Indian Journal of Chemistry Vol. 40A, April2001, pp. 340-344 Solvatochromic parameters and linear solvation energy relationships for hydrolysis of hydroxamic acids Kallol K Ghosh*, Pank aj Tamrakar & Santosh Singh Thakur School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur, 492 010, India Received 17 Apri/2000; revised 6 November 2000 Kinetic solvent effects on hydrolysis of hydroxamic ac ids of the type R.N (OH). C=O.R'[R=H; R'=CH 3 , C 6 H 5 , C 6 H4-0H; R=CH rC 6 H 4 R' = C 6 H 5 ] in aqueous mixtures (I 0-70% vl v) of some polar protic, polar aprotic and low polarity basic so lvents have been studied. The so lv atochromic parameters a for the hydrogen-bond-donating ability, 13. fo r the hydrogen- bond-accepting ability and, 1t *, for the dipolarity/polarizability and linea r solvation energy relationships have been used to quantify solvent effects. Kamlet and T aft's triparametric equation explain the greater than 70 % of the effect of so lv ent on the hydrolysis. The other parameters i.e., D, ET and Z have also been used. Many studies have been reported on the effect of solvents, mainly pure solvents, on reaction rate 1 - 3 Several studies have also been reported on the application of Kamlet-Taft parameters4- 6 . However, hydrolysis reactions in binary aqueous mixtures are limited?-9 . Investigation in mixed solvents, which are common in studies of reaction kinetics, have been hampered due to non-availability of solvatochromk parameters for the binary aqueous solvent mixtures 10 • The Kamlet-Taft parameters are solvatochromic parameters and the values for a particular co mpound may be different for different solvent and solute situations. Marcus and associates 11 - 13 extended solvato- chrornic parameters, such as a, n* [ET and Z] to mixed solvent s. The application of the techniques of multiple regression and Marcus values of the parameters have now proved successful m understanding the role of solvent on reaction mechanism. We have undertaken a detailed program of investigations on the effect of diverse solvents on the acid catalysed hydrolysis of hydroxamic acids, which are widely studied because of both their biological activity as well as their chemical properties 14 " 15 • It is also important to know how solvatochromic parameters rationalize multiple interacting so vent effects. The interpretation of the kinetic solvent effect on the basis of solvatochromic parameters of the medium does not always succeed. We have taken some low polarity basic, polar aprotic and polar protic solvents (Table 1 ). This chemometrics 16 classification is based primarily on solvent polarity/acidity, Jess on polarity/basicity, and least on polarity/polarizability. The quantitative correlation betwee n rate constant for the hydrolysis reaction of hydroxamic acids and solvatochromic parameters (a, n*, Z,ET) is still an elusive exercise. One, two, and three parameters equations involving different combinations of linear solvation free energy relationships have been attempted with a view to finding out an efficient correlation. Materials and Method All the hydroxamic acids were prepared by the standard method 17 • 1,4-Dioxane (Qualigens), DMSO (Merck), DMF (BDH), acetone (Qualigens ExcelaR), ethanol (Anilax Chern. ACS.), propan-2-ol (Qualigens), acetonitrile (Merck), tetrahydrofuron (Merck), methanol, ethylene glycol (Qualigens) and sulfolane (Merck) were used without further purification. All other chemicals used were of AnalaR grades. The kinetic measurements were followed spectrophotometrically by measuring the concentration of hydroxarnic acids by Fe 3 + ions and the rate constants were calculated as described before 18 • Results and Discussion The kinetics of acid catalysed hydrolysis of hydroxamk acids [acetohydroxamic acid (AHA), benzohydroxamic acid (BHA), salicylhydroxarnic acid (S HA), N-p-tolylb enzohydroxamk acid (p- TBHA)] in diverse organic solvents have been studied and the results are listed in Table 1. The kinetics were studied by varying the percentage of non-aqueous cosolvent from 10-70% (v/ v) in binary aqueous mixtures.
Transcript
Indian Journal of Chemistry Vol. 40A, April2001, pp. 340-344
Solvatochromic parameters and linear solvation energy relationships for hydrolysis of hydroxamic acids
Kallol K Ghosh*, Pankaj Tamrakar & Santosh Singh Thakur School of Studies in Chemistry, Pt. Ravishankar Shukla Uni versi ty, Raipur, 492 010, India
Received 17 Apri/2000; revised 6 November 2000
Kinetic solvent effects on hydrolysis of hydroxamic acids of the type R.N (OH). C=O.R'[R=H; R'=CH3, C6H5, C6H4-0H; R=CHrC6H4R' = C6H5] in aqueous mixtures (I 0-70% vlv) of some polar protic, polar aprotic and low polarity basic solvents have been studied. The solvatochromic parameters a for the hydrogen-bond-donating ability, 13. fo r the hydrogenbond-accepting ability and, 1t*, for the dipolarity/polarizability and linear solvation energy relationships have been used to quantify solvent effects. Kamlet and Taft's triparametric equation explain the greater than 70% of the effect of solvent on the hydrolysis. The other parameters i.e., D, ET and Z have also been used.
