Reaction of aniline with ammonium persulphate and concentrated hydrochloric acid: Experimental and DFT studies

Chemical Papers 66 (7) 699–708 (2012)
DOI: 10.2478/s11696-012-0163-1
ORIGINAL PAPER
Reaction of aniline with ammonium persulphate and concentrated
hydrochloric acid: Experimental and DFT studies
Maciej Przybylek*, Jerzy Gaca
Department of Chemistry and Environmental Protection, Faculty of Chemical Technology and Engineering,
University of Technology and Life Sciences, 85-326 Bydgoszcz, Poland
Received 31 October 2011; Revised 11 February 2012; Accepted 14 February 2012
In this paper, the reaction of aniline with ammonium persulphate and concentrated HCl was
studied. As a result of our experimental studies, 2,4,6-trichlorophenylamine was identified as the
main product. This shows that a high concentration of HCl does not favour oxidative polymerisation
of phenylamine, even though the ammonium persulphate/HCl system is widely used in polyaniline
synthesis. On the basis of the experimental data and density functional theory for reaction path
modelling, we proposed a mechanism for oxidative chlorination of aniline. We assumed that this
reaction proceeded in three cyclically repeated steps; protonation of aniline, formation of singlet
ground state phenylnitrenium cation, and nucleophilic substitution. In order to confirm this mech-
anism, kinetic, thermochemical, and natural bond orbital population analyses were performed.
c 2012 Institute of Chemistry, Slovak Academy of Sciences
Keywords: oxidative chlorination, 2,4,6-trichloroaniline, DFT
Introduction
The presence of oxidants, Cl−
and many differ-
ent kinds of organic compounds in the reaction mix-
ture can contribute to the formation of chloro–organic
species. Oxidative chlorination reactions have a great
significance in modern organic synthesis and the nat-
ural environment (Sasson, 1995; Gribble, 2003). In
general, the mechanisms for the formation of chloro–
organic compounds are very diverse because there are
many kinds of chlorinating agents such as Cl−
, Cl2,
HCl, HOCl, Cl.. This is why so many organic species
can be converted into chloro–organic compounds.
The reaction mechanism can be quantitatively and
qualitatively described by means of experimental and
theoretical tools. Probably mainly due to the low com-
putation cost, density functional theory (DFT) calcu-
lations are very common in reaction path modelling.
Nevertheless, the results of studies of reactions ob-
tained with the use of DFT methods are often in very
good agreement with experimental data (Zheng et al.,
2010; Szaleniec et al., 2006; Dračínský et al., 2010).
When considering the reaction of aromatic amine
with peroxides and aqueous HCl (HClaq), two main
pathways should be generally taken into account:
oxidative chlorination and oxidative polymerisation.
It is commonly known that aniline is very unstable
in the presence of oxidants. The ammonium persul-
phate ((NH4)2S2O8)/HClaq system is widely used in
polyaniline synthesis (Chiang & MacDiarmid, 1986;
MacDiarmid et al., 1987, 1991; Mattoso et al., 1994).
However, some of the N- and C-substituted aniline
derivatives can be chlorinated by Cl−
in the pres-
ence of peroxides without yielding large amounts
of oxidation by-products, e.g. oxidative chlorina-
tion of N-phenylacetamide (acetanilide) is an effec-
tive method of obtaining 2,4,6-trichlorophenylamine
(2,4,6-trichloroaniline, 2,4,6-TCA) (Gaca & ˙Zak,
1997). It is worth noting that C-substituted pheny-
lamine (aniline) derivatives can be chlorinated with
a good yield (up to 80 %) in the presence of
HClaq and H2O2 (Vyas et al., 2003). Studies on
the oxidative chlorination of p-nitrophenylamine (p-
nitroaniline) with the use of many different peroxides
*Corresponding author, e-mail: przybylek@mail.utp.edu.pl
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700 M. Przybylek, J. Gaca/Chemical Papers 66 (7) 699–708 (2012)
like M2S2O8 (where M = NH4, Na, K) and H2O2
showed that the yield of chlorinated products is the
highest when the reaction proceeds in the presence of
(NH4)2S2O8 (Kowalska & Gaca, 2002).
It is commonly accepted that the oxidative poly-
merisation of aniline is usually carried out in diluted
HClaq (Chiang & MacDiarmid, 1986; MacDiarmid et
al., 1987, 1991; Mattoso et al., 1994). To the best of
our knowledge, there is no information in the litera-
ture about the reaction of non-substituted aniline with
(NH4)2S2O8 in the presence of concentrated HClaq.
The purpose of this paper is to contribute towards fill-
ing this gap by presenting the results of our empirical
and theoretical studies.
Theoretical
All DFT calculations reported in this paper were
performed with the use of a Gaussian03 computer
software package (Frisch et al., 2004). The natural
bond orbital (NBO) charge distribution and opti-
mised molecular structures presented here were visu-
alised using GaussView 3.0 program (Dennington et
al., 2003).
Geometry optimisation, single point, and frequen-
cies calculations for all reactants, intermediates, tran-
sition states, and product were performed using the
Becke three-parameter (B3) hybrid method (Becke,
1988, 1993) with the Lee–Yang–Parr (LYP) func-
tional (Lee et al., 1988) and 6-311++G(2d,p) basis
set (McLean & Chandler, 1980; Krishnan et al., 1980;
Frisch et al., 1984; Clark et al., 1983). In order to
evaluate the reliability of the B3LYP method, addi-
tional G3MP2 (Curtiss et al., 1998) calculations were
performed for selected steps of the aniline oxidative
chlorination process. Because the reaction studied pro-
ceeds in an aqueous environment, the self-consistent
reaction field (SCRF) method with the polarised con-
tinuum model (PCM) of solvation (Miertuš et al.,
1981; Miertuš & Tomasi, 1982) and Bondi-type atomic
radii were applied. Boys–Bernardi counterpoise cal-
culations (Boys & Bernardi, 1970), which are neces-
sary for ion pair species, cannot be combined with
the PCM model. Therefore, basis set superposition er-
ror (BSSE) corrections were calculated in vacuum for
structures optimised in the PCM model according to
the approach presented in the paper (Hunter & Wet-
more, 2007). Vibrational analysis was performed in
order to confirm that reactants, product and interme-
diates were local minima (zero imaginary frequencies)
and transitional states are saddle points on the po-
tential energy surface of the molecules (one imaginary
frequency). Furthermore, the structures of the tran-
sitional states were verified by intrinsic reaction co-
ordinate (IRC) calculations (Fukui, 1981; Gonzales &
Schlegel, 1990). Gibbs free energy changes (∆G), and
the pK values derived from them, were calculated for
the standard conditions of 101.325 kPa and 298.15 K.
In order to obtain accurate results in reactions involv-
ing protonation and deprotonation, Gibbs free energy
of hydrated H+
(∆G(H+
)aq) was calculated from the
Eqs. (1) and (2).
Ggas(H+
) = Hgas(H
+
)−T Sgas(H
+
) (1)
∆G(H
+
)aq=Ggas(H
+
)+∆Ghydr(H
+
) (2)
Where Hgas(H+
) = 2.5RT (6.19 kJ mol−1
) and
TSgas(H+
) = 32.47 kJ mol−1
(Tawa et al., 1998; Topol
et al., 2000; Pokon et al., 2001; Liptak & Shields,
2001), R is gas constant, T represent temperature,
Hgas(H+
) is gas-phase enthalpy of H+
, Sgas(H+
) is
gas-phase entropy of H+
, and Ggas(H+
) is gas-phase
Gibbs free energy of H+
We used the experimental
value of Gibbs free energy of hydration ∆Ghydr(H+
)
= –1104.58 kJ mol−1
, which is accepted as the most
accurate (Camaioni & Schwerdtfeger, 2005; Palascak
& Shields, 2004; Król et al. 2006).
Experimental
All reagents, standards and solvents were pur-
chased from commercial suppliers. Ammonium per-
sulphate (purity ≥ 97 %), hydrochloric acid (37 %),
benzene (purity ≥ 99.5 %), and hexane (purity
≥ 99.0) from POCH (Poland). Aniline (purity ≥
99.5 %), 2,4,6-TCA (purity ≥ 98.0 %), tetramethyl-
silane (TMS), and zinc powder from Sigma–Aldrich
(USA). Chloroform-d (purity 99.96 %) from Deutero
GmbH (Germany) and used without purification ex-
cept aniline which was purified by distillation over
zinc powder before use. 1
H NMR (200 MHz) and
13
C NMR (50 MHz) spectra were recorded on a Var-
ian Gemini 200 spectrometer (Varian, USA), the IR
spectrum on a Bruker Alpha-PFT-IR spectrometer
(Bruker, Germany) with a diamond attenuated total
reflection (ATR) crystal, and the MS (EI) spectrum
on a Bruker 320-MS spectrometer (Bruker, Germany).
