ECW model: Difference between revisions
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{{Short description|Semi-quantitative model describing interactions between acids and bases of Lewis}} |
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In [[chemistry]], the '''ECW model''' is a semi-quantitative model that describes and predicts the strength of [[Lewis acid]]–[[Lewis base]] interactions. Many [[chemical reaction]]s can be described as [[acid–base reaction]]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]], which also seeks to rationalize the behavior of diverse acids and bases. |
In [[chemistry]], the '''ECW model''' is a semi-quantitative model that describes and predicts the strength of [[Lewis acid]]–[[Lewis base]] interactions. Many [[chemical reaction]]s can be described as [[acid–base reaction]]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]], which also seeks to rationalize the behavior of diverse acids and bases. |
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==History of the problem== |
==History of the problem== |
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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.<ref>Lewis G. N. (1938) Acids and bases Journal of the Franklin Institute, 226, 293-313.</ref> 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:. |
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.<ref>Lewis G. N. (1938) Acids and bases Journal of the Franklin Institute, 226, 293-313.</ref> 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:. |
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:4F-C<sub>6</sub>H<sub>4</sub>OH |
: 4F-C<sub>6</sub>H<sub>4</sub>OH + OEt<sub>2</sub> −Δ''H'' = 5.94 kcal/mole |
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:4F-C<sub>6</sub>H<sub>4</sub>OH |
: 4F-C<sub>6</sub>H<sub>4</sub>OH + SMe<sub>2</sub> −Δ''H'' = 4.73 kcal/mole |
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These data suggest that OEt<sub>2</sub> is a stronger base than SMe<sub>2</sub>. The opposite is found, however, when I<sub>2</sub> is the acid: |
These data suggest that OEt<sub>2</sub> is a stronger base than SMe<sub>2</sub>. The opposite is found, however, when I<sub>2</sub> is the acid: |
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:I<sub>2</sub> |
: I<sub>2</sub> + OEt<sub>2</sub> −Δ''H'' = 4.16 kcal/mole |
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:I<sub>2</sub> |
: I<sub>2</sub> + SMe<sub>2</sub> −Δ''H'' = 7.63 kcal/mole |
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==E and C equation== |
==''E'' and ''C'' equation== |
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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.<ref>{{cite journal|author1=Drago, R. S. |author2=Wayland, B. B. |year=1965|title=A Double-Scale Equation for Correlating Enthalpies of Lewis Acid-Base Interactions|journal=Journal of the American Chemical Society|volume=87|pages=3571–3577|doi=10.1021/ja01094a008}}</ref> Each acid is characterized by an E<sub>A</sub> and a C<sub>A</sub>. Each base is likewise characterized by its own E<sub>B</sub> and C<sub>B</sub>. |
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.<ref>{{cite journal|author1=Drago, R. S. |author2=Wayland, B. B. |year=1965|title=A Double-Scale Equation for Correlating Enthalpies of Lewis Acid-Base Interactions|journal=Journal of the American Chemical Society|volume=87|issue=16 |pages=3571–3577|doi=10.1021/ja01094a008}}</ref> Each acid is characterized by an ''E''<sub>A</sub> and a ''C''<sub>A</sub>. Each base is likewise characterized by its own ''E''<sub>B</sub> and ''C''<sub>B</sub>. The ''E'' and ''C'' parameters refer, respectively, to the [[ionic bond|electrostatic]] and [[covalent]] contributions to the [[bond energy|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 [[sigma bond|σ bonds]] between the acid and base as well as [[adducts]] that have no [[steric effects|steric repulsion]] between the acid and base. |
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: <math> -\Delta H= E_AE_B + C_AC_B</math> |
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This equation reproduces and predicts the [[enthalpy]], Δ''H'', of a reaction between many acids and |
This equation reproduces and predicts the [[enthalpy]], Δ''H'', of a reaction between many acids and 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]] effects are ignored. A matrix presentation of the equation enhances its utility.<ref>{{cite journal|author1=Drago, R. S. |author2= Vogel,G. C. |author3= Needham, T. E. |year=1971|title= A Four-Parameter Equation for Predicting Enthalpies of Adduct Formation |journal=Journal of the American Chemical Society|volume=93|issue= 23 |pages=6014–6026|doi=10.1021/ja00752a010}}</ref> |
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Four values, two E and two C were assigned as references. |
