Spectroscopic and theoretical studies on the nucleophilic substitution of 2,3-dichloronaphthoquinone with para-substituted anilines in solid state via initial charge transfer complexation

Article abstract of DOI:10.1016/j.saa.2012.08.056

Various spectroscopy techniques (UV-Vis, DRS, FT-IR, ¹H NMR, LC-MS) and theoretical computations have been employed to investigate the mechanism of the nucleophilic substitution reaction of 2,3- dichloronaphthoquinone (DCNQ) with para-substitut

Full text of DOI:10.1016/j.saa.2012.08.056

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 378–383
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic and theoretical studies on the nucleophilic substitution
of 2,3-dichloronaphthoquinone with para-substituted anilines
in solid state via initial charge transfer complexation
⇑
Angupillai Satheshkumar, Kuppanagounder P. Elango
Department of Chemistry, Gandhigram Rural Institute (Deemed University), Gandhigram 624302, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
"
"
Nucleophilic substitution of DCNQ
with anilines in solid state has been
demonstrated.
Driving force for the reaction is the
ease with which initial EDA adduct
forms between reactants.
It is an environment friendly attempt
for the preparation of
aminonaphthoquinone derivatives.
a r t i c l e i n f o
a b s t r a c t
Various spectroscopy techniques (UV–Vis, DRS, FT-IR, 1H NMR, LC–MS) and theoretical computations
have been employed to investigate the mechanism of the nucleophilic substitution reaction of 2,3-dichlo-
ronaphthoquinone (DCNQ) with para-substituted anilines in solid state under base- and solvent-free
conditions against traditional synthetic routes. The initial formations of electron donor acceptor (EDA)
adduct between DCNQ and aniline was found to be the driving force for the substitution reaction to occur
in solid phase.
Article history:
Received 26 April 2012
Received in revised form 11 August 2012
Accepted 21 August 2012
Available online 27 August 2012
Keywords:
EDA complex
Ó 2012 Elsevier B.V. All rights reserved.
Solid state
DCNQ
HOMO–LUMO calculations
Amino naphthoquinone
Nucleophilic substitution
Introduction
2-amino-1,4-naphthoquinone moiety can also be found in natural
products, such as echinamines A and B from the sea urchin [3–11].
Quinones are prevalent motifs in various natural products,
which are associated with diverse biological activities. Among
the naphthoquinones, 2-amino-1,4-naphthoquinone derivatives
are interesting molecules because of their molluscicidal, cytotoxic,
anti-tumor, anti-fungal and anti-bacterial activities [1,2]. The
Synthetic chemists continue to develop various techniques for
obtaining better products with less environmental impact. The
solid-state synthesis provides several advantages such as opera-
tional simplicity, neutral condition, high yield, environment
friendly and economically cheap. In recent years, wide varieties of
organic reactions have been studied in solid-state [12–16]. Many
of the nucleophilic substitution reactions between DCNQ and ani-
lines were performed in the presence of bases like NaHCO3, K2
⇑
Corresponding author. Tel.: +91 451 2452371; fax: +91 451 2454466.
E-mail address: drkpelango@rediffmail.com (K.P. Elango).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.08.056

A. Satheshkumar, K.P. Elango / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 378–383
379
Table 1
CO3, Et3N, etc. in organic solvents at high temperatures (or) under
Reaction of DCNQ with aniline on different condition.
microwave irradiation [17]. Different organic solvents such as eth-
anol [18,19], methanol [20], benzene, chloroform, dichloromethane,
dimethylformamide, dimethylsulfoxide, diethyl ether, tetrahy-
drofuran and toluene [21] have been used to carry out nucleophilic
substitution reactions of quinones.
Entry
Solvent
T (°C)
Time (h)
Yield (%)
Base
1
2
3
4
EtOH
H2O
MeOH
CH3CN
40–50
60–70
40–50
50–60
4–6
6
12
12
73
93
78
85
K2CO3
–
K2CO3
Et3N
In conventional method, DCNQ is heated with aniline in ben-
zene at 50 °C for 30 min to give the product around 60%. Sarhana
et al. [22] have reported a yield of 73% by changing the medium
from non polar to polar. Recently, the nucleophilic substitution
reaction of DCNQ with aniline was also carried out in aqueous
medium [23]. The main objective, therefore, of the present endea-
vor is to investigate the nucleophilic substitution reaction of DCNQ
with para-substituted anilines under environment friendly solid
state condition against conventional methods i.e. in organic sol-
vents in the presence of a base.
The results obtained were found to be in line with those re-
ported earlier. According to Tandon and Maurya [23], the yield of
the product is relatively higher in aqueous medium than that in
non-aqueous media even though DCNQ is insoluble in water. The
observed high yield, according to them, is due to the interaction
of non polar (or) hydrophobic regions of the reactants [23].
Since the interaction of non-polar regions play the key role for
the reaction to occur in a heterogeneous environment, we pre-
sumed that the same can be achieved in a non-conventional
homogenous environment i.e. in solid phase. The reaction of DCNQ
(1) with different para-substituted anilines possessing electron
donating and electron withdrawing substituents (2a–g) has been
carried out in solid phase (Scheme 2) by simply grinding the reac-
tants in a beaker in the absence of any base. The results obtained
are collected in Table 2.
Interestingly, the yields obtained in the solid-state are compa-
rable with that reported in aqueous and non-aqueous media but
within a short period of time at room temperature (or on slight
warming) and also in the absence of any base. Also, the results in
Table 2 indicated that the unsubstituted aniline (2a) and anilines
with electron donating substituents (2b and c) gave relatively
higher yields at room temperature. While anilines with electron
withdrawing substituents (2d–g) required warming of the vessel
and also gave relatively lower yields. This observation indicated
that the nature of the substituent present in aniline has a signifi-
cant role to play even in the solid state.
Experimental
General
The chemical reagents and solvents were purchased from
Aldrich and/or Merck, India and were used as received. The
UV–Vis spectra were recorded on a JASCO-V-630, Japan double
beam spectrophotometer. Millisecond UV–Vis absorption spectra
were recorded on an Ocean Optics (DH-2000) spectrograph in
absorption mode using an optical fiber probe. The infrared spectra
were recorded on a JASCO 460 Plus, Japan, FT-IR spectrometer.
Fluorescence spectra were recorded on a JASCO-FP-6200, Japan,
spectrofluorometer. 1H NMR spectra were recorded on a Bruker
NMR (300 MHz) with DMSO-d6 as solvent. DRS were performed
on a (Shimadzu UV-2550) UV–Vis spectrophotometer (DRS acces-
sory Model: ISR-2200).
Synthesis of compound 3a–g
Mechanism of the reaction
In a typical experiment, both DCNQ and given aniline (2a–g)
were taken in 1:1.2 M ratio in a clean beaker and mixed thoroughly
using a glass rod for 30 min. The color of the reaction mixture was
changed from yellow to dark red within 30 min. The progress of
the reaction was monitored by TLC. Then the reaction mixture
was washed with excess of water and dried under vacuum to ob-
tain the product as a dark red colored powder. The products ob-
tained were characterized using FT-IR, 1H NMR and HRMS
spectral techniques (Supplementary information).
The mechanism of the nucleophilic substitution of DCNQ with
anilines is a well established one [16,24]. A general scheme of
the mechanism of the reaction is shown in Scheme 3. The first step
is the formation of an EDA adduct between DCNQ and the nucleo-
phile. In a subsequent step substitution takes place with the re-
moval of chloride ion. Also it is well known that the reactivity of
aniline depends on the nature of the substituent. The reactivity is
higher with electron donating substituents than with electron
withdrawing ones [25]. This is due to the fact that anilines with
electron donating substituent readily form the EDA adduct and
hence the higher reactivity.
Results and discussion
As a representative case, the reaction of DCNQ (1) with p-chlor-
oraniline (Scheme 1) has been carried out under conventional con-
ditions reported earlier. The results obtained in different solvents,
including water, at varying time intervals and in presence different
bases are given in Table 1.
Electronic spectral studies
In order to ascertain the mechanism of the reaction, UV–Vis
spectra of the reaction mixture were recorded in ethanol. A
NH2
O
O
Cl
Cl
Cl
+
Cl
N
H
Cl
O
O
1
2d
3c
Scheme 1. Reaction of DCNQ (1) with p-chloroaniline (2c).

380
A. Satheshkumar, K.P. Elango / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 378–383
O
O
NH2
Cl
R
Cl
Cl
+
N
H
O
R
O
3a-g
1
2a-g
3a: R=H
2a: R=H
3b: R=Me
3c: R=OMe
3d: R=Cl
2b: R=Me
2c: R=OMe
2d: R=Cl
2e: R=Br
2f: R=F
3e: R=Br
3f: R=F
3g:R=COMe
2g:R=COMe
Scheme 2. The reaction of DCNQ (1) with different para-substituted anilines in solid phase.
in the interaction between aniline and DCNQ was identified using
the following experiment employing millisecond absorption spec-
trograph in absorbance mode. Chloroform solutions of the acceptor
and donor were extracted separately with water and the water ex-
tracts were mixed together. The electronic spectrum of the mix-
ture, recorded immediately after mixing the water extracts,
exhibited no absorption in the visible region. Whereas the chloro-
form solutions of the acceptor and donor were mixed together and
the red colored solution thus obtained was extracted with water.
The absorption spectrum of the aqueous extract, recorded immedi-
ately, is shown in Fig. 2. The curve C1 is characteristic of quinone
radical ion, which is converted to curve C3 (through curve C2) the
characteristic curve of the reaction product. This spectrum re-
mained unchanged after half an hour (curve C4). This observation
suggests the formation of a polar intermediate (the EDA complex)
in the interaction between the donor and acceptor [31–33].
The diffused reflectance spectra of the reactants (1 and 2b) and
the product (3b) were also recorded. A representative spectrum is
shown in Fig. 3. It is evident, from the figure, that the appearance of
a new broad peak (where either of the reactants are non-reflective)
in the 450–650 nm region indicated the formation of the product
in solid state.
Table 2
Reaction of DCNQ with anilines (2a–g) on solid state.
Entry Quinone Aniline Time (min) Temp (°C) Product Yield (%)
1
2
3
4
5
6
7
1
1
1
1
1
1
1
2a
2b
2c
2d
2e
2f
30
20
50
40
40
30
30
RT
RT
RT
40
60
40
40
3a
3b
3c
3d
3e
3f
88
93
90
80
82
83
81
2g
3g
representative UV–Vis spectrum is shown in Fig. 1. The electronic
spectrum of DCNQ exhibits a peak at 341 nm due to the
p⁄ tran-
p–
sition [26–28]. Immediately, on adding aniline to DCNQ, the elec-
tronic spectrum of the reaction mixture showed a new broad
peak centered around 479 nm and the absorbance of which in-
creased progressively with elapse of time indicating the formation
of the product. The broadness of the peak is due to the existence of
intramolecular charge transfer transition between N-atom and
quinone in the product [29].
Parallel to the observation made by Neelgund and Budni [30],
who reported that the formation of EDA complex between DCNQ
and n-butylamine is an instantaneous process and also the concen-
tration of which is very low, in the present study we could not
visualize the EDA complex using conventional electronic spectro-
scopic technique. Hence, the nature of the intermediate formed
1H NMR spectral studies
1H NMR spectra of a representative reactant (2b) and the reac-
tion mixture (1 and 2b in 1:1 ratio) in DMSO-d6 were recorded to
O
O
Cl
O
O
:Nu
Cl
Cl
Cl
Cl
:Nu
:Nu
Cl
O
O
EDA COMPLEX
-Cl -
O
O
O
Nu
Cl
Nu
Cl
Cl
o
Scheme 3. The mechanism of nucleophilic substitution of DCNQ with aniline.

A. Satheshkumar, K.P. Elango / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 378–383
381
elucidate the reaction mechanism. The signal at 4.764 ppm
(Fig. 4a) corresponds to the –NH2 protons of 2b. The 1H NMR spec-
tra of the reaction mixture recorded with elapse of time (in the
NMR tube) are shown in Fig. 4b–g. The deshielding of the –NH2
protons, immediately after mixing 2b and 1, may be due to the for-
mation of an EDA adduct between them. With elapse of time the
extent of deshielding was observed to increase. Concurrently, after
5 min, the new signal appeared at 9.314 ppm corresponds to the –
NH proton in the product. The intensity of the signal increased
with an increase in time as it is evidenced from the Fig. 4. The 1H
NMR spectrum of the product is also shown in the figure for com-
parison. The 1H NMR study of the DCNQ–2a system is also shown
in Fig. S2. The 1H NMR study supported the fact that the mecha-
nism of the reaction involves two steps: the first one being the for-
mation of an EDA adduct between the reactants and in the second
step the substitution occurs with the removal of chloride ion.
2.0
1.5
1.0
0.5
0.0
every 5 min
DCNQProduct peak
every 5 min
Aniline
300
400
500
600
700
Wavelength (nm)
Naked-eye detection
Fig. 1. Absorption spectra of DCNQ (A), Aniline (D), and CT-complexes (P) of DCNQ
with Aniline in ethanol.
It is interesting to note that the course of the solid state reaction
can even be visualized with naked eye. A representative system is
depicted in Fig. S1. A yellow colored solid DCNQ (A) on grinding
with dirty white p-methylaniline (B) gave a brown colored solid
(C) which subsequently changed to reddish-brown solid (D) within
10 min and finally to a red colored product (E) in 20 min. No fur-
ther color change in color was observed after 20 min indicating
completion of the reaction. The initially formed brown colored so-
lid may be the EDA adduct which is converted into the final
product.
0.5
0.4
Since the formation of an EDA adduct between DCNQ and
nucleophile is the driving force for the reaction to occur, the influ-
ence of an external inert electron donor, hexamethyl benzene
(HMB), on the course of the reaction is also studied in solid state.
The selection of HMB is based on the fact that it is a well known
electron donor in an EDA complex formation and also it is chemi-
cally inert towards DCNQ and the anilines. When colorless HMB (F)
was ground with DCNQ in forms an orange colored (G) EDA adduct.
On subsequent addition of p-methylaniline (2b) to this adduct, the
desired nucleophilic substitution reaction has occurred, but very
slowly. This may be due to the fact that the added nucleophile, ani-
line, has to replace HMB from the DCNQ–HMB EDA adduct for the
reaction to occur and hence the rate of the substitution reaction is
slow.
C4
C3
0.3
C2
C1
0.2
300
350
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 2. Millisecond UV–Vis absorption spectra of Aniline – DCNQ EDA complex (C1)
and product (C3).
Fluorescence study
With an aim to find out the relative stabilities of the DCNQ–
HMB and DCNQ-2b EDA adducts, fluorescence studies were carried
out. The fluorescence spectrum of the donors HMB and 2a were re-
corded at room temperature in the range of 230–730 nm upon
excitation at 271 and 284 nm, respectively. It is observed that in
both the cases addition of DCNQ quenched the fluorescence of
the donors through EDA complexation. The experimental results
indicated that the quenching efficiency, in both the cases, in-
creased with an increase in the concentration of DCNQ at a fixed
concentration of the donor (Figs. 5 and 6).
0.8
p-Toluidine
DCNQ
DCNQ+p-Toluidine
0.6
0.4
0.2
0.0
Fluorescence intensity data was analyzed according to the
Stern–Volmer law [34,35]:
F0=F ¼ 1 þ Ks
v
½Qꢀ
ð1Þ
200
300
400
500
600
700
800
where F0 is emission intensity in the absence of quencher (Q) and F
is emission intensity at quencher concentration [Q]. The linear
Stern–Volmer plots (Fig. 7) indicated that only one quenching
mechanism is operative and the quenching is bimolecular and
dynamic.
Wavelength (nm)
Fig. 3. UV–Vis diffuse reflectance spectra of DCNQ, p-toluidune and DCNQ +
p-toluidine system.

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A. Satheshkumar, K.P. Elango / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 378–383
Fig. 4. The 1H NMR spectra (300 MHz) of the reaction mixture of DCNQ with (2b) (1:1 M ratio) in DMSO-d6. a-only (2b). b–g are mixture of DCNQ-(2b) after 2, 5, 7, 10, 15 and
20 min respectively and p is the pure product.
16
14
12
10
8
140
120
100
80
D
a
D
a
b
c
b
c
d
e
f
60
6
d
e
f
40
4
20
2
0
0
300
350
300
350
400
450
Wavelength (nm)
Wavelength (nm)
Fig. 5. Fluorescence spectra for HMB–DCNQ system in ethanol at fixed concentra-
tions of [D] = {5 ꢁ 10ꢂ3 M (curved)} and variable concentration of [A] = {1.25 (a), 2.5
(b), 3.75 (c), 5.0 (d), 6.25 (e), 7.5 (f)} ꢁ 10ꢂ5 M at 298 K.
Fig. 6. Fluorescence spectra for aniline–DCNQ system in ethanol at fixed concen-
trations of [D] = {5 ꢁ 10ꢂ3 M (curved)} and variable concentration of [A] = {1.25 (a),
2.5 (b), 3.75 (c), 5.0 (d), 6.25 (e), 7.5 (f)} ꢁ 10ꢂ5 M at 298 K.
The binding constant values of DCNQ–HMB and DCNQ–2a EDA
adducts are found to be 2.9 ꢁ 104 and 3.2 ꢁ 104 l/M, respectively.
The results showed that, the binding constant for the formation
of DCNQ–HMB and DCNQ–2a EDA adduct are comparable. Hence,
addition of HMB to DCNQ, prior to the addition 2a, forms DCNQ–
HMB EDA adduct from which HMB is to be replaced by 2a to form
DCNQ–2a EDA adduct. Such a process is possible as the binding
constants are comparable but it requires few seconds and hence
the slow rate of the reaction, as observed in the naked eye detec-
tion section.
Theoretical calculations
To understand the effect of substituents present in the aniline
moiety on the neuclephilic substitution reaction, we have
performed the complete geometrical optimization of DCNQ and

A. Satheshkumar, K.P. Elango / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 378–383
383
4.0
3.5
3.0
2.5
2.0
1.5
1.0
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.08.056.
B
C
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To conclude, the driving force, for the nucleophilic substitution
reaction to occur at DCNQ, is the ease with which the initial EDA
adduct forms between the reactants. This can easily be achieved
in solid state itself against the conventional methods i.e. in the
absence of any solvent and base. Such an environment friendly
attempt would certainly be useful in the preparation of many bio-
logically significant amino naphthaquinone derivatives.
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Acknowledgement
The authors thank the University Grants Commission, New
Delhi for its financial assistance to carry out this research work.
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