Thin-layer chromatography (TLC) was performed on
silica-gel plates (Merck 60 F254, Merck, Germany).
The reaction of aniline with (NH4)2S2O8 and con-
centrated HClaq was carried out at ambient temper-
ature and under atmospheric pressure conditions, ac-
cording to the following procedure. First, a solution of
phenylammonium chloride was prepared by the addi-
tion of aniline (0.91 mL, 10.00 mmol) to concentrated
HClaq (50 mL, 37 %) with vigorous magnetic stirring.
Then, (NH4)2S2O8 (3.5 g, 15.35 mmol) was added to
this solution. The mixture was stirred for an hour and
then poured into 1500 mL of water. The precipitated
product was filtered and washed copiously with wa-
ter until the filtrate was colourless. After drying in
air, 2,4,6-TCA was obtained as a pinkish–white solid.
The yield was 20.1 %; m.p. 75◦
C. TLC, eluent ben-
zene/hexane (ϕr = 2 : 3); Rf = 0.88. IR, ˜ν /cm−1
:
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M. Przybylek, J. Gaca/Chemical Papers 66 (7) 699–708 (2012) 701
Fig. 1. Oxidation reactions of aniline and 2,4,6-TCA.
3456 (NH), 3367 (NH), 3075 (CH), 1610 (NH2),
1571 (Ph), 1391 (Ph), 1296 (C—N), 1072 (NH2), 783
(C—Cl), 555 (C—Cl). 1
H NMR (CDCl3), δ: 7.19 (s,
2H, benzene ring), 4.43 (bs, 2H, NH2). 13
C NMR
(CDCl3), δ: 139 (Car—N), 127.6 (CH), 121.8 (C—Cl),
119.7 (C—Cl). UV-VIS/nm (methanol): 304, 242, 218.
MS (EI) signals of 2,4,6-TCA in the recorded spec-
trum, m/z (Ir/%): 48.6 (6), 52.0 (10), 61.8 (15), 65.0
(13), 73.3 (4), 79.5 (7), 87.8 (12), 96.8 (21), 106.8 (3),
124 (29), 133.1 (4), 158.7 (7), 167.3 (3), 194.9 (100),
198.5 (25), 200.9 (10). We also identified peaks of m/z
values greater than those characteristic of 2,4,6-TCA
(Stein, 2011; AIST, 2011), m/z (Ir/%): 274.7 (7) and
355.1 (4).
Results and discussion
At the first stage of our studies, we sought to
establish what type of reaction occurred under ex-
perimental conditions and to propose a mechanism
for the main product formation. The main goal of
the experimental part of this study was to resolve
whether the oxidative chlorination of aniline occurred
in the presence of reagents which are typical of aniline
polymerisation. We found that a high concentration
of HCl did not favour the oxidative polymerisation
of aniline. Furthermore, good agreement with avail-
able spectral data (Stein, 2011; AIST, 2011; Badawi
et al., 2009; Davis, 2009) indicates that, in the re-
action of aniline with (NH4)2S2O8 and concentrated
HClaq, 2,4,6-TCA is mainly produced. We found that
the yield of chlorination of aniline and the purity of
obtained 2,4,6-TCA was most satisfactory when the
reaction was carried out according to the procedure
presented in “Experimental”, i.e. in the presence of a
maximal concentration of chloride ions, in contrast to
polymerisation which was carried out in diluted HCl.
However, the pinkish-white colour of the precipitate
and some mass spectrum peaks suggest that 2,4,6-
TCA is probably contaminated with small amounts
of aniline trimerisation (m/z = 274.7) and 2,4,6-
TCA dimerisation (m/z = 355.1) by-products. The
first of these reactions (Fig. 1) can take place under
conditions of the oxidative polymerisation of aniline
(Wei et al., 1990, 1989; Gospodinova & Terlemezyan,
1998; Ćirić-Marjanović et al., 2008a). The product
of the latter reaction was identical with the prod-
uct obtained by electrochemical oxidation of 2,4,6-
TCA (Jaworski & Kalinowski, 2007; Pusztai et al.,
2004).
