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Adsorption and Oxidation of Aniline and Anisidine by Chromium Ferrocyanide
Journal of Colloid and Interface Science 245, 251–256 (2002) doi:10.1006/jcis.2001.7968, available online at http://www.idealibrary.com on
Adsorption and Oxidation of Aniline and Anisidine by Chromium Ferrocyanide Tanveer Alam,∗ Hina Tarannum,† Shah Raj Ali,† and Kamaluddin†,1 ∗ K.L.D.A.V. Degree College, Roorkee 247 667, India; and †Chemistry Department, University of Roorkee, Roorkee 247 667, India Received June 12, 2001; accepted September 14, 2001; published online December 10, 2001
The interaction of aniline and p-anisidine with chromium ferrocyanide has been studied. Maximum uptake of both anilines was observed around pH 7. The adsorption data obtained at neutral pH were found to follow Langmuir adsorption. Anisidine was a better adsorbate because of its higher basicity. In alkaline medium (pH > 8) both aniline and anisidine reacted with chromium ferrocyanide to give colored products. Analysis of the products by GC– MS showed benzoquinone and azobenzene as the reaction products of aniline while p-anisidine afforded a dimer. IR analysis of the amine–chromium ferrocyanide adduct suggests that the outer metal ion of chromium ferrocyanide and amino group of amines are responsible for the interaction. A possible reaction mechanism for the product formation in alkaline medium has been proposed. The present study suggests that metal ferrocyanides might have played an important role in the stabilization of organic molecules through their surface activity in the prebiotic condensation reactions. °C 2002 Elsevier Science Key Words: adsorption; interaction; metal ferrocyanides; anisidine; aniline.
EXPERIMENTAL
Materials Potassium ferrocyanide (BDH), chromic chloride (BDH), aniline (BDH), p-anisidine (BDH), and all other chemicals used were of analytical reagent grade. Aniline was distilled and anisidine was recrystallized each time before use.
INTRODUCTION
Clays and clay minerals are proposed to be the most suitable materials that could have contributed to certain reactions producing polymeric substances from which life has emerged. Clay minerals are also known to interact with certain organic compounds producing color derivatives (1–6). It is assumed that transition metal ions, abundant in the primeval sea, would have formed complex compounds with the simple and readily available molecules (7, 8). It is, therefore, reasonable to assume that transition metal ions could have easily formed a number of soluble and insoluble complexes with the abundant CN− present in the primeval sea. The insoluble cyano metal complexes thus formed could have settled at the bottom of the sea or on the sea shore and catalyzed a number of reactions such as condensation–oligomerization and oxidation on their surfaces. The existence of metal ferri- and ferrocyanides on primitive earth has been proposed by Arrhenius (9).
1 To whom correspondence should be addressed. Fax: +91-1332-73560. E-mail: [email protected].
Amines are widely distributed in nature in the form of amino acids, alkaloids, and vitamins. The presence of amino acids containing aromatic rings on primitive earth has been proposed by Friedmann and Miller (10). One could, reasonably, postulate the presence of aromatic amines on primitive earth. Druing the past decade some studies on the catalytic role of cyano complexes and their possible role in chemical evolution have been carried out in the authors’ laboratory. A number of metal ferrocyanides have been synthesized and their interactions with organic monomers studied in detail (11–20). The present paper describes studies on the interaction of aniline and p-anisidine with chromium ferrocyanide. Spectral studies indicate that the outer metal ion of chromium ferrocyanide is responsible for the interaction.
Preparation of Chromium Ferrocyanide Chromium ferrocyanide was prepared following the method proposed by Malik et al. (21). It was characterized on the basis of elemental analysis and spectral studies. The percentage composition of the metals was determined by using a PerkinElmer 3100 AAS. Carbon, hydrogen, and nitrogen analysis was performed on a CEST-118 CHN analyzer. Methodology Electronic spectra of aniline and anisidine were recorded on a Beckman DU-6 spectrophotometer. The characteristic λmax values for aniline and anisidine are 280 and 295 nm, respectively. Infrared spectra of adsorbates, adsorbent, and adsorption adducts were recorded on a Perkin-Elmer 1600 FTIR spectrophotometer using KBr pellets. The adsorption of aniline and anisidine on chromium ferrocyanide as a function of pH (3.0–7.0) and amine concentration (10−3 –10−4 M) was studied at 25◦ C and represented in Figs. 1
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All rights reserved.
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ALAM ET AL.
