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ECW model
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ECW model
In chemistry, the ECW model is a semi-quantitative model that describes and predicts the strength of Lewis acid–Lewis base interactions. Many chemical reactions can be described as acid–base reactions, so models for such interactions are of potentially broad interest. The model initially assigned E and C parameters to each and every acid and base. The model was later expanded to the ECW model to cover reactions that have a constant energy term, W, which describes processes that precede the acid–base reaction. This quantitative model is often discussed with the qualitative HSAB theory, which also seeks to rationalize the behavior of diverse acids and bases.
As early as 1938, G. N. Lewis pointed out that the relative strength of an acid or base depended upon the base or acid against which it was measured. No single rank order of acid or base strength can predict the energetics of the cross reaction. Consider the following pair of acid–base reactions:.
These data suggest that OEt2 is a stronger base than SMe2. The opposite is found, however, when I2 is the acid:
The E-C model accommodates the failure of single parameter descriptions of acids and bases. In 1965 Russell S. Drago and Bradford Wayland published the two term equation such that each acid and each base is described by two parameters. Each acid is characterized by an EA and a CA. Each base is likewise characterized by its own EB and CB. The E and C parameters refer, respectively, to the electrostatic and covalent contributions to the strength of the bonds that the acid and base will form. These parameters have been empirically obtained by using enthalpies for adducts that form only σ bonds between the acid and base as well as adducts that have no steric repulsion between the acid and base.
This equation reproduces and predicts the enthalpy, ΔH, of a reaction between many acids and bases. ΔH is a measure of strength of the bond between the acid and the base, both in the gas phase and in weakly solvating media. Entropy effects are ignored. A matrix presentation of the equation enhances its utility.
Four values, two E and two C were assigned as references. EA and CA of I2 were chosen as standards. Since I2 has little tendency to undergo electrostatic bonding, the EA parameter was assigned a small value, 0.5, while the value of CA for the covalent property was set at 2.0. For the two base parameters, EB for CH3C(O)N(CH3)2 (DMA) was set at 2.35 and CB for (C2H5)2S, diethyl sulfide, was set at 3.92. Fixing the parameters in this way imposed the covalent-electrostatic model on the data set by fixing the EAEB and CACB products of the DMA and (C2H5)2S adducts with iodine, and these four values ensured that none of the parameters had negative values. Due to increasing enthalpy data that became available since the EC equation was first proposed the parameters have been improved. Mixing E and C numbers from the improved set of parameters with older parameters will result in incorrect calculations and is to be avoided. A select set of the improved E and C numbers is found in this article and the complete set is available in the literature. EB and CB parameters for phosphines that can be used in combination with the improved parameters for oxygen, nitrogen, and sulfur donors to measure σ-basicity have been reported.
In the ECW model, a new term W was added to the equation.
The W term represents a constant energy for cleavage of a dimeric acid or base. For example, the enthalpy of cleavage the [Rh(CO)2Cl]2 by base B involves two steps. The first step is cleavage of the dimer, which is W:
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ECW model
In chemistry, the ECW model is a semi-quantitative model that describes and predicts the strength of Lewis acid–Lewis base interactions. Many chemical reactions can be described as acid–base reactions, so models for such interactions are of potentially broad interest. The model initially assigned E and C parameters to each and every acid and base. The model was later expanded to the ECW model to cover reactions that have a constant energy term, W, which describes processes that precede the acid–base reaction. This quantitative model is often discussed with the qualitative HSAB theory, which also seeks to rationalize the behavior of diverse acids and bases.
As early as 1938, G. N. Lewis pointed out that the relative strength of an acid or base depended upon the base or acid against which it was measured. No single rank order of acid or base strength can predict the energetics of the cross reaction. Consider the following pair of acid–base reactions:.
These data suggest that OEt2 is a stronger base than SMe2. The opposite is found, however, when I2 is the acid:
The E-C model accommodates the failure of single parameter descriptions of acids and bases. In 1965 Russell S. Drago and Bradford Wayland published the two term equation such that each acid and each base is described by two parameters. Each acid is characterized by an EA and a CA. Each base is likewise characterized by its own EB and CB. The E and C parameters refer, respectively, to the electrostatic and covalent contributions to the strength of the bonds that the acid and base will form. These parameters have been empirically obtained by using enthalpies for adducts that form only σ bonds between the acid and base as well as adducts that have no steric repulsion between the acid and base.
This equation reproduces and predicts the enthalpy, ΔH, of a reaction between many acids and bases. ΔH is a measure of strength of the bond between the acid and the base, both in the gas phase and in weakly solvating media. Entropy effects are ignored. A matrix presentation of the equation enhances its utility.
Four values, two E and two C were assigned as references. EA and CA of I2 were chosen as standards. Since I2 has little tendency to undergo electrostatic bonding, the EA parameter was assigned a small value, 0.5, while the value of CA for the covalent property was set at 2.0. For the two base parameters, EB for CH3C(O)N(CH3)2 (DMA) was set at 2.35 and CB for (C2H5)2S, diethyl sulfide, was set at 3.92. Fixing the parameters in this way imposed the covalent-electrostatic model on the data set by fixing the EAEB and CACB products of the DMA and (C2H5)2S adducts with iodine, and these four values ensured that none of the parameters had negative values. Due to increasing enthalpy data that became available since the EC equation was first proposed the parameters have been improved. Mixing E and C numbers from the improved set of parameters with older parameters will result in incorrect calculations and is to be avoided. A select set of the improved E and C numbers is found in this article and the complete set is available in the literature. EB and CB parameters for phosphines that can be used in combination with the improved parameters for oxygen, nitrogen, and sulfur donors to measure σ-basicity have been reported.
In the ECW model, a new term W was added to the equation.
The W term represents a constant energy for cleavage of a dimeric acid or base. For example, the enthalpy of cleavage the [Rh(CO)2Cl]2 by base B involves two steps. The first step is cleavage of the dimer, which is W: