Methylene blue sensitized photodechlorination of isomeric mono- and dichloroanilines via molecular complex formation mechanism

Article abstract of DOI:10.1155/2011/827145

The photosensitized dechlorination of isomeric mono- and dichloroanilines has been studied using methylene blue as photosensitizer in alkaline medium. The dechlorination products have been identified and formation of molecular complex between aniline and methylene blue has been observed. The effects of the pH, concentration of the sensitizer, concentration of the substrate, the intensity of the light and the temperature on the rate of the reaction have been studied. The quantum efficiency of the photodechlorination has been evaluated. The mechanism of the photodechlorination has been suggested.

Full text of DOI:10.1155/2011/827145

ISSN: 0973-4945; CODEN ECJHAO
E-Journal of Chemistry
2011, 8(3), 1086-1093
http://www.e-journals.net
Methylene Blue Sensitized Photodechlorination of
Isomeric Mono- and Dichloroanilines via Molecular
Complex Formation Mechanism
U. C. PANDE and A. H. DWIVEDI*
Department of Chemistry, University School of Science
Gujarat University, Navrangpura, Ahmedabad-380 009, Gujarat, India
dwivedi_ank@indiatimes.com
Received 5 December 2010; Accepted 23 January 2011
Abstract: The photosensitized dechlorination of isomeric mono- and
dichloroanilines has been studied using methylene blue as photosensitizer in
alkaline medium. The dechlorination products have been identified and formation
of molecular complex between aniline and methylene blue has been observed.
The effects of the pH, concentration of the sensitizer, concentration of the
substrate, the intensity of the light and the temperature on the rate of the reaction
have been studied. The quantum efficiency of the photodechlorination has been
evaluated. The mechanism of the photodechlorination has been suggested.
Keywords: Photosensitized, Dechlorination, Chloroanilines, Dichloroanilines, Methylene blue
Introduction
Anilines and their derivatives are used in the manufacture of azo dyes, medicines, pesticides,
polymers, resins, varnishes, perfumes, shoe blacks, vulcanizing rubber and as solvent.
Aniline hydrochloride is used in pharmaceuticals and as an intermediate1-3. Chloroanilines
and other aniline derivatives are used for the inhibition of copper metal corrosion in
hydrochloric acid medium4. Chloroanilines are grouped as hazardous compounds and show
toxicity by inhalation, ingestion or cutaneous absorption. The compounds show acute
toxicity such as cyanosis, vertigo, headache and mental confusion. Chronic toxicity like
anemia, anorexia, weight loss and cutaneous lesions are also reported5-7. Plenty of work is
being carried out nowadays on aniline and its derivatives. A number of articles are published
in the journals, magazines and books today all over the world explaining the different types
of reactions of aniline and its compounds8-10. Amongst all aniline derivatives, halo-
derivatives have been reported as an environment pollutant in the literature11-12. Different
techniques have been used to degrade these compounds. Several methods for biodegradation

1087
A. H. DWIVEDI et al.
of chloroanilines (CA) and dichloroanilines (DCA) have been adopted13-15. Many articles
have been published for the treatment of wastewater with ozone and different chemicals
contaminated by haloanilines.
Amongst all these techniques, the photochemical degradation technique has been found
very effective for the decomposition of chloroanilines especially by using UV-vis radiation
in different solvents16-17. In our previous publications, we have reported the photosensitized
dechlorination of isomeric mono- and dichloronitrobenzenes, isomeric chloroanisidines and
isomeric mono- and dichlorobenzoic acids18-20.. The present study has been undertaken for
the photosensitized dechlorination of isomeric mono- and dichloroanilines in alkaline
medium in visible light using 100 W tungsten lamp for the irradiation. Methylene blue (MB)
has been used as photosensitizer, which has also been used as sensitizer in a number of
photochemical reactions21-23
.
The dechlorination products have been identified and the formation of a molecular
complex between aniline and methylene blue has been observed. Several molecular
complexes have been reported in literature24-26. The effect of the different parameters e.g.
pH, concentration of the sensitizer, concentration of the substrate, the intensity of the light
and the temperature on the rate of the photodechlorination have been studied and reaction
conditions have been established. The quantum efficiency of the photodechlorination has
been evaluated using potassium ferrioxalate actinometer. The effect of the substrate
concentration on quantum efficiency has been studied to evaluate the different excited state
of the substrate molecule. The removal of chlorine from the substrate has been tested with
silver nitrate. The mechanism of the photodechlorination has been suggested.
Experimental
All chemicals that have been used for the experiments are of AR grade (Aldrich) and no
further purification has been done. Absolute alcohol (99%) has been used after distillation.
Double distilled water has been used for the dilution.
Instrumentation
100 W tungsten filament light source (Phillips) has been used for the exposure of the sample
solution. A convex lens has been used to converge the irradiation. A glass water jacket has
been used to decrease the temperature of the solution. All the spectral measurements have
been done on UV Vis spectrophotometer (Spectrascan-2800-Cibacorning, England). pH of
the solution has been measured using pH meter (Systronics, India).
Procedure
Four sets of the experiment were prepared by pipetting 4 mL of 1.0x10-3 M solution of
chloroanilines, 10 mL of 1.0x10-1 M solution of sodium hydroxide in each 50 mL volumetric
flask and 2 mL 1.0x10-4 M solution of methylene blue was added to two flasks and solutions
of all the four flasks were made up to the mark with double distilled water. The final
concentrations of chloroanilines and methylene blue were 8.0x10-5 M and 4.0x10-6
M
respectively. The measurements were carried out at pH 12.
Two flasks, each containing methylene blue and without methylene blue were kept in
the dark for 24 h while remaining similar flasks were exposed to visible light from 100 W
tungsten lamp. The course of the reaction was followed by recording the spectrum of the
exposed solution with a control solution in the range of 200-400 nm against reagent blank.
The flask kept in the dark and flask exposed without sensitizer did not show difference in the

Methylene Blue Sensitized Photodechlorination
1088
spectrum when compared to the control; while the exposed flask containing sensitizer
showed shift in λmax from the original spectrum. This confirms that reaction takes place only
in the presence of the methylene blue on irradiation. Thus, the reaction is photosensitized
dechlorination and not normal or thermal dechlorination.
Free chloride was tested in the exposed solution with silver nitrate by observing white
precipitate of AgCl. This was compared with the control experiment run as blank without
sample in which the Cl¯ test was found negative. The temperature of the solution was
maintained at 298 K using water jacket. The quantum efficiency (φ) of the photodechlorination
has been evaluated using potassium ferrioxalate actinometer.
Molar absorptivities of the reactants and products
The molar absorptivities of the substrates and the products have been calculated by
measuring the absorbance of the solutions of known concentrations (Table 1). It has been
observed that in the case of p-CA, the reaction goes to the completion but it is comparatively
slower for m-CA and very slow and doesn’t go to the completion in the case of o-CA.
2,6-DCA does not show any reaction on exposure.
The reaction rate
The increase in the absorbance of the reaction mixture at 310 nm has been measured by
withdrawing 5 mL aliquot of the solution at different time intervals. The absorbance of the
reaction mixture increased and became constant, indicating the completion of the reaction.
The increase in the absorbance has been used to calculate the rate of the reaction. A plot
of 2 + log (OD) (Optical Density) vs. time has been plotted and was found to be straight line
with a positive slope. This indicates that the reaction follows first order reaction kinetics.
The rate constant of the reaction has been determined by using the following expression:
Rate constant = 2.303 x slope
Results and Discussion
Spectral characteristics
The spectra of all chloro and dichloroanilines were recorded in the range of 200-400 nm
under the experimental conditions against the reagent blank. The absorption bands at 232 nm,
235 nm, 238 nm and 240 nm correspond to π - π* transition for o-CA, m-CA p-CA and
2,4-DCA respectively, which have higher molar absorptivity; while absorption bands at
283 nm, 285 nm, 289 nm and 295 nm correspond to non-bonding transition for o-CA, m-CA
p-CA and 2,4-DCA respectively, which have lower molar absorptivities (Table 1). The
spectra of unexposed solutions of aniline and chloroanilines containing MB were recorded
and it was observed that the spectra matched with the original samples.
The spectra of exposed solutions of o-CA, m-CA, p-CA and 2,4-DCA were recorded
after withdrawing aliquots at different time intervals. The π - π* bands changed to 230 nm
and n - π* bands were shifted to 310 nm for all the compounds. The molar absorptivity of
non-bonding transition band also increased (Table 1). The spectrum of aniline has also been
recorded under the similar experimental conditions to study the photochemical behavior of
aniline, which showed λmax at 230 nm and 280 nm. The spectrum of the exposed solution of
aniline showed λmax at 230 nm and 310 nm. The spectra of the chloroanilines after exposure
show the similar characteristics of λmax and absorptivity as of the pure aniline. Therefore it
has been concluded that all chloro- and dichloroanilines are converted into aniline after
dechlorination. 2,6-DCA does not undergo decomposition under the experimental conditions
and does not form molecular complex. o-CA molecule shows extremely slow reaction.