FIG. 1. Adsorption of aromatic amines on chromium ferrocyanide as a function of pH.
and 2, respectively. A series of 50-ml glass test tubes were employed and each tube was filled with 10 ml of amine solution of varying concentration. The pH of the solution was adjusted to the desired value using acetate or borax buffers. Species of these buffers do not get adsorbed onto the chromium ferrocyanide surface. This was verified by conductivity measurements as there was no change in the inflection point of buffers with and without chromium ferrocyanide (11). Chromium ferrocyanide (100 mg) was added to each tube. The tubes were shaken with an Expo shaker initially for 1 h and then allowed to equilibrate at 25◦ C with intermittent shaking at fixed time intervals. The equilibrium was attained within 6 h. The equilibrium time and concentration range were, however, decided after a good deal of preliminary investigations. The concentration of aniline and anisidine was measured spectrophotometrically at 280 and 295 nm, respectively. The amount of amine adsorbed was calculated from the difference between the amine concentration before
FIG. 2. Adsorption isotherms of aromatic amines on chromium ferrocyanide.
and after adsorption. The equilibrium concentration of amine and the amount adsorbed were fitted in the adsorption isotherm (Fig. 2). However, in alkaline medium (pH > 8) brown-red colored products were deposited on the chromium ferrocyanide surface within 24 h. These colored products were concentrated for GC–MS analysis by extraction in benzene. Analysis of the reaction products was performed on a Shimadzu gas chromatograph coupled directly to a Shimadzu QP-2000 quadrupole mass spectrometer system. Separation was performed on a fused silica capillary column Ulbon HR-1 equivalent to OV-1 (0.25 mm × 50 m with film thickness 0.25 µm). The conditions for GC were as follows: injector temperature, 250◦ C; transfer line temperature, 250◦ C. The capillary column temperature was programmed as follows: 100◦ C for 2 min; from 100 to 250◦ C at 10◦ C min−1 , held at 250◦ C for 15 min. Helium was used as a carrier gas with a flow rate of 2 ml min−1 . The mass spectrometer conditions were ion source 250◦ C and ionization energy 40 eV. The BET method was used to determine the surface area of chromium ferrocyanide. In this technique the surface area is determined by physical adsorption of N2 at its boiling temperature. The calculated surface area of chromium ferrocyanide is found to be 22.85 m2 /g. RESULTS AND DISCUSSION
A wide pH range (3.0–7.0) was selected for priliminary adsorption studies of amines on chromium ferrocyanide. A remarkable change in the amount of adsorbate was observed by varying the pH of the solution. Subsequent studies were performed at pH 7.0 which was optimum for the maximum sorption of both the amines. The neutral pH is physiologically significant as most of the reactions in living systems take place in neutral media. The molecular formula of chromium ferrocyanide was found to be KCr[Fe (CN)6 ] · 5H2 O. The species [Fe(CN)6 ]4− in chromium ferrocyanide exists with octahedral geometry where the central iron atom is surrounded by six CN− ligands (22). Due to the strong field of the CN− ligands, all six electrons paired up to give the electronic configuration t62g . Although the CN− ligands bond with Fe via σ -donation, there is sufficient back bonding from iron metal dπ orbitals to the CN− ligand antibonding pπ orbitals. Transition metal ferrocyanide complexes usually have a polymeric lattice structure with [Fe(CN)6 ]4− anions where the outer metal ion may be coordinated through the nitrogen end of the cyanide ligand. In earlier studies on the adsorption of aromatic amines with metal ferrocyanide, the interation has been proposed to occur between the amino group and the outer ions of metal ferrocyanides (14, 15). In the lower pH range aniline showed greater uptake in comparison to anisidine. This can be explained on the basis of the fact that in acidic media anisidine has a greater tendency for salt formation and thus a lesser tendency to interact with chromium ferocyanide. On the other hand, aniline has a lesser tendency for
253
ANILINE–CHROMIUM FERROCYANIDE INTERACTION
TABLE 1 Langmuir Constants for the Adsorption of Amines on Chromium Ferrocyanide Amine
K L (1 mol−1 )
X m (mg g−1 )
p-Anisidine Aniline
2.01 6.34
48.72 27.23
salt formation resulting in a greater tendency to interact with ferrocyanide. As the pH rises to neutral value, anisidine has more electrons available to interact with ferrocyanide and thus there is a drastic increase in uptake. Adsorption isotherms (Fig. 2) of aniline and anisidine in the present case show that adsorption is fast in both cases and the isotherms are regular, positive, and concave to the concentration axis. Adsorption data can be represented through a Langmuir adsorption isotherm which assumes the formation of a monolayer of solute molecules on the surface of the adsorbent and given by
FIG. 3. Mass spectrum of benzoquinone.