Many studies have been reported on the effect of solvents, mainly pure solvents, on reaction rate 1
-3
Several studies have also been reported on the application of Kamlet-Taft parameters4-6
. However, hydrolysis reactions in binary aqueous mixtures are limited?-9. Investigation in mixed solvents, which are common in studies of reaction kinetics, have been hampered due to non-availability of solvatochromk parameters for the binary aqueous solvent mixtures 10
•
The Kamlet-Taft parameters are solvatochromic parameters and the values for a particular compound may be different for different solvent and solute situations. Marcus and associates 11
-13 extended solvato
chrornic parameters, such as a, ~. n* [ET and Z] to mixed solvents. The application of the techniques of multiple regression and Marcus values of the parameters have now proved successful m understanding the role of solvent on reaction mechanism.
We have undertaken a detailed program of investigations on the effect of di verse solvents on the acid catalysed hydrolysis of hydroxamic acids, which are widely studied because of both their biological activity as well as their chemical properties 14
"15
• It is also important to know how solvatochromic parameters rationalize multiple interacting so vent effects. The interpretation of the kinetic solvent effect on the basis of solvatochromic parameters of the medium does not always succeed. We have taken some low polarity basic, polar aprotic and polar protic solvents (Table 1 ). This chemometrics 16 classification is based primarily on solvent polarity/acidity, Jess on polarity/basicity, and least on polarity/polarizability.
The quantitative correlation between rate constant for the hydrolysis reaction of hydroxamic acids and solvatochromic parameters (a, ~. n*, Z,ET) is still an elusive exercise. One, two, and three parameters equations involving different combinations of linear solvation free energy relationships have been attempted with a view to finding out an efficient correlation.
Materials and Method All the hydroxamic acids were prepared by the
standard method 17•
1,4-Dioxane (Qualigens), DMSO (Merck), DMF (BDH), acetone (Qualigens ExcelaR), ethanol (Anilax Chern. ACS.), propan-2-ol (Qualigens), acetonitrile (Merck), tetrahydrofuron (Merck), methanol, ethylene glycol (Qualigens) and sulfolane (Merck) were used without further purification. All other chemicals used were of AnalaR grades.
The kinetic measurements were followed spectrophotometrically by measuring the concentration of hydroxarnic acids by Fe3
+ ions and the rate constants were calculated as described before 18
•
Results and Discussion The kinetics of acid catalysed hydrolysis of
hydroxamk acids [acetohydroxamic ac id (AHA), benzohydroxamic acid (BHA), salicylhydroxarnic acid (S HA), N-p -toly lbenzohydroxamk acid (pTBHA)] in diverse organic solvents have been studied and the results are listed in Table 1. The kinetics were studied by varying the percentage of non-aqueous cosolvent from 10-70% (v/v) in binary aqueous mixtures.
GHOSH eta/.: SOL V ATOCHROMISM IN THE HYDROLYSIS OF HYDROXAMIC ACIDS
Table 1-The observed rnte constant (koo,, 105 s' 1) of acid catalysed hydrolysis of hydroxamic acids (I) in the solvent given at 55°C unless otherwise specified
The accepted mechanism for acid catalysed hydrolysis of hydroxamic acids is represented by Eqs I and 2
K R N(OH)(C = O)R' + H+~ RN(OH)(C = o +H)R'
.. . (I) k
R N(OH)(C = o +H)R' + H20 ¢=> R' COOH + RNH;OH
... (2) Under pseudo-first order conditions [excess
catalytic acid (2.9 mol dm-3) and water] the observed
first order rate constant, kobs. is given by Eq. 3.
(3)
The order of reaction with respect to catalytic acid has been established previously for the conditions employed in this study 18·19. The hydrated proton is the catalytic acid under the conditions employed.
A perusal of the kinetic results (first order rate constants) show following two types of behaviour. For all the substrates, the rate accelerating effect is shown by dioxane, MeOH and MeCN, while DMSO, DMF, THF, EtOH, i-PrOH and Me,CO show a rate retarding effect. A retarding effect was also shown by DMSO in the case of AHA and SHA, and by DMSO in the case of AHA; BHA and SHA. DMSO showed an accelerating effect in the case of p-TBHA.