1089
A. H. DWIVEDI et al.
Table 1. Experimental values of λmax and molar absorptivity (ε) of aniline and chloroanilines
Experimental
max (εvalue)
Product
Compound
λ
λ
max (εvalue)
230 (8490)
280 (1420)
232 (7790)
283 (2200)
235 (9240)
285 (1980)
238 (11080)
289 (1310)
240 (9940)
295 (2080)
210 (18910)
290 (2710)
230 (8490)
310 (4090)
230 (8100)
310 (3540)
230 (8760)
310 (3808)
230 (8530)
310 (4000)
230 (8990)
310 (3928)
-
Ph-NH2
o-CA
m-CA
p-CA
2,4-DCA
2,6- DCA
-
The rate of the reaction
The increase in the absorbance at 310 nm has been measured for isomeric chloroanilines at
different time intervals; which increased and became constant indicating the completion of
the reaction. The increase in the product concentration increases the absorbance at 310 nm,
which has been used to calculate the rate constant. The rates of the dechlorination for
o-, m-, p-chloroanilines and 2,4-dichloroaniline have been calculated (Table 2). The
photosensitized dechlorination shows different rates for different mono- and dichloroanilines
in experimental conditions (Table 2). The order of the dechlorination follows
p- > m- > o-chloroaniline. However 2,6-DCA does not undergo dechlorination reaction. The
2,4-DCA shows a reaction rate that is higher than m-CA but lower than p-CA. The
experimental results show that both the chlorine atoms are removed simultaneously.
Table 2. Rates of photosensitized dechlorination of chloroanilines
kx101, min-1
Compound
o-CA
m-CA
0.3915
0.8060
2.8780
2.303
-
p-CA
2,4-DCA
2,6- DCA
Ph-NH2
-
[o-CA] = [m-CA] = [p-CA] = 5.0x10-5 M, [MB] = 4x10-6 M, Light intensity = 11.18x10-8 E s-1;
pH = 12; Temperature = 298 K
The effect of the variables on the rate of the photodechlorination of chloroanilines
Effect of the pH
Methylene blue doesn’t work as a photosensitizer in the acidic and neutral medium for this
reaction. The rate constant of the photosensitized reaction has been calculated at different
pH for o-, m- and p-chloroanilines between 8-13 pH. It has been observed that as pH
increases, the rate of the reaction increases slowly, but between 11 to 12 pH, showed a large
increase in the rate of the reaction has been observed and then it became constant. Therefore
the pH 12 has been selected for the subsequent studies.