Ceq Ceq 1 = + Xe KL Xm Xm or 1 = Xe
µ
1 Ceq
¶µ
1 KL Xm
¶
FIG. 4. Mass spectrum of azobenzene.
1 , + Xm
where X e is the amount of solute adsorbed per gram of adsorbent; Ceq , the equilibrium concentration of the solute; X m , the moles of solute required per gram weight of chromium ferrocyanide for the formation of complete monolayer on the surface; and K L , a constant related to the heat of adsorption or enthalpy. The values of X m and K L were also calculated (Table 1). X m values also indicate that anisidine adsorption is more in comparison to that of aniline. The percent uptake of anisidine and aniline is 87.2 and 73.4, respectively. It is observed from percent binding that anisidine
FIG. 5. Mass spectrum of 4,40 -dimethoxyazobenzene.
TABLE 2 Typical Infrared Spectral Frequencies (cm−1 ) of Amines and Chromium Ferrocyanide before and after Adsorption Amine characteristic frequencies Amines Aniline
p-Anisidine
Chromium ferrocyanide characteristic frequencies
νN–H
δN–H
νC–N
νCH of–OCH3
νC≡ N
νFe–C
δFe–CN
3463 (3374) 3555 (3508) 3515 (3433) 3555 (3503)
1618 (1631)
1330 (1259)
—
2099 (2098)
602 (601)
494 (494)
1614 (1632)
1259 (1232)
2926 (2837)
2101 (2098)
604 (601)
496 (494)
Note. Values in parentheses represent frequencies before adsorption.
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ALAM ET AL.
SCHEME 1
is strongly adsorbed on chromium ferrocyanide in comparison to aniline. The observed adsorption trend is also related to the basicities of amines. The pK b values of anisidine and aniline are 8.8 and 9.3, respectively. This indicates that anisidine is a stronger base than aniline which reflects the greater availability of electrons for the interaction. In the present study it seems that strong chemical forces are responsible for adsorption of aniline and anisidine on chromium ferrocyanide. Efforts to extract the adsorbed amines from chromium ferrocyanide with either 0.1 M NH4 Cl or 0.1 M NH4 OH gave negligible desorption indicating the presence of strong chemical forces in the adsorption process. It is unlikely for amine molecules to enter into the inner sphere of chromium ferrocyanide simply by replacing a strong ligand such as cyanide. Ohno (23) reported that the replacement of cyanide by another strong ligand is possible only under UV light. Chromium ferrocyanide–amine adducts were washed several times with water to remove excess amine and dried. The
infrared spectra of chromium ferrocyanide before and after adsorption were recorded and analyzed. The results are given in Table 2. It was observed that νN–H (doublet 3374, 3508 and 3433, 3503 cm−1 ) shifted to 3463, 3555 and 3515, 3555 cm−1 , whereas δN–H frequencies (1631 and 1632 cm−1 ) shifted to 1618 and 1614 cm−1 for aniline and anisidine, respectively. The weak band of νC–N (1259 and 1232 cm−1 ) shifted to 1330 and 1259 cm−1 in the case of aniline and anisidine, respectively. The change in characteristic frequencies of chromium ferrocyanide after adsorption was not remarkable. The pronounced change in the characteristic frequencies of amino group indicates that the interaction occurred through the chromium ion with an amino group. Analysis of Reaction Products The product extracted in benzene was subjected to gas chromatography. Three major peaks with retention time (Rt ) 2.61, 4.16, and 20.97 min corresponding to aniline, p-benzoquinone,
SCHEME 2
255
ANILINE–CHROMIUM FERROCYANIDE INTERACTION
SCHEME 3
and azobenzene were observed, respectively. Mass spectra of the reaction products corresponding to peaks at 4.16 and 20.97 min are shown in Figs. 3 and 4. The formation of benzoquinone (Rt = 4.16 min) is clearly evidenced by the peaks corresponding to m/z 108, 82, and 54, in accordance with its fragmentation pattern determined by electron bombardment. However, GC–MS study of the peak with Rt = 20.97 min showed the formation of azobenzene which is confirmed by a peak at m/z 182. Some other mass peaks in the fragmentation pattern of the product at m/z 105, 77, and 51 were also observed. A well-known mechanism (24) is proposed for the formation of benzoquinone and azobenzene (Schemes 1 and 2). In the case of anisidine the chromatogram showed two major peaks with Rt = 2.81 and 14.72 min corresponding to anisidine and its dimer, respectively. Mass analysis of the product with Rt = 14.72 min showed major peaks at m/z 242, 135, and 107 which correspond to fragmentation of anisidine dimer (Fig. 5). A possible mechanism for the formation of dimer of anisidine may be proposed as shown in Scheme 3. The role of Cr3+ –ferrocyanide in the proposed transformations of aniline and anisidine is significant. As a consequence of the direct coordination of amine through its amino group to the Cr3+ cation, the oxidation of amine to a free radical takes place possibly by the removal of a hydrogen atom from the amine group and the reduction of Cr3+ to Cr2+ . Condensation of the amine free radical (type I, Scheme 1) produces azo-compounds probably through hydrazobenzene as an intermediate (Schemes 2 and 3). The anilino radical may exist in another canonical form (type III) which might undergo coupling to afford benzoquinone (Scheme 1).
