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Mechanism of acid catalysis in the dissociation of tetra-aquo-mono-oxalato- and tetra-aquo-mono-malonato-chromium(III) ions
J. inorg,nucl.Chem.. 1970,Vol.3Z pp. 2985 to 2992. PergamonPress. Printedin Great Britain
MECHANISM OF ACID CATALYSIS IN THE DISSOCIATION OF TETRA-AQUO-MONO-OXALATOAND TETRA-AQUO-MONO-MALONATO-CHROMIUM(I IONS
I I)
D. BANERJEA and S. DUTTA C H A U D H U R I Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta-32, India (Received 16 October 1969) A b s t r a c t - T h e kinetics of the acid catalysed dissociation of tetra-aquo-mono-oxalato- and tetraaquo-mono-malonato-chromium(lll) ions in aqueous perchloric acid media have been investigated spectrophotometrically and activation parameters for the reactions have been evaluated. The observed dependence of the rates on acid concentration and acidity function, Ho, suggests the operation of different mechanisms, in different regions of acid concentration, due to different types of solvent participation in the transformations of the protonated complexes. INTRODUCTION
THE acid-catalysed aquations of the tris-[1,2] and the bis-[3,4] oxalato and malonato complexes of chromium(Ill) have been investigated. Similar studies on the tetra-aquomono-oxalato and tetra-aquomono-malonato complexes of chromium(l l l) in aqueous perchloric acid media are now reported. Preliminary observations indicated that dissociation of these species into the hexa-aquochromium(IIl) ion can be followed conveniently at a moderately high temperature in strongly acid media (see Table 1). The rates of the reactions were followed spectrophotometrically at 415 m/x and 565 mp., for the mono-oxalato and monomalonato complexes, respectively, where the molar absorbance of the hexaaquochromium(l I I) ion is much lower than that of the reacting complexes. EXPERIMENTAl_ Materials and reagents. Solutions of tetra-aquomono-oxalato-chromium{11I) [3], tetra-aquomonomalonato-chromiumlllI)[4] and hexa-aquochromium(IIl)[5] perchlorates were prepared, analysed and stored as described previously. Reagent grade IG. R., E. Merck) perchloric acid was used. Distilled water was redistilled before use [5]. Apparatus and procedure. A Hilger Uvispek Spectrophotometer {H-700)was used for absorbance measurements. Samples of reacting solution removed at suitable intervals were quenched [6] before measuring the absorbances. RESULTS AND DISCUSSION
The pseudo-first-order rate-constant, kobs, for each experiment was evaluated graphically by plotting log (A o-,4 ~)/tAt-A ~) vs. time It) using for A ~ the calculated 1. 2. 3. 4. 5. 6.
D. D. D. D. D. D.
Banerjea andM. S. Mohan, J. inorg, nucl. Chem. 27, 1643 (1965). Banerjea and C. Chatterjee, J. inorg, nucl. Chem. 30, 3354 (1968). Banerjea and M. S. Mohan, J. inorg, nucl. Chem. 26, 613 (1964). Banerjea and C. Chatterjee, J. inorg, nucL Chem. 29, 2387 11967). Banerjea and S. Dutta Chaudhuri, J. inorg, nucl. Chem. 30, 871 ~1968). Banerjea and P. Chaudhuri, J. inorg, nucl. Chem. 30, 3259 (1968). 2985
2986
D. B A N E R J E A and S. D. C H A U D H U R I
Table 1. kobs values under different conditions {Accuracy _+ 2%). Cr(OHe)4A+; 0-002M (a = O x 2- or Mal 2-) Temp. (°C) 60
70
80
85 90
94.8
HCIO4 (M) 1.0 1.5 2-0 2-5 3.0 3.5 3.7 4.0 4.5 5.0 8.0 1-0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7-0 8.0 9.0 1.0 4.0 8.0 2.0 1.0 1.5 2.0 2.5 3.0 3.5 4-0 4-5 2.0
103 kobs (min -1) Oxalato complex Malonato complex 0.38 0.87 1.67 2.90 4.26 5-80 6.52 8.28 11-30 14.17 2.37 1-27 0.31 0.48 0.75 1.00 1.62 2.26 3.27 4.50 6-85 14.00 1-4 19.1 0.3
25.00
3 -46 74.50
9.72 0.21 0.45 0.87 1.56 2.53 3-86 5-84 0.73
value of the absorbance corresponding to complete transformation of the complex into Cr(OH2)63+. The kobs values obtained under various conditions for both the systems are given in Table 1. The plot of kobs vs. perchloric acid concentrations is a highly curved line with continuously increasing (positive) slope. In both the systems the Zucker and Hammett[7] plot of log kobs vs. log[HC104] is a straight line of unusually high slope (2.32 for the mono-malonato complex at 60°C and 3.96 for the monooxalato complex at 70°C) (Fig. 1).'The alternative Zucker and Hammett[7] plot of log kobs vs. -- Ho [8] is again a curved line more or less resolvable into initial 7. L. Zucker and L. P. Hammett, J. A m . chem. Soc. 61,2791 (1939). 8. Ho values have been taken from M. A. Paul and F. A. Long, Chem. Rev. 57, 1 (1957).
M e c h a n i s m of acid catalysis
_98,
and final linear portions with unusual slopes. T h u s , for the m o n o - m a l o n a t o c o m p l e x at 60°C the initial and final slopes are 1.16 and 0.5, and t h o s e for the m o n o - o x a l a t o c o m p l e x at 70°C are 0.7 and 0.34 respectively (Fig. 2). Clearly, i
i
i
i
i
i
~
i
i
C
o J
°
_1 + 0
-Jo
L
o.J
L
o,z
L
0.3
~
o4
L
o.5
~
o.~
t
t
o7
o.o
0.9
Log [HClO4 ] Fig. 1. A-Oxalato complex at 70°C; B-Oxalato complex at 90°C: C-Malonato complex at 60°C. I
I
i
i
'1
i
i
i
C
÷FO
i
j o
l + to)
-I.O
I
I
0"6
1'2
I
1.8
I
I
I
k
1
I
24
30
36
42
4-8
54
-
H o
Fig. 2. A - O x a l a t o complex at 70°C: B-Oxalato complex at 90°C: C - M a l o n a t o at 60°C.
complex
2988
D. BANERJEA and S. D. C H A U D H U R I
therefore, the Zucker-Hammett plots do not lead to any definite conclusion as such large discrepancies cannot be entirely due to the use of Ho values for 25°C instead of H+ [9] values at the experimental temperatures [10]. An attempt has been made to elucidate the mechanism from a plot of log kobs+ H o vs. log arho [ 11 ] (slope = w) [ 12] and of log kobs+ Ho vs. log [H+] + Ho (slope = ~b)[13] as suggested by Bunnett[12, 13] (Figs. 3 and 4). Regions of different slope observed in these plots suggest the operation of different mechanisms[14]. The observed oJ and ~b values for each of the systems in different -I.0
I
I
t
(A)
4- o ..,1 ÷
+ 1.0 I
I
-0.3
I
-0.6
-0'9
I
I
Log OH20
0'5 I
~
(B)
o
._.1 4-
+0.5
I
-oq
-0.2 -0-3 Log a H20 Fig. 3. (A)-Oxa]ato complex at 70°C (in 3-5-9.0M HCIO4); (B)-ll-Oxalato complex at 90°C (in 1.5-4.5 M HCIO4), O-Malonato complex at 60°C (in 1.0-5.0 M HCIO4). 9. T. G. Bonner and J. C. Lockhart, J. chem. Soc., 364, 2840 t1957). 10. A. I. Gelbstein, G. G. Shcheglova and M. I. Temkin, Zh. neorg. Khim. 1, 282, 506 (1956); Dokl. Acad. N a u k 107, 108 (1956). 11. Log an.2o values were taken from Ref. [ 12]. 12. J. F. Bunnett, J. Am. chem. Soc. 83, 4956 (1961). 13. J. F. Bunnett and F. P. Olsen, Can. J. Chem. 44, 1917 (1966). 14. J. F. Bunnett, J. Am. chem. Soc. 83, 4968 (1961).
