ECW model
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In chemistry, the ECW model is a semi-quantitative model that describes and predicts the strength of Lewis acidLewis base interactions. Many
chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
s can be described as
acid–base reaction An acid–base reaction is a chemical reaction that occurs between an acid and a base. It can be used to determine pH via titration. Several theoretical frameworks provide alternative conceptions of the reaction mechanisms and their applica ...
s, 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 HSAB concept is a jargon for "hard and soft (Lewis) acids and bases". HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemic ...
, which also seeks to rationalize the behavior of diverse acids and bases.


History of the problem

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:. : 4F-C6H4OH + OEt2   −Δ''H'' = 5.94 kcal/mole : 4F-C6H4OH + SMe2   −Δ''H'' = 4.73 kcal/mole These data suggest that OEt2 is a stronger base than SMe2. The opposite is found, however, when I2 is the acid: : I2 + OEt2   −Δ''H'' = 4.16 kcal/mole : I2 + SMe2   −Δ''H'' = 7.63 kcal/mole


''E'' and ''C'' equation

The ''E''-''C'' model accommodates the failure of single parameter descriptions of acids and bases. In 1965
Russell S. Drago Russell Stephen Drago (November 5, 1928 – December 5, 1997) was an American professor of inorganic chemistry.(see http://www.ACS.org for the publications) American Chemical Society Website He mentored more than 130 PhD students, authored over a ...
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 ''E''A and a ''C''A. Each base is likewise characterized by its own ''E''B and ''C''B. The ''E'' and ''C'' parameters refer, respectively, to the
electrostatic Electrostatics is a branch of physics that studies electric charges at rest ( static electricity). Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for amb ...
and
covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
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 An adduct (from the Latin ''adductus'', "drawn toward" alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all co ...
that have no
steric repulsion Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...
between the acid and base. : -\Delta H= E_AE_B + C_AC_B This equation reproduces and predicts the
enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
, Δ''H'', of a reaction between many acids and a bases. Δ''H'' 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 Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...
effects are ignored. A matrix presentation of the equation enhances its utility. Four values, two ''E'' and two ''C'' were assigned as references. ''E''A and ''C''A of I2 were chosen as standards. Since I2 has little tendency to undergo electrostatic bonding, the ''E''A parameter was assigned a small value, 0.5, while the value of ''C''A for the covalent property was set at 2.0. For the two base parameters, ''E''B for CH3C(O)N(CH3)2 ( DMA) was set at 2.35 and ''C''B 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 ''E''A''E''B and ''C''A''C''B 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. ''E''B and ''C''B 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.


''ECW'' model

In the ''ECW'' model, a new term W was added to the equation. : -\Delta H = E_AE_B + C_AC_B + W The W term represents a constant energy for cleavage of a dimeric acid or base. For example, the enthalpy of cleavage the h(CO)2Clsub>2 by base B involves two steps. The first step is cleavage of the dimer, which is ''W'': : h(CO)2Clsub>2 → Rh(CO)2Cl ''W'' = −10.39 kcal/mol The second step is the binding of B to RhCl(CO)2 monomer. In this case, ''W'' = −10.39 kcal/mol. In other cases, W is the enthalpy needed to cleave the internal hydrogen bonding of the H-bonding acid (CF3)3COH. W is also useful for a base displacement reaction in poorly solvating media: : F3B-OEt2 → BF3 + OEt2 For any base, a constant energy contribution is observed for the breaking of the F3B-OEt2 bond. An ECW study of the enthalpies of a series of bases produces a W value that corresponds to the enthalpy of dissociation of the F3B-OEt2 bond. The ''E''A and ''C''A parameters that result are those for uncomplexed BF3.