CONCLUSION
The present and earlier works (11–20) have shown metal ferrocyanides to be reasonable and plausible candidates in studies involving adsorption and condensation of biologically important molecules. The implications of this research in prebiotic chemistry are of significance because it has shown experimentally that lower valent metal ions substituted in potassium ferrocyanide provide a good adsorption site that could have played an important role in early stages of chemical evolution. The insoluble metal ferrocyanides, either settled at the bottom of sea or on the sea shore, on coming in contact with biomolecules could have acted as active surfaces for concentrating them.
REFERENCES 1. Theng, D. K. G., Clays Clay Miner. 19, 383 (1971). 2. Furukawa, T., and Brindley, G. W., Clays Clay Miner. 21, 279 (1973). 3. Govindraj, N., Mortland, M. M., and Boyd, S. A., Environ. Sci. Technol. 21, 1119 (1987). 4. Issacson, P. J., and Sawhney, B. L., Clay Miner. 18, 253 (1983). 5. Soma, Y., and Soma, M., Chemosphere 18, 1895 (1989). 6. Boyd, S. A., and Mortland, M. M., Environ. Sci. Technol. 20, 1056 (1996). 7. Kobayashi, K., and Ponnamperuma, C., Orig. Life 16, 41 (1985). 8. Egami, F., J. Biochem. 77, 1165 (1975). 9. Arrhenius, G., “Fourth Symposium on Chemical Evolution and Origin and Evolution of Life.” NASA Am. Research Centre, Moffet Field, July 24–27, 1990. 10. Friedmann, N., and Miller, S. L., Science 166, 766 (1969). 11. Kamaluddin, Nath, M., and Sharma, A., Orig. Life Evol. Biosphere 24, 469 (1994). 12. Viladkar, S., Agarwal, R., and Kamaluddin, Bull. Chem. Soc. Jpn. 69, 95 (1996).
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13. Kamaluddin, Nath, M., Deopujari, S., and Sharma, A., Orig. Life Evol. Biosphere 20, 259 (1990). 14. Viladkar, S., Alam, T., and Kamaluddin, J. Inorg. Biochem. 53, 69 (1994). 15. Alam, T., Tarannum, H., Ravi Kumar, M. N. V., and Kamaluddin, Talanta 51, 1097 (2000). 16. Alam, T., and Kamaluddin, Bull. Chem. Soc. Jpn. 72, 1697 (1999). 17. Alam, T., and Kamaluddin, Colloids Surf. 162, 89 (2000). 18. Alam, T., Tarannum, H., Kumar, N., and Kamaluddin, J. Colloid Interface Sci. 224, 133 (2000).
19. Tiwari, B. B., Mohan, D., and Kamaluddin, Colloids Surf. 131, 89 (1998). 20. Tiwari, B. B., and Kamaluddin, J. Colloid Interface Sci. 193, 167 (1997). 21. Malik, W. U., Srivastava, S. K., Bhandari, V. M., and Kumar, S., J. Inorg. Nucl. Chem. 38, 342 (1976). 22. Chandra, K., Raj, D., and Puri, S. P., J. Phys. Chem. 46, 1466 (1967). 23. Ohno, S. I., Bull. Chem. Soc. Jpn. 40, 1765 (1967). 24. Zimmer, H., Lanki, D. C., and Horgan, S. W., Chem. Rev. 71, 229 (1971).