Mechanismof acid catalysis -I 0
i
2989
F
E
_~©
-45 I
(A)
+
0
+l© _115
Coq [HCIO~ + Ho
-05
(S)
o
_g' o
+0'5
~
-
I -05
L - t0
I -h5
Log [HCIO4 ] + Ho
Fig. 4. (A)-Oxalato complex at 70°C (in 3-5-9.0 M HCIO~); (B)-Q-Oxalato complex at 90°C fin 1.5-4.5 M HCIO4), ©-Malonato complex al 60°C (in 1.0-5-0 M HCIO~).
ranges of acid concentrations and the corresponding mechanistic conclusions (see Refs. [ 13] and [ 14]) are indicated in Table 2. Values of the activation parameters, AH$ and ASS (Table 3), evaluated as usual (Fig. 5) from the following relation in the different ranges of acid concentration, are in agreement with the mechanistic conclusions based on the 4' values: - log kobs" h / K T = -- log [H +] + AH:~/2.3RT -- AS$/2.3R. It is significant that while in both the monooxalato and the monomalonato systems the AH$ value is independent of the acid concentration, the ASS values are different in the different regions of acid concentration in conformity with
2990
D. BANERJEA and S. D. C H A U D H U R I
Table 2. co and q5 values in different regions of acid concentration and the corresponding mechanistic conclusions
Complex
Experimental temp. (°C)
Cr(OH~)4(Mal) +
60
Cr(OH2h(Ox) +
70
Range of HCIO4 Concn.
90
(M)
oJ
4)
1.0-2.5 2.5-5.0 3.5-9"0
-2.5 3-1 2.5
1.5-3.0 3-0-4.5
- 3.0 2.1
-0.23 0.54 0.48 (at 3"5 -6M) 0.7 (at 6.5-9M) -- 0'4 0-38
Possible mechanism* SgI Sx2
Sx l S~-2
*The designations SN2 and S~I denote mechanisms in which the formation of the transition state from the protonated complex does or does not involve the participation of a molecule of the solvent, H20, respectively. -~o~value suggests S~2 mechanism in the entire range 3.5-.9.0 M HCIO4, while qb values suggest SN2 mechanism in the range 3-5-6.0 M HCIO4, and a different mechanism with the solvent molecule acting as a proton transfer agent in the range 6.5-9.0M HC104. This latter may perhaps involve a change in which H30 + acts as a nucleophile attacking the protonated complex and simultaneously transfering a proton (H +) to the leaving oxalate group (see Equations (1) and (2c)). Excepting in this case in all the other cases w and ~bvalues lead to identical mechanistic conclusion.
I
i
i
17.6
..J I
16.4
16.0 15.6
~ /
27-0
I
28 -0
29.0
I/T
I
30.0
xIO 4
Fig. 5. Eyring plots: O-Oxalato complex; ~ O-Malonato complex. HCIO4, molar: 1 (line 3), 2 (line 1), 4 (lines 2 and 5) 8 (line 4).
2991
M e c h a n i s m of acid catalysis Table 3. Values of the activation parameters in different regions of acid concentration for the oxalato and the malOnato c o m p l e x e s Monooxalato complex HCIO4, molar
2'0 23.9 (_+0.5) --18.1 (± 1-5)
.5115. kcal/mole AS:i:, e.u.
4"0 24-4 1+0.5) -- 14.1 (+_ 1.2)
M o n o m a l o n a t o complex
8"0 24-2 1±1)-5) --10.9 (± 1.2)
1"0 25.0 (+__0.3) --7.8 (±0.8)
4"0 25.0 (_+0.3) 4-5 I± 0,9)
the operation of different mechanisms due to different types of solvent participation as suggested by the 6 values (lot. cit.). The set of changes in the systems may be represented as follows taking the oxalato complex as an example: O--C=O {H.,O)~Cr
+/
(-)--('~-O H +
+ H:~O+ ~ (H~OhCr
\
q- H . 2 0 .
\ O--C=O
(I)
()--C~---O (Rapid pre-equilibrium
1
Sxl (1-5-3"0 M HC10,): COOH s|o'o,
1
2+
) (H,_,O)4Cr
fast ) Cr(OH2)B3+-t -HC20 21I,O
\
4 .
121a})
O---C=O Sx2 (H.,O)(3"5-6"0 M HCIO~): COOH
OH2 slow ~H()
2+
fast
(H2OhCr
H_,O
O
) Cr(OH2)03+ + HC.~,O4 . (2(b))
C = O
SN2 (H:¢O+) (6"5-9"0 M HCIO4): OHz slow
__COOH
2+ J
/
+H:()~
~O
fast
/
) (H20)4Cr
C
- OH +
+ H.,O
Cr(OH2)63+ +
H2C204. (2(C))
On the basis of the present observations it seems that 4) value is better diagnostic of mechanism of acid catalysis than the co value, at least in the high acid concentration region. In the case of the oxalato c o m p l e x while the ~o value is
2992
D. BANERJEA and S. D. C H A U D H U R i
the same in the range 3.5-9M HC104, the 4~ values are significantly different in the regions 3.5-6M and 6.5-9M HCIO4 respectively (Table 2). In agreement with this the AS~: values are also different in the two regions (Table 3).
