.050
0.010
0.010
0.010
0.010
0.010
0.010
0.010
0.010
0.900
0.786
0.872
0.860
0.875
0.887
0.920
0.905
0.867
0.912
0.915
0.867
0.912
0.915
intermediates. To provide further insight into this aspect, we conducted kinetic
experiments separately in presence of efficient radical scavengers such as mercuric
chloride (HgCl2) and allyl acetate. Addition of these additives over a wide
concentration range (Table 2) did not alter the rate constant (k0) of nitration to any
significant extent. If radical intermediates were predominant, the rate of nitration
would have decreased, because active free radicals should have been trapped by the
scavengers. More particularly, HgCl2 (mercuric chloride) should have been reduced
Table 2 Effect of variation of [Radical Scavengers] on pseudo-first-order rate constant (k0) of SPB/
NaNO2-mediated nitration reaction of phenol. 103[Phenol] = 5.0 mol/dm3; 104 [SPB] = 5.00 (mol/dm3);
103 [NaNO2] = 8.00 (mol/dm3); [KHSO4] = 0.010 (mol/dm3); MeCN (% V/V) = 5.0;
temperature = 303 K
Radical scavenger
103[Radical Scavenger] (mol/dm3)
102k0/min
Normal reaction
HgCl2
Nil
0.912
0.905
0.898
0.926
0.902
0.925
0.912
0.910
0.904
1.00
2.00
4.00
8.00
1.00
2.00
4.00
8.00
Allyl acetate
123

K. C. Rajanna et al.
to HgCl (mercurous chloride), forming a precipitate. We did not observe any such
precipitate. This observation may further lend support to our contention that
nitration occurs through an ionic path but not by a free radical mechanism.
Results and discussion
Data in Table 3 show that phenols underwent nitration smoothly and gave
corresponding nitration products in excellent yield, using both conventional and
microwave-assisted (MWA) protocols. However, the reactions using the conven-
tional protocol required longer reaction time. This difference can be explained by
considering the fractions of reactive species which are activated under the
conventional and MWA protocols. When using the MWA protocol, the fraction of
activated species is far greater than obtained in the conventional reaction. This can
be attributed to the instantaneously increased high temperature in MWA reactions,
supplied directly to all reactant species without any heat loss. Such an increase in
temperature is not possible in conventional thermal reactions, as explained in
literature reports [36, 37].
Reactive species and mechanism of nitration
Literature reports on X-ray crystallographic studies reveal that the structure of
sodium perborate (SPB) is Na2XÁ6H20 [38, 39], exhibiting two Na? ions, six water
molecules, and the centrosymmetric cyclic anion, X2- (where X2- = [B2(O2)2(-
OH)4]2-). In the anion, boron atoms are joined by two peroxo bridges and their
Table 3 SPB/NaNO2/KHSO4-mediated nitration of organic compounds
Entry
Substrate
Product
Conventional
MWAS
R.t. (s)
R.t. (h)
Yield (%)
Yield (%)
1
Phenol
2-NO2 Phenol
7
7
68
70
75
65
75
75
70
74
70
75
75
75
75
90
70
70
70
75