Adsorption and Oxidation of Aniline and Anisidine by Chromium Ferrocyanide Tanveer Alam,∗ Hina Tarannum,† Shah Raj Ali,† and Kamaluddin†,1 ∗ K.L.D.A.V. Degree College, Roorkee 247 667, India; and †Chemistry Department, University of Roorkee, Roorkee 247 667, India Received June 12, 2001; accepted September 14, 2001; published online December 10, 2001
The interaction of aniline and p-anisidine with chromium ferrocyanide has been studied. Maximum uptake of both anilines was observed around pH 7. The adsorption data obtained at neutral pH were found to follow Langmuir adsorption. Anisidine was a better adsorbate because of its higher basicity. In alkaline medium (pH > 8) both aniline and anisidine reacted with chromium ferrocyanide to give colored products. Analysis of the products by GC– MS showed benzoquinone and azobenzene as the reaction products of aniline while p-anisidine afforded a dimer. IR analysis of the amine–chromium ferrocyanide adduct suggests that the outer metal ion of chromium ferrocyanide and amino group of amines are responsible for the interaction. A possible reaction mechanism for the product formation in alkaline medium has been proposed. The present study suggests that metal ferrocyanides might have played an important role in the stabilization of organic molecules through their surface activity in the prebiotic condensation reactions. °C 2002 Elsevier Science Key Words: adsorption; interaction; metal ferrocyanides; anisidine; aniline.
EXPERIMENTAL
Materials Potassium ferrocyanide (BDH), chromic chloride (BDH), aniline (BDH), p-anisidine (BDH), and all other chemicals used were of analytical reagent grade. Aniline was distilled and anisidine was recrystallized each time before use.
INTRODUCTION
Clays and clay minerals are proposed to be the most suitable materials that could have contributed to certain reactions producing polymeric substances from which life has emerged. Clay minerals are also known to interact with certain organic compounds producing color derivatives (1–6). It is assumed that transition metal ions, abundant in the primeval sea, would have formed complex compounds with the simple and readily available molecules (7, 8). It is, therefore, reasonable to assume that transition metal ions could have easily formed a number of soluble and insoluble complexes with the abundant CN− present in the primeval sea. The insoluble cyano metal complexes thus formed could have settled at the bottom of the sea or on the sea shore and catalyzed a number of reactions such as condensation–oligomerization and oxidation on their surfaces. The existence of metal ferri- and ferrocyanides on primitive earth has been proposed by Arrhenius (9).
1 To whom correspondence should be addressed. Fax: +91-1332-73560. E-mail: [email protected].
Amines are widely distributed in nature in the form of amino acids, alkaloids, and vitamins. The presence of amino acids containing aromatic rings on primitive earth has been proposed by Friedmann and Miller (10). One could, reasonably, postulate the presence of aromatic amines on primitive earth. Druing the past decade some studies on the catalytic role of cyano complexes and their possible role in chemical evolution have been carried out in the authors’ laboratory. A number of metal ferrocyanides have been synthesized and their interactions with organic monomers studied in detail (11–20). The present paper describes studies on the interaction of aniline and p-anisidine with chromium ferrocyanide. Spectral studies indicate that the outer metal ion of chromium ferrocyanide is responsible for the interaction.
Preparation of Chromium Ferrocyanide Chromium ferrocyanide was prepared following the method proposed by Malik et al. (21). It was characterized on the basis of elemental analysis and spectral studies. The percentage composition of the metals was determined by using a PerkinElmer 3100 AAS. Carbon, hydrogen, and nitrogen analysis was performed on a CEST-118 CHN analyzer. Methodology Electronic spectra of aniline and anisidine were recorded on a Beckman DU-6 spectrophotometer. The characteristic λmax values for aniline and anisidine are 280 and 295 nm, respectively. Infrared spectra of adsorbates, adsorbent, and adsorption adducts were recorded on a Perkin-Elmer 1600 FTIR spectrophotometer using KBr pellets. The adsorption of aniline and anisidine on chromium ferrocyanide as a function of pH (3.0–7.0) and amine concentration (10−3 –10−4 M) was studied at 25◦ C and represented in Figs. 1
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0021-9797/02 $35.00
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All rights reserved.
252
ALAM ET AL.