Studies on oxidative chlorination of p-nitroaniline
(Kowalska & Gaca, 2002) and anodic chlorination of
aniline (Matsuda et al., 1984) suggest that Cl−
is the
most probable chlorinating agent. It appears obvious
that, although Cl2 is formed under reaction condi-
tions, it cannot act as a chlorinating agent in o- and
p-chlorination of aniline. According to Henderson–
Hasselbalch equation:
pKa = pH + log([BH]/[B]) (3)
in the presence of concentrated HClaq (pH < 1) almost
all of aniline, pKa = 4.57 (Hall et al., 1972), exists in
a protonated form and NH+
3 substituent directs elec-
trophilic attack on the meta position.
According to many papers, the oxidation of ani-
line with (NH4)2S2O8 in an acidic medium leads
to the formation of phenylnitrenium cation (Ćirić-
Marjanović et al., 2006, 2008a, 2008b; Kovaľchuk et
al., 2001; Kulkarni et al., 2011). This species may
react with various nucleophiles (McClelland et al.,
1996; Tordeux & Wakselman, 1995; Borodkin & Shu-
bin, 2005). It is worth emphasising that only the sin-
glet ground state phenylnitrenium cation exhibits elec-
trophilic properties in the reaction with Cl−
(McClel-
land et al., 1996). Based on these facts, we assumed
that the oxidative chlorination of aniline consisted
of three steps which were cyclically repeated; proto-
nation of aniline, formation of singlet ground state
phenylnitrenium cation, and nucleophilic substitution
(Fig. 2).
At the next stage of our studies, DFT reaction
path modelling was performed. The results of the ther-
mochemical analysis of p-chloroaniline (p-CA), 2,4-
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Fig. 2. Proposed mechanistic pathway for oxidative chlorination of aniline.
Table 1. Results of thermochemical analysis of proposed aniline oxidative chlorination steps: in the cases of K1, K5, K9, K13, pK
values are equal to negative logarithm of equilibrium constant of deprotonation reaction (pKa); values in parentheses are
calculated at G3MP2 level
∆G pK
Reaction
kJ mol−1 Calc. Exp.
C6H5NH+
3
K1
←→ C6H5NH2 + H+ 23.01 4.03
4.57a
(30.51) (5.35)
C6H5NH+
3 + S2O2−
8
K2
←→ C6H5NH+
+ 2HSO−
4
–117.07 –20.51 –
(–87.90) (–15.4) –
C6H5NH+
3 Cl−
+ S2O2−
8
K3
←→ C6H5NH+
Cl−
+ 2HSO−
4 –140.58 –24.63 –
C6H5NH+
3 Cl− K4
←→ p−CA –108.91 –19.08 –
p-CAH+ K5
←→ p-CA + H+
18.03 3.16 3.99a
p-CAH+
+ S2O2−
8
K6
←→ p-Cl-C6H4NH+
+ 2HSO−
4 –127.78 –22.39 –
p-CAH+
Cl−
+S2O2−
8
K7
←→ p-Cl-C6H4NH+
Cl−
+ 2HSO−
4 –125.56 –22.00 –
p-Cl-C6H4NH+
Cl− K8
←→ 2,4-DCA –122.93 –21.54 –
2,4-DCAH+ K9
←→ 2,4-DCA + H+ 6.07 1.06
2.05a
(17.35) (3.04)
2,4-DCAH+
+ S2O2−
8
K10
←→ 2,4-di-Cl-C6H3NH+
+ 2HSO−
4
–131.17 –22.98
–
(–100.50) (–17.60)
2,4-DCAH+
Cl−
+S2O2−
8
K11
←→ 2,4-di-Cl-C6H3NH+
Cl−
+ 2HSO−
4 –130.75 –22.91 –
2, 4-di-Cl-C6H3NH+
Cl− K12
←→ 2,4,6-TCA –124.93 –21.89 –
2,4,6-TCAH+ K13
←→ 2,4,6-TCA + H+
–9.54 –1.67 –0.03b
a) pKa value from (Hall et al., 1972); b) pKa value from (Angioi et al., 2005); Calc. – calculated, Exp. – experimental.
dichloroaniline (2,4-DCA), and 2,4,6-TCA formation
are given in Table 1.