70
75
75
78
75
78
82
75
82
75
2
o-Cresol
2-Me-4-NO2 Phenol
2-NO2 4-Me Phenol
3-Me-4-NO2 Phenol
4-NO2 2-Cl Phenol
4-NO2 3-Cl Phenol
2-NO2 4-Cl Phenol
2-NO2 4-Br Phenol
2,4-Dinitrophenol
3
p-Cresol
7
70
4
m-Cresol
7
75
5
o-Cl Phenol
m-Cl Phenol
p-Cl Phenol
p-Br Phenol
p-Nitrophenol
Catechol
8
90
6
8
100
90
7
8
8
8
120
180
65
9
10
7
10
11
12
13
4-Nitrobenzene-1,2-diol
2-Nitrobenzene-1,3-diol
2-Nitrobenzene-1,4-diol
2-OH 4-NO2 Benzoic acid
Resorcinol
Quinol
7
75
7
65
Salicylic acid
7
120
123

Sodium perborate/NaNO2/KHSO4-triggered synthesis and…
tetrahedral coordination is completed by terminal OH groups [40–45]. In aqueous
acid medium, SPB may exist as perborate anion, perboric acid, and H2O2. Earlier, it
was also mentioned that sodium perborate is not peroxoborate but borate
peroxyhydrate; H2O2 acquires stronger electrophilicity by coordination with boric
acid, which acts as a Lewis rather than protic acid. Extensive conductometric and
cryoscopic investigations [44, 45] on borate—H2O2 systems revealed that H2O2 is
associated with borate, thus supporting the above contention. It is also interesting to
note [36, 37] that perborate exists as monomer, in solution of below 0.05 mol/dm-3
concentration in [B(OH)4(H2O2)]- and/or [B(OH)3(H2O2)] forms of SPB, according
to the following dissociation equilibrium:
Â
Ã
Â
ÃÀ
Kh
BðOHÞ3ðH2O2Þ þ H2O ꢀ BðOHÞ4ðH2O2Þ þHþ
ð1Þ
The hydrolytic constant (Kh) of the above step was found to be in the range of
10-8 [40–43]. NMR and Raman spectroscopy indicate that, in dilute solution, an
equilibrium exists that still contains peroxoborate anions [40–43]. This equilibrium
is faster than the NMR time scale and is detected by a downfield shift in the time-
averaged B-peak for the whole system [8]. In the present study, the reactions were
conducted in presence of protic KHSO4 salt, which dissociates easily to release H?
according to the following equilibrium:
Kd
HSOÀ4 ꢀ Hþ þ SOÀ4 2
ð2Þ
ð3Þ
K1
Hþ þ NOÀ2 ꢀ HNO2
The dissociation constant (Kd) of (HSO-4 ) is about 10-2, and the Kd/Kh ratio is
about 106, i.e., Kd/Kh = 106. Therefore, hydrolysis of [H3BO3(H2O2)] can be
suppressed in presence of KHSO4. In otherwards equilibrium (1) shifts to the left,
suggesting formation of [B(OH)3(H2O2)] or [H3BO3(H2O2)] during the course of
the reaction. The reactive species [B(OH)3(H2O2)] thus formed in turn might
oxidize nitrite (NO-2 ) and generate in situ transient nitronium ion (NO?2 )
electrophile through the following sequence of steps:
Â
Ã
Â
Ã
K2
BðOHÞ ðH2O2Þ þ HNO2 ꢀ NOþ þ BðOHÞ Àþ H2O
ð4Þ
3
2
4
In presence of H?, transient HNO3 might generate active nitronium (NO?2 )
species responsible for electrophilic nitration of aromatic compounds
NOþ2 þ RÀC6H4ÀXSlo!wðk1ÞRÀC6H3ÀXÀðNO2Þ þ Hþ
ð5Þ
For the above sequence of mechanistic steps (2–5), the rate law can be derived
according to the following steps:
Â
Ã
Rate ¼ Àd½RÀC6H4ÀXŠ=dt ¼ k1 NOþ2 ½RÀC6H4ÀXŠ
ð6Þ
123