Methylene Blue Sensitized Photodechlorination
1090
Effect of the concentration of the sensitizer
The effect of the different concentrations of methylene blue on the rate of the dechlorination
has been studied by calculating the rate constants for o-, m- and p-chloroanilines using the
different concentrations of methylene blue in the range of 0.4x10-5 - 2.0x10-5 M. The
increase in the concentration of the sensitizer does not show change in the rate of the
reaction. However the rate slightly decreases at the higher concentration of the sensitizer. It
appears that the self-deactivation of the sensitizer takes place at higher concentration.
Effect of the concentration of the substrate
The effect of the different initial concentrations of o-, m- and p-chloroanilines on the rate of
the reaction has been studied in the range of 0.4x10-4 - 1.2x10-4 M. The rate of the reaction
remains constant with increase in the initial concentration of the substrate. This shows that
the reaction is independent of the initial concentration of the substrate. However the rate of
the dechlorination of p-CA is highest and o-CA is lowest.
Effect of the light intensity
The increase of the light intensity [Einstein / second (E /s)] shows positive effect and the rate
of the dechlorination increases as the light intensity increases. The number of the excited
molecules of the sensitizer increases with higher light intensity as the number of photons
increases and corresponding the rate of the reaction increases (Table 3). A linear relationship
is observed between the light intensity and the rate of the reaction.
Table 3. Effect of the light intensity
Light intensity,
Ix108, E sec-1
4.47
kx101, min-1
o-CA
0.1948
kx101 min-1
kx101, min-1
p-CA
1.3810
,
m-CA
0.3768
0.4915
0.8060
6.72
11.18
0.2405
0.3838
1.8420
2.8180
[o-CA] = [m-CA] = [p-CA] = 5.0x10-5 M, [MB] = 4x10-6 M, pH = 12; Temperature = 298 K
Table 4. Quantum efficiencies of chloroanilines
Compound
Φ-Value
o-CA
0.4041
m-CA
1.0377
p-CA
1.1579
2,4-DCA 2,6-DCA
0.9831
-
Effect of the temperature
The effect of the variation of the temperature has been studied between 298-328 K on the
rate of the reaction. The rate of the reaction remains constant with the increase in the
temperature in the above range. This indicates that the rate of the reaction is independent of
the temperature and thermal condition does not affect the photodechlorination. The
temperature independent nature of the rate of the reaction also indicates that the product
formation takes place directly from the excited species without any intermediate stage.
φ
- Value
The quantum efficiency has been calculated by using standard potassium ferrioxalate
actinometer. φ values have been also calculated for the chloroaniline at different initial
concentrations of the substrate (Table 4). There is a linear relationship between the initial
concentration of the substrate and φvalue, suggesting that the φvalues are dependent on the
initial concentration of the substrate. The plot of the inverse of the quantum efficiency
versus the inverse of the initial concentration of the substrate has been found to be linear
with a positive slope which suggests that there is a direct product formation from the singlet
excited state and not via triplet intermediate state27.

1091
A. H. DWIVEDI et al.
Molecular complex formation
The spectrum of the exposed solution of aniline and isomeric chloroanilines compare well
with each other. The formation of anilines from chloroanilines takes place. The band at
280 nm in aniline corresponds to n-π* transition and shows lower absorptivity (Table 1). The
exposed solution of aniline shows that this band is shifted to longer wavelength 310 nm with
higher molar absorptivity. The formation of a molecular complex between excited aniline
molecules with MB molecule has been suggested. There is no absorption band change in
ground state. However excited chloroaniline molecule does not form a complex with MB.
The excited molecule of chloroaniline undergoes two simultaneous processes viz.
dechlorination and donation of nitrogen lone pair to MB molecule. The molecular complex
thus formed exists in ground state. The molecular complex formation between aniline and
MB on exposure takes place very fast while the formation of a molecular complex in the
case of chloroanilines takes longer time, suggesting that complex formation takes place only
after the dechlorination. The exposure of the isomeric chloroanilines with MB in alkaline
medium excites the chloroaniline molecules to singlet-excited state by energy transfer from
MB molecule. The presence of chlorine does not allow the formation of a molecular
complex. The excited molecule of chloroaniline undergoes first dechlorination and then
forms molecular complex. The rate of complex formation is proportional to the rate of
dechlorination and thus the formation of aniline takes place. The absorption spectrum of MB
was recorded before and after the exposure taking aniline as a blank, which did not show
any change before and after exposure. The band at 280 nm of aniline gets shifted to 310 nm
of aniline. Thus it can be concluded that aniline molecule only gets perturbed and forms
charge transfer complex with MB.
Mechanism
Aniline and chloroanilines have their λmax below 300 nm. These molecules do not absorb
the visible light and they are not photo-degraded directly. Methylene blue (MB) has λmax
at 665 nm; which absorbs visible radiation and forms excited molecule. The absorption of
the radiation at this λmax results in the singlet excited state of methylene blue23. The energy
transfer from singlet excited state of MB to aniline and isomeric chloroanilines takes
place, resulting in the ground state of MB and singlet excited state of chloroanilines and
aniline. The change in the spectrum of aniline has been observed after exposure along
with MB. The absorption band of π- π* of aniline remains unchanged, but n - π* bands
shows red shift and with increased absorptivity28-31. It can be concluded that non-bonding
electrons of aniline are excited in the lower vacant orbital of MB forming a molecular
complex. The molecular complex shows higher absorptivity28-31 and longer wavelength
absorption at 310 nm (Scheme 1). Energy transfer takes place from singlet-excited state of
MB to isomeric chloroanilines. The excited chloro molecule undergoes C-Cl bond
cleavage forming Cl ¯ ion. The excited chloroaniline molecule simultaneously undergoes
dechlorination and transfer of non-bonding electrons of nitrogen atom to MB molecule,
forming a molecular complex26,32. The molecular complex is in the ground state. The
absorbance of the molecular complex remains constant with time. The spectral
characteristics of dechlorinated chloroanilines are similar to the exposed aniline (Scheme 2).
The new absorption band at 310 nm is due to transfer of non-bonding electrons of nitrogen
atom of aniline to MB. The plot of the inverse of the quantum efficiency versus the
inverse of the initial concentration of the substrate is linear with a positive slope; which
indicates that there is a direct product formation from the singlet-excited state of the
substrate and not via intermediate triplet state27.