MECHANISM OF ACID CATALYSIS IN THE DISSOCIATION OF TETRA-AQUO-MONO-OXALATOAND TETRA-AQUO-MONO-MALONATO-CHROMIUM(I IONS
I I)
D. BANERJEA and S. DUTTA C H A U D H U R I Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta-32, India (Received 16 October 1969) A b s t r a c t - T h e kinetics of the acid catalysed dissociation of tetra-aquo-mono-oxalato- and tetraaquo-mono-malonato-chromium(lll) ions in aqueous perchloric acid media have been investigated spectrophotometrically and activation parameters for the reactions have been evaluated. The observed dependence of the rates on acid concentration and acidity function, Ho, suggests the operation of different mechanisms, in different regions of acid concentration, due to different types of solvent participation in the transformations of the protonated complexes. INTRODUCTION
THE acid-catalysed aquations of the tris-[1,2] and the bis-[3,4] oxalato and malonato complexes of chromium(Ill) have been investigated. Similar studies on the tetra-aquomono-oxalato and tetra-aquomono-malonato complexes of chromium(l l l) in aqueous perchloric acid media are now reported. Preliminary observations indicated that dissociation of these species into the hexa-aquochromium(IIl) ion can be followed conveniently at a moderately high temperature in strongly acid media (see Table 1). The rates of the reactions were followed spectrophotometrically at 415 m/x and 565 mp., for the mono-oxalato and monomalonato complexes, respectively, where the molar absorbance of the hexaaquochromium(l I I) ion is much lower than that of the reacting complexes. EXPERIMENTAl_ Materials and reagents. Solutions of tetra-aquomono-oxalato-chromium{11I) [3], tetra-aquomonomalonato-chromiumlllI)[4] and hexa-aquochromium(IIl)[5] perchlorates were prepared, analysed and stored as described previously. Reagent grade IG. R., E. Merck) perchloric acid was used. Distilled water was redistilled before use [5]. Apparatus and procedure. A Hilger Uvispek Spectrophotometer {H-700)was used for absorbance measurements. Samples of reacting solution removed at suitable intervals were quenched [6] before measuring the absorbances. RESULTS AND DISCUSSION
The pseudo-first-order rate-constant, kobs, for each experiment was evaluated graphically by plotting log (A o-,4 ~)/tAt-A ~) vs. time It) using for A ~ the calculated 1. 2. 3. 4. 5. 6.
D. D. D. D. D. D.
Banerjea andM. S. Mohan, J. inorg, nucl. Chem. 27, 1643 (1965). Banerjea and C. Chatterjee, J. inorg, nucl. Chem. 30, 3354 (1968). Banerjea and M. S. Mohan, J. inorg, nucl. Chem. 26, 613 (1964). Banerjea and C. Chatterjee, J. inorg, nucL Chem. 29, 2387 11967). Banerjea and S. Dutta Chaudhuri, J. inorg, nucl. Chem. 30, 871 ~1968). Banerjea and P. Chaudhuri, J. inorg, nucl. Chem. 30, 3259 (1968). 2985
2986
D. B A N E R J E A and S. D. C H A U D H U R I
Table 1. kobs values under different conditions {Accuracy _+ 2%). Cr(OHe)4A+; 0-002M (a = O x 2- or Mal 2-) Temp. (°C) 60
70
80
85 90
94.8
HCIO4 (M) 1.0 1.5 2-0 2-5 3.0 3.5 3.7 4.0 4.5 5.0 8.0 1-0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7-0 8.0 9.0 1.0 4.0 8.0 2.0 1.0 1.5 2.0 2.5 3.0 3.5 4-0 4-5 2.0
103 kobs (min -1) Oxalato complex Malonato complex 0.38 0.87 1.67 2.90 4.26 5-80 6.52 8.28 11-30 14.17 2.37 1-27 0.31 0.48 0.75 1.00 1.62 2.26 3.27 4.50 6-85 14.00 1-4 19.1 0.3
25.00
3 -46 74.50
9.72 0.21 0.45 0.87 1.56 2.53 3-86 5-84 0.73
value of the absorbance corresponding to complete transformation of the complex into Cr(OH2)63+. The kobs values obtained under various conditions for both the systems are given in Table 1. The plot of kobs vs. perchloric acid concentrations is a highly curved line with continuously increasing (positive) slope. In both the systems the Zucker and Hammett[7] plot of log kobs vs. log[HC104] is a straight line of unusually high slope (2.32 for the mono-malonato complex at 60°C and 3.96 for the monooxalato complex at 70°C) (Fig. 1).'The alternative Zucker and Hammett[7] plot of log kobs vs. -- Ho [8] is again a curved line more or less resolvable into initial 7. L. Zucker and L. P. Hammett, J. A m . chem. Soc. 61,2791 (1939). 8. Ho values have been taken from M. A. Paul and F. A. Long, Chem. Rev. 57, 1 (1957).