K. C. Rajanna et al.
Â
Ã
Â
Ã
Â
ÃÀ
From step (4Þ NOþ2 ¼ K2 BðOHÞ3ðH2O2Þ ½HNO2Š= BðOHÞ4
However, from step (3), in situ-produced [HNO2] = K1[H?][NO2-]; substituting
for [HNO2] in Eq. (7) gives
ð7Þ
Â
Ã
Â
Ã
NOþ2 ¼ K1K2½SPBŠ NOÀ2 ½HþŠ=½B0Š
ð8Þ
À
Â
Ã
Â
ÃÀÁ
Where½SPBŠ ¼ BðOHÞ3ðH2O2Þ ; ½B0Š ¼ BðOHÞ4
Now, [H?] can be obtained from the dissociation step of (HSO-4 ) as
À
Â
ÃÁ Â
Ã
½HþŠ ¼ Kd HSO4À = SO4À2
Upon further substitution of [H?] from Eq. (9) into Eq. (8), followed by proper
simplification, active species [NO?2 ] can be obtained as
ð9Þ
Â
à Â
Ã
KdK1K2½SPBŠ NOÀ2 HSO4À
Â
Ã
NOþ2
¼
ð10Þ
t
Â
Ã
½B0Š SOÀ4 2
Substituting the active species [NO2?] from Eq. (10) into Eq. (6), the final rate
law can be written as
Â
à Â
Ã
k1KdK1K2½SPBŠ NOÀ2 HSO4À ½RÀC6H4ÀXŠ
t
t
t
Â
Ã
Rate ¼ Àd½RÀC6H4ÀXŠ=dt ¼
½B0Š SOÀ4 2
ð11Þ
The above rate equation is in accordance with the observed kinetic results, viz.
first order in [Substrate] (i.e., R–C6H4–X) and [SPB]. Since [NO-2 ]t is taken in large
excess over [SPB]t and [KHSO4], it implies that the order in [active NO2? species]
is also one. The bisulfate term ([HSO-4 ]) in the numerator of the rate law is canceled
by the sulfate (SO-4 2) term in the denominator, thus accounting for the observed
marginal (HSO4-) effect on the reaction rate. At constant (HSO-4 ) concentration and
known excess, the rate law reduces to
Â
Ã
Rate ¼ Àd½RÀC6H4ÀXŠ=dt ¼ k NOþ2 ½RÀC6H4ÀXŠ
Â
Ã
À
Â
à Â
ÃÁ À
Â
ÃÁ
ðWhere NOþ2 ¼ KdK1K2½SPBŠ NO2À HSO4À = ½B0Š SO4À2
t
t
On the basis of the foregoing discussion, the sequential mechanistic steps for the
nitration of aromatic compounds in this study can be summarized as shown in
Scheme 2.
Temperature effect on rate of nitration
The SPB-catalyzed KHSO4/NaNO2-triggered nitration of various aromatic com-
pounds was studied at different temperatures in the range of 300–325 K. The free
energy of activation (DG#) at various temperatures was calculated for the second-
123

Sodium perborate/NaNO2/KHSO4-triggered synthesis and…
Scheme 2 Mechanism of nitration of aromatic compounds
order rate constant (k) using Eyring’s theory of reaction rates [40–43] using
Eyring’s equation according to the following steps:
DG# ¼ RT ln ðRT=NhkÞ
Substituting R = 8.314 J mol-1 K-1, N = 6.022 9 1023 mol-1, h = 6.626 9
10-34 J s gives
Â
À
ÁÃ
ðR=NhÞ ¼ 8:314= 6:022 Â 1023 Â 6:626 Â 10À34 ¼ 2:0836 Â 1010
À
Á
lnðR=NhÞ ¼ ln 2:0836 Â 1010 ¼ 0:7341 þ 23:03 ¼ 23:7641
DG# could be simplified as: DG# ¼ 8:314T½23:7641 þ lnðT=kފ
Proper substitution of the temperature (T) in Kelvin and second-order rate
constant (k), DG# is obtained in J/mol, which in turn can be easily converted to
kJ/mol by dividing by 1000. The enthalpy and entropy of activation (DH# and DS#)
were obtained from the slope and intercept values of the Gibbs–Helmholtz plot of
DG# versus temperature (T) (Fig. 5), according to the following relationship:
DG# ¼ DH# À TDS#:
Activation parameters for the nitration of various phenols are compiled in
Table 4.
Quantitative structure and reactivity study
Closer inspection of the kinetic data pertaining to the SPB-catalyzed nitration of
phenols reveals that introduction of electron-donating groups (EDG) into the
aromatic ring accelerated the rate of nitration, while electron-withdrawing groups
123

K. C. Rajanna et al.
79000
78000
77000
76000
75000
74000
73000
72000
(C)o-Cl Phenol;R² = 0.993
(B) p-Cresol;R² = 0.984
(A) Phenol;R² = 0.987
300
305
310
Temp (K)
315
320
325
Fig. 5 Gibbs–Helmholtz plot (DG# versus temperature) for nitration of some phenols.