In this paper we compared two diﬀerent compu-
tational methods: B3LYP/6-311++G(2d,p) based on
the density functional theory and G3MP2 based on
second order M¨oller–Plesset perturbation theory. ∆G
values calculated at these two levels of computational
theory vary from each other. Nevertheless, the results
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Fig. 3. Visual representation of optimised structures of ammonium cation/Cl−: C6H5NH+
3 Cl− (A); p-CAH+ Cl− (C); 2,4-DCAH+
Cl− (E) and phenylnitrenium/Cl−: C6H5NH+Cl− (B); p-Cl-C6H4NH+ Cl− (D); 2,4-di-Cl-C6H3NH+ Cl− (F); ion pair
intermediates.
obtained with both of these methods confirmed that
the process studied could proceed under experimen-
tal conditions. The theoretical values of pK for K1,
K5, K9, and K13 reactions are found to be close to
the experimental data (Hall et al., 1972; Angioi et
al., 2005). However, the pK1 value calculated by the
G3MP2 method is slightly less accurate than by the
DFT method. All acid–base equilibria except for K13
favour the phenylammonium cation. This is obvious
because 2,4,6-TCA has a very weak basic character
(pKa < 0), and hence a large amount of water added
to the reaction mixture leads to precipitation of the
neutralised product.
With a high concentration of Cl−
, phenylammo-
nium/Cl−
, and phenylnitrenium/Cl−
ion pairs forma-
tion seems to be probable and so it should be taken
into consideration. DFT calculations showed that the
K3 reaction was more thermodynamically favourable
than the K2 reaction, which meant that ion-pairing
was advantageous for the non-substituted aniline ox-
idation step. The same cannot hold true for the oxi-
dation of chlorinated anilines, because ion pair inter-
mediates do not affect the values of pK significantly.
The structures of complexes involved in the proposed
mechanism are summarised in Fig. 3.
Energy of interaction (Eint) (Krygowski et al.,
2005; Szatylowicz et al., 2007) of phenylammonium/
Cl−
A, C, and E adducts is –8.33 kJ mol−1
,
–8.95 kJ mol−1
, and –10.00 kJ mol−1
, respectively.
The negative values of Eint indicate that an ion pair
has lower energy than the sum of separate energies
of individual ions. It is worth noting that the forma-
tion of complex A was proved via various NMR spec-
troscopy techniques (Suezawa et al., 1991). Cl−
· · ·C
interaction in B, D, and F is the cause of the increased
tetrahedral character of orto- and para-carbon atoms
in phenylnitrenium/Cl−
complex. This loss of pla-
narity is probably due to charge transfer and can be il-
lustrated by the value of C-3 C-4 C-5 C-6 dihedral an-
gle in B (Fig. 3) which is equal to –0.346◦
. Eint values
calculated for B, D, F complexes are –39.29 kJ mol−1
,
–12.72 kJ mol−1
, –14.77 kJ mol−1
, respectively. This
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Fig. 4. NBO charges in phenylnitrenium cation and its chlorinated derivatives.
shows that the formation of phenylnitrenium/Cl−
ion pair intermediates is energetically favourable.
BSSE error calculated for A–F ion pair species
is 1.92 kJ mol−1
, 3.97 kJ mol−1
, 1.88 kJ mol−1
,
4.48 kJ mol−1
, 2.26 kJ mol−1
, 4.73 kJ mol−1
, respec-
tively. This means that the BSSE error is generally
higher for phenylnitrenium/Cl−
complexes than in the
case of phenylammonium/Cl−
ion pairs. Interaction
between phenylnitrenium cation and Cl−
seems to be
fundamental in K4, K8, and K12 substitution steps.
NBO charge distribution in phenylnitrenium cation
(Fig. 4) indicates that chloride ion would be likely
to interact with ortho- and para-carbon atoms. Be-
cause there is lower electron density in para-position
in C6H5NH+
than in ortho-position, our studies do
not include o-chlorination of non-substituted aniline,
as we sought to present only the most favourable path-
way for 2,4,6-TCA formation. It is worth noting that,
according to the similar mechanism of electrochemi-
cal oxidative chlorination proposed by Matsuda et al.