M e c h a n i s m of acid catalysis
_98,
and final linear portions with unusual slopes. T h u s , for the m o n o - m a l o n a t o c o m p l e x at 60°C the initial and final slopes are 1.16 and 0.5, and t h o s e for the m o n o - o x a l a t o c o m p l e x at 70°C are 0.7 and 0.34 respectively (Fig. 2). Clearly, i
i
i
i
i
i
~
i
i
C
o J
°
_1 + 0
-Jo
L
o.J
L
o,z
L
0.3
~
o4
L
o.5
~
o.~
t
t
o7
o.o
0.9
Log [HClO4 ] Fig. 1. A-Oxalato complex at 70°C; B-Oxalato complex at 90°C: C-Malonato complex at 60°C. I
I
i
i
'1
i
i
i
C
÷FO
i
j o
l + to)
-I.O
I
I
0"6
1'2
I
1.8
I
I
I
k
1
I
24
30
36
42
4-8
54
-
H o
Fig. 2. A - O x a l a t o complex at 70°C: B-Oxalato complex at 90°C: C - M a l o n a t o at 60°C.
complex
2988
D. BANERJEA and S. D. C H A U D H U R I
therefore, the Zucker-Hammett plots do not lead to any definite conclusion as such large discrepancies cannot be entirely due to the use of Ho values for 25°C instead of H+ [9] values at the experimental temperatures [10]. An attempt has been made to elucidate the mechanism from a plot of log kobs+ H o vs. log arho [ 11 ] (slope = w) [ 12] and of log kobs+ Ho vs. log [H+] + Ho (slope = ~b)[13] as suggested by Bunnett[12, 13] (Figs. 3 and 4). Regions of different slope observed in these plots suggest the operation of different mechanisms[14]. The observed oJ and ~b values for each of the systems in different -I.0
I
I
t
(A)
4- o ..,1 ÷
+ 1.0 I
I
-0.3
I
-0.6
-0'9
I
I
Log OH20
0'5 I
~
(B)
o
._.1 4-
+0.5
I
-oq
-0.2 -0-3 Log a H20 Fig. 3. (A)-Oxa]ato complex at 70°C (in 3-5-9.0M HCIO4); (B)-ll-Oxalato complex at 90°C (in 1.5-4.5 M HCIO4), O-Malonato complex at 60°C (in 1.0-5.0 M HCIO4). 9. T. G. Bonner and J. C. Lockhart, J. chem. Soc., 364, 2840 t1957). 10. A. I. Gelbstein, G. G. Shcheglova and M. I. Temkin, Zh. neorg. Khim. 1, 282, 506 (1956); Dokl. Acad. N a u k 107, 108 (1956). 11. Log an.2o values were taken from Ref. [ 12]. 12. J. F. Bunnett, J. Am. chem. Soc. 83, 4956 (1961). 13. J. F. Bunnett and F. P. Olsen, Can. J. Chem. 44, 1917 (1966). 14. J. F. Bunnett, J. Am. chem. Soc. 83, 4968 (1961).
Mechanismof acid catalysis -I 0
i
2989
F
E
_~©
-45 I
(A)
+
0
+l© _115
Coq [HCIO~ + Ho
-05
(S)
o
_g' o
+0'5
~
-
I -05
L - t0
I -h5
Log [HCIO4 ] + Ho
Fig. 4. (A)-Oxalato complex at 70°C (in 3-5-9.0 M HCIO~); (B)-Q-Oxalato complex at 90°C fin 1.5-4.5 M HCIO4), ©-Malonato complex al 60°C (in 1.0-5-0 M HCIO~).