Methylene Blue Sensitized Photodechlorination
1092
hν
A
A*1
A*1 + D
+ D*1
A + D*1
A
A
D
Molecular complex
Scheme 1
h
υ
MB *1
MB
Methylene Blue
*1
. .
H2N
. .
H2N
MB *1
+
+
MB
Aniline
*1
MB
. .
H2N
+
H2N
MB
Molecular Complex
*1
. .
H2N
. .
H2N
MB *1
+
MB
+
Cl
Cl
Chloroaniline
*1
*1
.
. .
H2N
. .
H2N
Cl
+
Cl
*1
MB
. .
H2N
H2N
MB
+
.
-
-
e
Cl
Cl
+
Scheme 2
Conclusion
Isomeric chloro- and dichloroanilines undergo photosensitized dechlorination in the
presence of methylene blue and visible light in alkaline medium. The spectral profiles of the
exposed solution suggest that the product of the reactions is only aniline. The molecular complex

1093
A. H. DWIVEDI et al.
between excited state of aniline and ground state of MB is formed. The study of different
parameters concludes that the rate of the dechlorination is dependent on light intensity and
pH and is independent of temperature, initial concentration of substrate and concentration of
sensitizer. φvalue is dependent on initial concentration of substrate. The rate of reaction
follows the order p-CA > m-CA > o-CA.
References
1.
2.
3.
The Merck Index, Version 12.3, Merck & Co. Inc., NJ, USA, 1999.
Jen J-F, Chang C-T and Yang T C, J Chromatogr A, 2001, 930(1-2), 119-125.
Sarasa J, Roche M P, Ormad M P, Gimeno E, Puig A and Ovelleiro J L, Water Res.,
1998, 32, 2721.
4.
5.
Khaled K F and Hackerman N, Electrochim Acta, 2004, 49, 485.
Gosselin R E, Hodge H C, Smith R P and Gleason M N, Clinical Toxicology of
Commercial Products, 4th Eds., (Williams and Wilkins, Baltimore), Section III, 1976, 29.
Adema D M M and Vink G J, Chemosphere, 1981, 10, 533-554.
Crossland N O, Chemosphere, 1990, 21, 1489-1497.
6.
7.
8.
9.
Chesta C A, Cosa J J and Previtali C M, J Photochem., 1987, 39, 251.
Soltermann A T, Chesta C A, Cosa J J and Previtali C M, An Asoc Quim Argent.,
1989, 77(6), 487-498.
10. Szczepanik B and Latowski T, Pol J Chem., 1997, 71, 807.
11. Raymond J W, Rogers T N, Shonnard D R and Kline A A, J Haz Mat., 2001, 84, 189-215.
12. Barr D B and Needham L L, J Chromatography B, Anal Technol Biomed Life Sci.,
2002, 778, 5.
13.
Hwang H-M, McCullum D and Slaughter L, Bull Environ Contam Toxicol., 1998, 60, 81-87.
14. Susarla S, Masunaga S and Yonezawa Y, Microb Ecol., 1997, 33(3), 252-256.
15. Susarla S, Masunaga S and Yonezawa Y, Wat Sci Technol., 1996, 34, 489.
16. Hwang H-M, Hodson R E and Lee R F, Appl Environ Microbiol., 1986, 50, 1177-1180.
17. Ishikawa S, Baba K, Hanada Y, Uchimura Y and Kido K, Bull Environ Contam
Toxicol., 1989, 42, 65.
18. Dwivedi A H and Pande U C, J Photochem Photobiol A Chem., 2003, 154, 303-309.
19. Dwivedi A H and Pande U C, J Indian Chem Soc., 2005, 82, 424-427.
20. Dwivedi A H and Pande U C, E-J Chem., 2010, 7(S1), S111- S120.
21. Jockusch S, Timpe H J, Schnabel W and Turro N J, J Phys Chem A, 1997, 101, 440-445.
22. Kojima M, Takagi T, Matsubara C and Nakamura K, Chem Lett., 2000, 4, 354-355.
23. Kojima M, Takagi T, Matsubara C and Nakamura K, J Photochem Photobiol A
Chem., 2001, 74(3), 369-377.
24. Enescu M and Lindqvist L, J Phys Chem B, 1995, 99, 8405.
25. Enescu M, Ridard J, Gheorghe V and Levy B, J Phys Chem B, 2002, 106, 176-184.
26. Enescu M, Ridard J, Gheorghe V and Levy B, J Phys Chem B, 2000, 104, 1073-1077.
27. Freeman P K, Jang J S and Ramnath N, J Org Chem., 1991, 56, 6072.
28. Kross R D and Fassel V A, J Am Chem Soc., 1957, 79, 38.
29. Issa R M and El-Essawey M M, Z Phys Chem., (Leipzig), 1973, 253, 96.
30. Hindawey A M, Nassar A M G and Issa R M, Acta Chim., 1976, 88, 341.
31. Hindawey A M, Nassar A M G and Issa R M, Acta Chim., 1977, 92, 263.
32. Issa Y M, El-Kholy A E, Hindawey A M and Issa R M, J Indian Chem Soc., 1980,
LVII, 216.

International Journal of
Advances in
Organic Chemistry
International
International Journal of
International Journal of
Photoenergy
Medicinal Chemistry
Analytical Chemistry
Physical Chemistry
Hindawi Publishing Corporation
http://www.hindawi.com
Hindawi Publishing Corporation
Hindawi Publishing Corporation
http://www.hindawi.com
Hindawi Publishing Corporation
Hindawi Publishing Corporation
http://www.hindawi.com
Volume 2014
Volume 2014
http://www.hindawi.com
Volume 2014
http://www.hindawi.com
Volume 2014
Volume 2014
International Journal of
Carbohydrate
Chemistry
Journal of
Quantum Chemistry
Hindawi Publishing Corporation
Hindawi Publishing Corporation
http://www.hindawi.com
http://www.hindawi.com
Volume 2014
Volume 2014
Submit your manuscripts at
http://www.hindawi.com
Journal of
The Scientific
Analytical Methods
in Chemistry
World Journal
Hindawi Publishing Corporation
Hindawi Publishing Corporation
http://www.hindawi.com
http://www.hindawi.com
Volume 2014
Volume 2014
International Journal of
Journal of
Journal of
International Journal of
Bioinorganic Chemistry
Spectroscopy
Inorganic Chemistry
Electrochemistry
Applied Chemistry
and Applications
Hindawi Publishing Corporation
http://www.hindawi.com
Hindawi Publishing Corporation
Hindawi Publishing Corporation
Hindawi Publishing Corporation
Hindawi Publishing Corporation
Volume 2014
http://www.hindawi.com
Volume 2014
http://www.hindawi.com
Volume 2014
http://www.hindawi.com
Volume 2014
http://www.hindawi.com
Volume 2014
Journal of
Journal of
International Journal of
Chromatography
Journal of
Theoretical Chemistry
Catalysts
Research International
Chemistry
Spectroscopy
Hindawi Publishing Corporation
http://www.hindawi.com
Hindawi Publishing Corporation
http://www.hindawi.com
Hindawi Publishing Corporation
http://www.hindawi.com
Hindawi Publishing Corporation
http://www.hindawi.com
Hindawi Publishing Corporation
http://www.hindawi.com
Volume 2014
Volume 2014
Volume 2014
Volume 2014
Volume 2014