FIG. 1. Adsorption of aromatic amines on chromium ferrocyanide as a function of pH.
and 2, respectively. A series of 50-ml glass test tubes were employed and each tube was filled with 10 ml of amine solution of varying concentration. The pH of the solution was adjusted to the desired value using acetate or borax buffers. Species of these buffers do not get adsorbed onto the chromium ferrocyanide surface. This was verified by conductivity measurements as there was no change in the inflection point of buffers with and without chromium ferrocyanide (11). Chromium ferrocyanide (100 mg) was added to each tube. The tubes were shaken with an Expo shaker initially for 1 h and then allowed to equilibrate at 25◦ C with intermittent shaking at fixed time intervals. The equilibrium was attained within 6 h. The equilibrium time and concentration range were, however, decided after a good deal of preliminary investigations. The concentration of aniline and anisidine was measured spectrophotometrically at 280 and 295 nm, respectively. The amount of amine adsorbed was calculated from the difference between the amine concentration before
FIG. 2. Adsorption isotherms of aromatic amines on chromium ferrocyanide.
and after adsorption. The equilibrium concentration of amine and the amount adsorbed were fitted in the adsorption isotherm (Fig. 2). However, in alkaline medium (pH > 8) brown-red colored products were deposited on the chromium ferrocyanide surface within 24 h. These colored products were concentrated for GC–MS analysis by extraction in benzene. Analysis of the reaction products was performed on a Shimadzu gas chromatograph coupled directly to a Shimadzu QP-2000 quadrupole mass spectrometer system. Separation was performed on a fused silica capillary column Ulbon HR-1 equivalent to OV-1 (0.25 mm × 50 m with film thickness 0.25 µm). The conditions for GC were as follows: injector temperature, 250◦ C; transfer line temperature, 250◦ C. The capillary column temperature was programmed as follows: 100◦ C for 2 min; from 100 to 250◦ C at 10◦ C min−1 , held at 250◦ C for 15 min. Helium was used as a carrier gas with a flow rate of 2 ml min−1 . The mass spectrometer conditions were ion source 250◦ C and ionization energy 40 eV. The BET method was used to determine the surface area of chromium ferrocyanide. In this technique the surface area is determined by physical adsorption of N2 at its boiling temperature. The calculated surface area of chromium ferrocyanide is found to be 22.85 m2 /g. RESULTS AND DISCUSSION
A wide pH range (3.0–7.0) was selected for priliminary adsorption studies of amines on chromium ferrocyanide. A remarkable change in the amount of adsorbate was observed by varying the pH of the solution. Subsequent studies were performed at pH 7.0 which was optimum for the maximum sorption of both the amines. The neutral pH is physiologically significant as most of the reactions in living systems take place in neutral media. The molecular formula of chromium ferrocyanide was found to be KCr[Fe (CN)6 ] · 5H2 O. The species [Fe(CN)6 ]4− in chromium ferrocyanide exists with octahedral geometry where the central iron atom is surrounded by six CN− ligands (22). Due to the strong field of the CN− ligands, all six electrons paired up to give the electronic configuration t62g . Although the CN− ligands bond with Fe via σ -donation, there is sufficient back bonding from iron metal dπ orbitals to the CN− ligand antibonding pπ orbitals. Transition metal ferrocyanide complexes usually have a polymeric lattice structure with [Fe(CN)6 ]4− anions where the outer metal ion may be coordinated through the nitrogen end of the cyanide ligand. In earlier studies on the adsorption of aromatic amines with metal ferrocyanide, the interation has been proposed to occur between the amino group and the outer ions of metal ferrocyanides (14, 15). In the lower pH range aniline showed greater uptake in comparison to anisidine. This can be explained on the basis of the fact that in acidic media anisidine has a greater tendency for salt formation and thus a lesser tendency to interact with chromium ferocyanide. On the other hand, aniline has a lesser tendency for
253
ANILINE–CHROMIUM FERROCYANIDE INTERACTION
TABLE 1 Langmuir Constants for the Adsorption of Amines on Chromium Ferrocyanide Amine
K L (1 mol−1 )
X m (mg g−1 )
p-Anisidine Aniline
2.01 6.34
48.72 27.23
salt formation resulting in a greater tendency to interact with ferrocyanide. As the pH rises to neutral value, anisidine has more electrons available to interact with ferrocyanide and thus there is a drastic increase in uptake. Adsorption isotherms (Fig. 2) of aniline and anisidine in the present case show that adsorption is fast in both cases and the isotherms are regular, positive, and concave to the concentration axis. Adsorption data can be represented through a Langmuir adsorption isotherm which assumes the formation of a monolayer of solute molecules on the surface of the adsorbent and given by
FIG. 3. Mass spectrum of benzoquinone.