(1984), aniline is chlorinated to p-CA and then p-CA
is chlorinated to 2,4-DCA.
High negative values of pK indicate that all oxida-
tion and substitution steps equilibria favour the prod-
ucts (Table 1). This means that the formation of chlo-
rinated aniline derivatives is probable, as our thermo-
chemical modelling predicts but, in order to confirm
that the proposed mechanism can really take place, ki-
netic calculations were also performed. The values of
Gibbs free energy of activation and transitional state
(TS) decomposition are presented in Fig. 5.
Protonation of the nitrogen atom in the transi-
tional state plays a key role in the kinetics of the nu-
cleophilic attack on the phenylnitrenium cation (Mc-
Clelland et al., 1996). Our calculations showed that
Gibbs free energy barriers of proton addition to B
and D intermediates (Figs. 5a and 5b) were the lowest
in comparison with other reaction paths. It is worth
noting that 2,4-DCA and 2,4,6-TCA can be formed
Table 2. Geometrical parameters of selected intermediates and
transitional states
Cl—H Cl—H—N Cl—C
Structure
˚A ◦ ˚A
A 2.136 176.302 –
B – – 1.887
TS1 – – 1.815
D – – 1.890
TS2 – – 1.811
TS4 – – 1.859
E 2.104 175.929 –
in the inter-molecular and intra-molecular proton-
transfer steps. The high positive values of Gibbs free
energy of activation suggest that inter-molecular pro-
ton transfers in D and F intermediates (Figs. 5d and
5e) are kinetically unfavourable, hence the formation
of TS2 and TS3 (Figs. 5b and 5c) in the alternative
pathway is preferred. Although TS3 is formed with
a relatively high value of Gibbs free energy of acti-
vation, this barrier can be overcome by the energy
released in the TS2 decomposition step. Summarising
these considerations, in accordance with our DFT cal-
culations, the most reasonable reaction path consists
of the intra-molecular proton transfers presented in
Figs. 5a–5c.
The geometrical parameters of selected intermedi-
ates and transitional states are presented in Table 2.
C—Cl bond distances in transitional states are longer
than in intermediates. However, the differences be-
tween values in this geometrical parameter are not
significantly high. Interactions of phenylammonium
cation/Cl−
and phenylnitrenium cation/Cl−
interme-
diates can be characterised by Cl—H and Cl—C bond
distances. It is worth noting that Cl—H and Cl—C
values calculated for A–F are lower than the sum of
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Fig. 5. Transitional states formation and decomposition in substitution steps of oxidative chlorination of aniline. Gibbs free energy
values are given in kJ mol−1.
the appropriate van der Waals radii. Moreover, the
Cl—H—N bond angle in these complexes is close to
180◦
. Both of the geometrical parameters referred to
above (bond distances and angles) suggest relatively
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good stability of the ion pairs studied.
Conclusions
The reaction between aniline, (NH4)2S2O8, and
concentrated HClaq has never been studied before. As
a result of our experimental investigations, we found
that, in a large excess of concentrated hydrochloride
acid, nucleophilic attack of chloride anion to phenyl-
nitrenium cation was more probable than chemical
polymerisation of aniline, although (NH4)2S2O8 and
HClaq are reagents widely used in polyaniline syn-
thesis. Oxidative chlorination of aniline under exper-
imental conditions results in the formation of 2,4,6-
TCA. Unfortunately, the reaction studied is not a
very effective method of chlorination of aniline be-
cause 2,4,6-TCA is formed in rather low yield and
it is contaminated with small amounts of probably
oxidative dimerisation and trimerisation by-products.
In order to establish the mechanism of the chlori-
nation of aniline with the use of (NH4)2S2O8 and
HClaq, B3LYP/6-311++G(2d,p) reaction path cal-
culations were performed. The results of this study
showed that formation of 2,4,6-TCA by the proposed
pathway (protonation of aniline, formation of phenyl-
nitrenium cation/Cl−
ion pair intermediate, and nu-
cleophilic substitution involving intra-molecular pro-
ton transfer) appears to be thermodynamically and
kinetically feasible.
Acknowledgements. The authors gratefully acknowledge the
help received from the Academic Computer Centre in Gdańsk
in providing its facility to perform all the calculations published
in this study.
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