Substrate = (a) phenol, (b) p-cresol, (c) o-Cl phenol
(EWG) retarded the reaction rate with a decreasing trend: m-Me [ p-MeO [ -
H [ p-Me [ m-OH [ & p-Br & p-OH [ p-Cl. On the other hand, ortho-substi-
tuted phenols indicated the following sequence: o-OH [ o-Me [ -H. The obtained
data were cast into Hammett’s quantitative structure and reactivity relationship
[46, 47] with a view to analyze the kinetic data quantitatively, which in turn would
be useful to understand the reaction mechanism.
log(kÞ ¼ logðk0Þ Æ rq:
According to Hammett, a plot of log(k) versus r (Hammett’s substituent
constant) should show a straight line with either positive or negative slope (q;
Hammett’s rho). A negative q value indicates that electron-donating substituent
accelerates the reaction rate, with electron flow away from aromatic ring in the rate-
determining step, thus producing electron deficiency (often a positive charge) in the
activated complex. In the present study, Hammett’s plots of log(k) versus r
indicated poor linear relationships with very low correlation coefficient (R2) and
negative slope value with (q \ 0), indicating attack of an electrophile on the
aromatic ring. Closer inspection of the data revealed that para-substituted
derivatives scattered from linearity. The observed deviations can be explained by
the mesomeric para interaction energy (DDGp) parameter. For this purpose, the
effective sigma (rꢀ or reff) values were calculated as suggested by Brown, Okamoto,
van Bekkum, Webster, and others [48–55].
ðrꢀ or reffÞ ¼ ðlog k À log k0Þ=qm;
where qm is the modified reaction constant obtained for the Hammett’s plot after
removing the scattered points.
123

Sodium perborate/NaNO2/KHSO4-triggered synthesis and…
Table 4 Second-order rate constants and activation parameters for nitration of phenols
Substrate
Temp.
(K)
k (dm3/mol/ DG#
Equation and R2
DH# -DS# (J/
(kJ/mol) K/mol)
s)
(kJ/mol)
Phenol
303
308
313
318
323
303
308
313
318
323
303
308
313
318
323
303
308
313
318
323
303
308
313
318
323
303
308
313
318
323
303
308
313
303
308
313
323
303
308
313
323
1.80
2.41
2.60
2.82
3.02
1.23
1.62
1.81
2.20
3.01
0.400
0.601
0.800
1.20
1.60
2.41
2.80
3.21
3.02
3.21
2.60
3.62
5.00
6.61
8.61
0.402
0.602
0.801
1.02
1.22
0.140
0.151
0.200
1.02
1.21
1.61
2.21
3.02
3.81
5.22
8.61
72.8
73.3
74.3
75.3
76.4
73.8
74.3
75.2
76.0
76.4
76.6
76.8
77.4
77.6
78.1
72.0
72.9
73.8
75.2
76.2
71.8
72.2
72.6
73.0
73.5
76.6
76.8
77.4
78.0
78.8
79.2
80.4
80.0
74.2
75.0
75.6
77.2
71.5
72.1
72.5
73.5
y = 185.2x ? 16,428 16.43
R2 = 0.987
185.2
136.3
75.0
p-Cresol
y = 136.3x ? 32,455 32.45
R2 = 0.985
p-Bromophenol
y = 75.00x ? 53,797 53.80
R2 = 0.988
p-
y = 211.5x ? 7792
R2 = 0.990
7.79
211.5
84.59
115.5
Methoxyphenol
m-Cresol
y = 84.59x ? 46,178 46.18
R2 = 0.996
Resorcinol
y = 115.5x ? 41,358 41.36
R2 = 0.970
m-Chlorophenol
o-Cresol
y = 177.0x ? 25,659 25.66
R2 = 0.966
177.0
141.6
y = 141.6x ? 31,329 31.33
R2 = 0.988
Catechol
y = 101.7x ? 40,677 40.68
R2 = 0.996
101.7
123

K. C. Rajanna et al.
Table 4 continued
Substrate
Temp.