A graphical presentation of the ''ECW'' model

A graphical presentation of this model clearly shows that there is no single rank order of acid or base strength, a point often overlooked, and emphasizes that the magnitude of acid and base interactions requires two parameters (''E'' & ''C'') to account for the interactions. The EC equation from the ECW Model : -\Delta H= E_AE_B + C_AC_B can be rearranged into a form which can be plotted as a straight line. In a Cramer–Bopp plot for Lewis bases, the parameter ''Ra'' reflects the mode of bonding of a potential Lewis acid partner, from purely electrostatic interactions () to purely covalent interactions (). The parameter \scriptstyle\frac reflects the strength of the bonding interaction. The plot shown here allows comparison of three chosen Lewis bases: acetonitrile,
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
, and dimethyl sulfide. The Lewis acid iodine () will interact most strongly with dimethyl sulfide and least strongly with acetonitrile, whereas triethylgallium () will interact most strongly with ammonia and least strongly with dimethyl sulfide. The plot also shows that ammonia is a stronger Lewis base than acetonitrile irrespective of its Lewis acid partner, whereas the relative strengths of ammonia and dimethyl sulfide as Lewis bases depends on the bonding characteristics of the Lewis acid, swapping order when . The Cramer–Bopp plot was developed as a visual tool for comparing Lewis base strength with the range of possible Lewis acid partners, and a similar plot can be constructed to examine selected Lewis acids against the range of possible Lewis bases. References 5, 8, 12, and 14 contain graphical presentations that define the ranking order of strength of many Lewis acids and bases.


Other aspects and extensions of the ''ECW'' model

As mentioned above the ''E'' and ''C'' parameters are obtained from enthalpies of adduct formation in which the bond between the acid and base is a σ interaction and adducts that have no steric repulsion between the acid and base. As a results, ''E'' and ''C'' parameters can be used to glean information about
pi bond In chemistry, pi bonds (π bonds) are covalent chemical bonds, in each of which two lobes of an orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally. Each of these atomic orbitals ...
ing. When pi bonding contributes to the measured enthalpy, the enthalpy calculated from the ''E'' and ''C'' parameters will be less than the measured enthalpy and the difference provides a measure of the extent of the pi bonding contribution. The ᐃH calculated for the reaction of Me3B with Me3N is larger than the observed. This discrepancy is attributed to steric repulsion between the methyl groups on the B and N. The difference between the calculated and observed values can then be taken as the amount of the
steric effect Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...
, a value otherwise not attainable. Steric effects have also been identified with (CH3)3SnCl and with Cu(HFacac)2. The use of ''E'' and ''C'' parameters have been extended to analyze spectroscopic changes occurring during adduct formation. For example, the shift of the phenol OH stretching frequency, , that occurs upon adduct formation has been analyzed using the following equation: : where asterisks on the ''E''A and ''C''A for phenol indicate that the acceptor is held constant and the frequency shift is measured as the base is varied. The asterisks also indicate that the phenol parameters are those for frequency shifts and not those for enthalpies. An analysis like this provides a basis for using ''E''B and ''C''B parameters as a reference scale of donor strengths for frequency shifts. This type analysis has also been applied to other spectroscopic shifts (
NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...
, EPR, UV-vis, IR, etc.) accompanying adduct formation. Any physicochemical property, , that is dominated by σ donor-acceptor interaction can be correlated with the enthalpy-derived ''E'' and ''C'' parameters. The ''ECW'' equations enables one to correlate and predict the enthalpies of adduct formation of neutral donor-acceptor interactions for which the electron-transfer is limited. For gas-phase reactions between cations and neutral donors, there is significant electron-transfer. The extension of the ''ECW'' model to cation-neutral Lewis base interactions has led to the ''ECT'' model. Others have concluded that the ECW model "is generally found helpful in many fields of solution chemistry and biochemistry”.


Charge-transfer complexes of I2

The enthalpies of formation of some Donor-I2 adducts are listed below. I2 is a Lewis acid classified as a soft acid and its acceptor properties are discussed in the ECW model. The relative acceptor strength of I2 toward a series of bases, versus other Lewis acids, can be illustrated by C-B plots.Laurence, C. and Gal, J-F. Lewis Basicity and Affinity Scales, Data and Measurement, (Wiley 2010) pp 50-51 ISBN 978-0-470-74957-9


Notes


References

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See also

*
Acid–base reaction An acid–base reaction is a chemical reaction that occurs between an acid and a base. It can be used to determine pH via titration. Several theoretical frameworks provide alternative conceptions of the reaction mechanisms and their applica ...
* Lewis acidLewis base * Acid *
Russell S. Drago Russell Stephen Drago (November 5, 1928 – December 5, 1997) was an American professor of inorganic chemistry.(see http://www.ACS.org for the publications) American Chemical Society Website He mentored more than 130 PhD students, authored over a ...
Acid–base chemistry Inorganic chemistry