ranges of acid concentrations and the corresponding mechanistic conclusions (see Refs. [ 13] and [ 14]) are indicated in Table 2. Values of the activation parameters, AH$ and ASS (Table 3), evaluated as usual (Fig. 5) from the following relation in the different ranges of acid concentration, are in agreement with the mechanistic conclusions based on the 4' values: - log kobs" h / K T = -- log [H +] + AH:~/2.3RT -- AS$/2.3R. It is significant that while in both the monooxalato and the monomalonato systems the AH$ value is independent of the acid concentration, the ASS values are different in the different regions of acid concentration in conformity with
2990
D. BANERJEA and S. D. C H A U D H U R I
Table 2. co and q5 values in different regions of acid concentration and the corresponding mechanistic conclusions
Complex
Experimental temp. (°C)
Cr(OH~)4(Mal) +
60
Cr(OH2h(Ox) +
70
Range of HCIO4 Concn.
90
(M)
oJ
4)
1.0-2.5 2.5-5.0 3.5-9"0
-2.5 3-1 2.5
1.5-3.0 3-0-4.5
- 3.0 2.1
-0.23 0.54 0.48 (at 3"5 -6M) 0.7 (at 6.5-9M) -- 0'4 0-38
Possible mechanism* SgI Sx2
Sx l S~-2
*The designations SN2 and S~I denote mechanisms in which the formation of the transition state from the protonated complex does or does not involve the participation of a molecule of the solvent, H20, respectively. -~o~value suggests S~2 mechanism in the entire range 3.5-.9.0 M HCIO4, while qb values suggest SN2 mechanism in the range 3-5-6.0 M HCIO4, and a different mechanism with the solvent molecule acting as a proton transfer agent in the range 6.5-9.0M HC104. This latter may perhaps involve a change in which H30 + acts as a nucleophile attacking the protonated complex and simultaneously transfering a proton (H +) to the leaving oxalate group (see Equations (1) and (2c)). Excepting in this case in all the other cases w and ~bvalues lead to identical mechanistic conclusion.
I
i
i
17.6
..J I
16.4
16.0 15.6
~ /
27-0
I
28 -0
29.0
I/T
I
30.0
xIO 4
Fig. 5. Eyring plots: O-Oxalato complex; ~ O-Malonato complex. HCIO4, molar: 1 (line 3), 2 (line 1), 4 (lines 2 and 5) 8 (line 4).
2991
M e c h a n i s m of acid catalysis Table 3. Values of the activation parameters in different regions of acid concentration for the oxalato and the malOnato c o m p l e x e s Monooxalato complex HCIO4, molar
2'0 23.9 (_+0.5) --18.1 (± 1-5)
.5115. kcal/mole AS:i:, e.u.
4"0 24-4 1+0.5) -- 14.1 (+_ 1.2)
M o n o m a l o n a t o complex
8"0 24-2 1±1)-5) --10.9 (± 1.2)
1"0 25.0 (+__0.3) --7.8 (±0.8)
4"0 25.0 (_+0.3) 4-5 I± 0,9)
the operation of different mechanisms due to different types of solvent participation as suggested by the 6 values (lot. cit.). The set of changes in the systems may be represented as follows taking the oxalato complex as an example: O--C=O {H.,O)~Cr
+/
(-)--('~-O H +
+ H:~O+ ~ (H~OhCr
\
q- H . 2 0 .
\ O--C=O
(I)
()--C~---O (Rapid pre-equilibrium
1
Sxl (1-5-3"0 M HC10,): COOH s|o'o,
1
2+
) (H,_,O)4Cr
fast ) Cr(OH2)B3+-t -HC20 21I,O
\
4 .
121a})
O---C=O Sx2 (H.,O)(3"5-6"0 M HCIO~): COOH
OH2 slow ~H()
2+
fast
(H2OhCr
H_,O
O
) Cr(OH2)03+ + HC.~,O4 . (2(b))
C = O
SN2 (H:¢O+) (6"5-9"0 M HCIO4): OHz slow
__COOH
2+ J
/
+H:()~
~O
fast
/
) (H20)4Cr
C
- OH +
+ H.,O
Cr(OH2)63+ +
H2C204. (2(C))
On the basis of the present observations it seems that 4) value is better diagnostic of mechanism of acid catalysis than the co value, at least in the high acid concentration region. In the case of the oxalato c o m p l e x while the ~o value is
2992
D. BANERJEA and S. D. C H A U D H U R i
the same in the range 3.5-9M HC104, the 4~ values are significantly different in the regions 3.5-6M and 6.5-9M HCIO4 respectively (Table 2). In agreement with this the AS~: values are also different in the two regions (Table 3).