(K)
k (dm3/mol/ DG#
Equation and R2
DH# -DS# (J/
(kJ/mol) K/mol)
s)
(kJ/mol)
o-Chlorophenol
303
308
313
323
0.601
0.803
1.04
75.5
76.1
76.8
77.7
y = 109.7x ? 42,296 42.30
R2 = 0.993
109.7
1.83
ÀDDGp ¼ 2:3RTqmðrꢀ À rÞ:
The DDGp value indicates a less pronounced electron-withdrawing effect
between the substituent and reaction center in the transition state than predicted. The
Yukawa–Tsuno equation [46, 47] is one of the best modifications to the Hammett
equation, quantifying the role of enhanced resonance effects on the reactivity of
meta- and para-substituted aromatic compounds. This equation explains the
enhanced resonance effects in electrophilic reactions of para- and meta-substituted
organic compounds by introducing a new term (r) into the original Hammett
relation. The Yukawa–Tsuno equation can take the following forms:
logk ¼ logk0 þ q½r þ rðrþ À rފ;
logk ¼ logk0 þ q½r þ rðrÀ À rފ;
where kX and k0 represent the rate constants for an X-substituted and unsubstituted
compound, respectively, q represents the Hammett reaction constant, r represents
the Hammett substituent constant, r? and r- represent the Hammett substituent
constants for reactions in which positive or negative charge is built up at the reactive
center, respectively, and r represents the Yukawa–Tsuno parameter. This term
provides a measure of the extent of resonance stabilization for a reactive structure
that builds up charge (positive) in its transition state. The negative entropy of
activation (-DS#) suggests greater solvation and/or cyclic transition state before
yielding products (Table 5).
Conclusions
Sodium perborate (SPB) was investigated as efficient green catalyst for nitration of
aromatic compounds (phenols) using NaNO2/KHSO4. The reaction followed
second-order kinetics with first-order dependence on [SPB] and [Phenol]. Closer
inspection of the kinetic data revealed that the reaction is sensitive to the structural
variation of phenol. Reaction rates accelerated with introduction of electron-
donating groups but retarded with electron-withdrawing groups in the following
order: m-Me [ p-MeO [ -H [ p-Me [ m-OH [& p-Br & p-OH [ p-Cl. On the
other hand, ortho-substituted phenols indicated the following sequence: o-Me [o-
123

Sodium perborate/NaNO2/KHSO4-triggered synthesis and…
Table 5 Exalted Hammett’s constants and mesomeric para interaction energy (DDGp) parameter
Temp. Substituent Effective sigma (rꢀ or
reff
Hammett’s sigma
(r)
DDGp
(kJ/mol)
Y–T parameter
(r)
)
303 K p-OMe
p-Me
- 0.043
0.060
- 0.27
- 0.17
0.43
- 3.85
- 3.90
3.50
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
p-Br
0.224
p-Cl
0.327
0.23
- 1.64
- 1.00
- 4.67
- 4.23
4.49
p-OH
0.224
- 0.37
- 0.27
- 0.17
0.43
308 K p-OMe
p-Me
- 0.021
0.056
p-Br
0.190
313 K p-OMe
p-Br
- 0.029
0.164
- 0.27
0.43
- 4.51
- 4.98
- 5.02
8.76
318 K p-Me
p-Br
0.025
- 0.17
0.43
0.088
323 K p-Me
p-Br
0.0003
0.064
- 0.17
0.43
-4.57
9.83
OH [ -H. However, the results did not fit well with Hammett’s quantitative linear
free energy relationship. Deviations were suitably addressed by calculating exalted
Hammett’s constants (rꢀ or reff), mesomeric para interaction energy (DDGp), and
Yukawa–Tsuno parameters (r). The Yukawa–Tsuno parameter (r) provides a
measure of the extent of resonance stabilization for a reactive structure that builds
up charge (positive or negative) in its transition state.
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