Most probably the organic cosolvent exerts two types of opposite effects on the rate. The first type of effect due to which the rate is enhance is the greater solvation of the transition state and the increase of true water molecules from water clusters, while the other type of effects responsible for decreasing the rate constant is (i) decrease of the bulk dielectric constant of the medium, (ii) decrease in the polarity of the solvent, and, (iii) decrease in the relative catalytic activity of H+. From an electrostatic viewpoint, a rate decrease might be expected because of destabilization of the polar transition state when the bulk dielectric constant is lowered by successive addition of the solvent. Since the highly polar transition state is more strongly solvated relative to the less polar ground state, it is expected that as the solvent polarity decreases the reaction rate decreases20. Aqueous DMF, EtOH, i-PrOH, Me,CO and THF show perfect agreement with such theory, but the reaction in the other solvents show anomalous results.
The plots of kobs versus liD give almost straight
lines. Deviation from this linearity are observed at higher solvent composition. These deviations are probably due to the preferential solvation of the activated complex by water, the higher dielectric constant compound. Two types of curves have been observed. This indicates that there is no simple correlation between the reaction rate constant and the dielectric constant of the medium.
It is hardly surprising that a single parameter fails to sum up the complexities of solvation. Due to limitations of dielectric constant different solvent parameters have been developed which are based on actual solvent senstttve chemical or physical processes. Majority of these are based on linear free energy relationships and solvatochromic parameters.
Univariate linear solvation energy relationships (LSERs) may possess the conventional linear free energy relationship form21 (LFER) as shown in Eq 4 or they may simply be plots of log kobs against a solvent parameter such as Z and ET etc.
k log-=RS
ko .. . (4)
where R is characteristic of the reaction and S is the function (ET or Z) of the solvent. The rate constants have been related with Kosower21 , Z, and DirnrothReichardt22, ET, parameters. Good linear correlation have been observed. In the present case correlation with ET and Z are good. Therefore the properties ET and Z appear to mimic the solvation effects on the reaction rate better than does the bulk property, D. Which any mechanistic information from the correlation must be very limited, the solvatochromic correlation has expanded our ability to comprehend this reaction by relating it to quite a different phenomenon.
The most familiar multivariate LSER is that of the Kamlet-Taft4 given as Eq. 5.
XYZ = [XYZ]0+aa + bf3+s1t* + h()" + e~ ... (5)
where [XYZ]0, a, b and s are solvent independent coefficients, characteristics of the process and indicative of its sensitivity to the accompanying solvent properties, a is the hydrogen bond donation, HBD, (acidity) ability of the solvent, f3 is its hydrogen bond acceptance HBA (basicity) parameter, n* is its dipolarity/polarizability parameter, ()H is the Hildebrand solubility parameter and ~ is a coordinate covalency
GHOSH et al.: SOLVATOCHROMISM IN THE HYDROLYSIS OF HYDROXAMIC ACIDS 343
Table 2-Triparametric analysis of the solvent effects using Kamlet and Taft parameter for hydrolysis of AHA, BHA, SHA andp-TBHA
index. For rate processes, Eq. (5) can be written as
log kobs =log k. +a a+ b B + s n* +minor terms . . . (6)
Attempts have been made to correlate the variations in reaction rate. Simple linear regression analysis was tried first and later multiple regressions. The results of correlation analyses in terms of Eq. 5, a triparametric equation (a, B. n *) are given in Table 2 for all the hydroxamic acids studied.
The rate constants were also correlated separately against individual parameters i.e., a, B and n* (Data not shown). All the values of solvatochromic parameters have been taken from literature23
-25
• TMS was not considered as the complete range of solvatochromic parameters are not available. It was found that multiple linear regression gives better correlation. In dilute regions the properties are not very different. Kamlet's triparametric equation explains the greater than 70% (p-TBHA = 71%, BHA = 70%, SHA = 85%, AHA= 70%) of the effect of solvent on the hydrolysis in higher composition. The major contribution is of solvent polarity which sums up all the specific and non-specific interactions of the media with initial and transition states. A minor role is played by a whereas B plays an insignificant role. Similarly biparametric equation involving a and B. accounted for 70% of the effect. Monoparametric equation also gave similar results, but the correlation is not very satisfactory. The choice of the best
parameter for every type of interaction is critical because of the complexity of the corresponding empirical solvent parameters, and also because of their susceptibility to more than one of the multiple facets of solvent polarity.
Acknowledgements Financial assistance from Department of Science
and Technology (SP/S-1/G-28/94 New Delhi) and Council of Scientific and Industrial Research, New Delhi (India) are gratefully acknowledged. The authors are deeply grateful to Prof. Yizhak Marcus, University of Jerusalem, Israel for providing literature on solvatochromic parameter and Prof. Christian Reichardt, Philipps University, FRG, for sending his extremely useful book "Solvents & Solvent Effect in Organic Chemistry." The authors are grateful to Head SOS in Chemistry, Pt. Ravishankar Shukla University, Raipur for providing laboratory facilities .
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