Ceq Ceq 1 = + Xe KL Xm Xm or 1 = Xe
µ
1 Ceq
¶µ
1 KL Xm
¶
FIG. 4. Mass spectrum of azobenzene.
1 , + Xm
where X e is the amount of solute adsorbed per gram of adsorbent; Ceq , the equilibrium concentration of the solute; X m , the moles of solute required per gram weight of chromium ferrocyanide for the formation of complete monolayer on the surface; and K L , a constant related to the heat of adsorption or enthalpy. The values of X m and K L were also calculated (Table 1). X m values also indicate that anisidine adsorption is more in comparison to that of aniline. The percent uptake of anisidine and aniline is 87.2 and 73.4, respectively. It is observed from percent binding that anisidine
FIG. 5. Mass spectrum of 4,40 -dimethoxyazobenzene.
TABLE 2 Typical Infrared Spectral Frequencies (cm−1 ) of Amines and Chromium Ferrocyanide before and after Adsorption Amine characteristic frequencies Amines Aniline
p-Anisidine
Chromium ferrocyanide characteristic frequencies
νN–H
δN–H
νC–N
νCH of–OCH3
νC≡ N
νFe–C
δFe–CN
3463 (3374) 3555 (3508) 3515 (3433) 3555 (3503)
1618 (1631)
1330 (1259)
—
2099 (2098)
602 (601)
494 (494)
1614 (1632)
1259 (1232)
2926 (2837)
2101 (2098)
604 (601)
496 (494)
Note. Values in parentheses represent frequencies before adsorption.
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SCHEME 1
is strongly adsorbed on chromium ferrocyanide in comparison to aniline. The observed adsorption trend is also related to the basicities of amines. The pK b values of anisidine and aniline are 8.8 and 9.3, respectively. This indicates that anisidine is a stronger base than aniline which reflects the greater availability of electrons for the interaction. In the present study it seems that strong chemical forces are responsible for adsorption of aniline and anisidine on chromium ferrocyanide. Efforts to extract the adsorbed amines from chromium ferrocyanide with either 0.1 M NH4 Cl or 0.1 M NH4 OH gave negligible desorption indicating the presence of strong chemical forces in the adsorption process. It is unlikely for amine molecules to enter into the inner sphere of chromium ferrocyanide simply by replacing a strong ligand such as cyanide. Ohno (23) reported that the replacement of cyanide by another strong ligand is possible only under UV light. Chromium ferrocyanide–amine adducts were washed several times with water to remove excess amine and dried. The
infrared spectra of chromium ferrocyanide before and after adsorption were recorded and analyzed. The results are given in Table 2. It was observed that νN–H (doublet 3374, 3508 and 3433, 3503 cm−1 ) shifted to 3463, 3555 and 3515, 3555 cm−1 , whereas δN–H frequencies (1631 and 1632 cm−1 ) shifted to 1618 and 1614 cm−1 for aniline and anisidine, respectively. The weak band of νC–N (1259 and 1232 cm−1 ) shifted to 1330 and 1259 cm−1 in the case of aniline and anisidine, respectively. The change in characteristic frequencies of chromium ferrocyanide after adsorption was not remarkable. The pronounced change in the characteristic frequencies of amino group indicates that the interaction occurred through the chromium ion with an amino group. Analysis of Reaction Products The product extracted in benzene was subjected to gas chromatography. Three major peaks with retention time (Rt ) 2.61, 4.16, and 20.97 min corresponding to aniline, p-benzoquinone,
SCHEME 2
255
ANILINE–CHROMIUM FERROCYANIDE INTERACTION
SCHEME 3
and azobenzene were observed, respectively. Mass spectra of the reaction products corresponding to peaks at 4.16 and 20.97 min are shown in Figs. 3 and 4. The formation of benzoquinone (Rt = 4.16 min) is clearly evidenced by the peaks corresponding to m/z 108, 82, and 54, in accordance with its fragmentation pattern determined by electron bombardment. However, GC–MS study of the peak with Rt = 20.97 min showed the formation of azobenzene which is confirmed by a peak at m/z 182. Some other mass peaks in the fragmentation pattern of the product at m/z 105, 77, and 51 were also observed. A well-known mechanism (24) is proposed for the formation of benzoquinone and azobenzene (Schemes 1 and 2). In the case of anisidine the chromatogram showed two major peaks with Rt = 2.81 and 14.72 min corresponding to anisidine and its dimer, respectively. Mass analysis of the product with Rt = 14.72 min showed major peaks at m/z 242, 135, and 107 which correspond to fragmentation of anisidine dimer (Fig. 5). A possible mechanism for the formation of dimer of anisidine may be proposed as shown in Scheme 3. The role of Cr3+ –ferrocyanide in the proposed transformations of aniline and anisidine is significant. As a consequence of the direct coordination of amine through its amino group to the Cr3+ cation, the oxidation of amine to a free radical takes place possibly by the removal of a hydrogen atom from the amine group and the reduction of Cr3+ to Cr2+ . Condensation of the amine free radical (type I, Scheme 1) produces azo-compounds probably through hydrazobenzene as an intermediate (Schemes 2 and 3). The anilino radical may exist in another canonical form (type III) which might undergo coupling to afford benzoquinone (Scheme 1).
CONCLUSION
The present and earlier works (11–20) have shown metal ferrocyanides to be reasonable and plausible candidates in studies involving adsorption and condensation of biologically important molecules. The implications of this research in prebiotic chemistry are of significance because it has shown experimentally that lower valent metal ions substituted in potassium ferrocyanide provide a good adsorption site that could have played an important role in early stages of chemical evolution. The insoluble metal ferrocyanides, either settled at the bottom of sea or on the sea shore, on coming in contact with biomolecules could have acted as active surfaces for concentrating them.
REFERENCES 1. Theng, D. K. G., Clays Clay Miner. 19, 383 (1971). 2. Furukawa, T., and Brindley, G. W., Clays Clay Miner. 21, 279 (1973). 3. Govindraj, N., Mortland, M. M., and Boyd, S. A., Environ. Sci. Technol. 21, 1119 (1987). 4. Issacson, P. J., and Sawhney, B. L., Clay Miner. 18, 253 (1983). 5. Soma, Y., and Soma, M., Chemosphere 18, 1895 (1989). 6. Boyd, S. A., and Mortland, M. M., Environ. Sci. Technol. 20, 1056 (1996). 7. Kobayashi, K., and Ponnamperuma, C., Orig. Life 16, 41 (1985). 8. Egami, F., J. Biochem. 77, 1165 (1975). 9. Arrhenius, G., “Fourth Symposium on Chemical Evolution and Origin and Evolution of Life.” NASA Am. Research Centre, Moffet Field, July 24–27, 1990. 10. Friedmann, N., and Miller, S. L., Science 166, 766 (1969). 11. Kamaluddin, Nath, M., and Sharma, A., Orig. Life Evol. Biosphere 24, 469 (1994). 12. Viladkar, S., Agarwal, R., and Kamaluddin, Bull. Chem. Soc. Jpn. 69, 95 (1996).
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ALAM ET AL.
13. Kamaluddin, Nath, M., Deopujari, S., and Sharma, A., Orig. Life Evol. Biosphere 20, 259 (1990). 14. Viladkar, S., Alam, T., and Kamaluddin, J. Inorg. Biochem. 53, 69 (1994). 15. Alam, T., Tarannum, H., Ravi Kumar, M. N. V., and Kamaluddin, Talanta 51, 1097 (2000). 16. Alam, T., and Kamaluddin, Bull. Chem. Soc. Jpn. 72, 1697 (1999). 17. Alam, T., and Kamaluddin, Colloids Surf. 162, 89 (2000). 18. Alam, T., Tarannum, H., Kumar, N., and Kamaluddin, J. Colloid Interface Sci. 224, 133 (2000).
19. Tiwari, B. B., Mohan, D., and Kamaluddin, Colloids Surf. 131, 89 (1998). 20. Tiwari, B. B., and Kamaluddin, J. Colloid Interface Sci. 193, 167 (1997). 21. Malik, W. U., Srivastava, S. K., Bhandari, V. M., and Kumar, S., J. Inorg. Nucl. Chem. 38, 342 (1976). 22. Chandra, K., Raj, D., and Puri, S. P., J. Phys. Chem. 46, 1466 (1967). 23. Ohno, S. I., Bull. Chem. Soc. Jpn. 40, 1765 (1967). 24. Zimmer, H., Lanki, D. C., and Horgan, S. W., Chem. Rev. 71, 229 (1971).