Four values, two ''E'' and two ''C'' were assigned as references. ''E''<sub>A</sub> and ''C''<sub>A</sub> of [[iodine|I<sub>2</sub>]] were chosen as standards. Since I<sub>2</sub> has little tendency to undergo electrostatic bonding, the ''E''<sub>A</sub> parameter was assigned a small value, 0.5, while the value of ''C''<sub>A</sub> for the covalent property was set at 2.0. For the two base parameters, ''E''<sub>B</sub> for CH<sub>3</sub>C(O)N(CH<sub>3</sub>)<sub>2</sub> ([[dimethylacetamide|DMA]]) was set at 2.35 and ''C''<sub>B</sub> for (C<sub>2</sub>H<sub>5</sub>)<sub>2</sub>S, [[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''<sub>A</sub>''E''<sub>B</sub> and ''C''<sub>A</sub>''C''<sub>B</sub> products of the DMA and (C<sub>2</sub>H<sub>5</sub>)<sub>2</sub>S 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.<ref>{{cite journal|author1=Drago, R. S. |author2=Ferris, D. C..P.|author3=Wong, N..M. |year=1990|title= A method for the analysis and prediction of gas-phase ion-molecule enthalpies |journal= Journal of the American Chemical Society |volume=112|issue=24 |pages=8953–8961–2479|doi= 10.1021/ja00180a047 }}</ref> A select set of the improved ''E'' and ''C'' numbers is found in this article and the complete set is available in the literature.<ref>{{cite journal|author1=Vogel G. C. |author2=Drago, R. S. |year=1996|journal=Journal of Chemical Education|volume=73|pages=701–707|title=The ECW Model|issue=8 |bibcode=1996JChEd..73..701V|doi=10.1021/ed073p701}}</ref> |
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<ref>Drago, R. S. |
<ref>{{cite journal|author1=Drago, R. S. |author2=Dadmun, A. P.|author3=Vogel, G. C. |year=1992|title= Addition of new donors to the E and C model.|journal= Inorganic Chemistry|volume=32|issue=11 |pages=2473–2479|doi= 10.1021/ic00063a045}}</ref> ''E''<sub>B</sub> and ''C''<sub>B</sub> 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.<ref>{{cite journal|author1=Drago, R. S. |author2= Joerg,S. |year=1996|title= Phosphine E<sub>B</sub> and C<sub>B</sub> Values |journal=Journal of the American Chemical Society|volume=118|issue= 11 |pages=2654–2663|doi=10.1021/ja953581e}}</ref> |
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==ECW model== |
==''ECW'' model== |
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In the ECW model, a new term W was added to the equation. |
In the ''ECW'' model, a new term W was added to the equation. |
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: <math> -\Delta H = E_AE_B + C_AC_B + W </math> |
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The W term represents a constant energy for cleavage of a dimeric acid or base. |
The W term represents a constant energy for cleavage of a dimeric acid or base. For example, the enthalpy of cleavage the [[rhodium carbonyl chloride|[Rh(CO)<sub>2</sub>Cl]<sub>2</sub>]] by base B involves two steps. The first step is cleavage of the dimer, which is ''W'': |
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: |
: {{sfrac|1|2}} [Rh(CO)<sub>2</sub>Cl]<sub>2</sub> → Rh(CO)<sub>2</sub>Cl {{pad}} ''W'' = −10.39 kcal/mol |
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:F<sub>3</sub>B-OEt<sub>2</sub> → BF<sub>3</sub> + OEt<sub>2</sub> |
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⚫ | For any base, a constant energy contribution is observed for the breaking of the [[Boron trifluoride etherate|F<sub>3</sub>B-OEt<sub>2</sub>]] 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 F<sub>3</sub>B-OEt<sub>2</sub> bond. The ''E<sub>A</sub> |
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⚫ | For any base, a constant energy contribution is observed for the breaking of the [[Boron trifluoride etherate|F<sub>3</sub>B-OEt<sub>2</sub>]] 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 F<sub>3</sub>B-OEt<sub>2</sub> bond. The ''E''<sub>A</sub> and ''C''<sub>A</sub> parameters that result are those for uncomplexed BF<sub>3</sub>. |
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⚫ | 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.<ref>Cramer, R. E. |
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⚫ | 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.<ref>{{cite journal|author1=Cramer, R. E. |author2=Bopp, T. T. |year=1977|title= Graphical display of the enthalpies of adduct formation for Lewis acids and bases |journal= Journal of Chemical Education |volume=54|pages=612–613|doi= 10.1021/ed054p612}} The plots shown in this paper used older parameters. Improved E&C parameters are listed in the tables and in references 5 and 6.</ref> |
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The EC equation from the ECW Model |
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: <math> -\Delta H= E_AE_B + C_AC_B</math> |
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can be rearranged into a form which can be plotted as a straight line. |
can be rearranged into a form which can be plotted as a straight line. |
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|center|thumb]] |
|center|thumb]] |
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In a Cramer–Bopp plot for Lewis bases, the parameter ''R<sub>a</sub>'' |
In a Cramer–Bopp plot for Lewis bases, the parameter ''R<sub>a</sub>''{{efn|(''C''<sub>A</sub> − ''E''<sub>A</sub>)/(''C''<sub>A</sub> + ''E''<sub>A</sub>)}} reflects the mode of bonding of a potential Lewis acid partner, from purely electrostatic interactions ({{math|size=100%|1=''R''<sub>a</sub> = −1}}) to purely covalent interactions ({{math|size=100%|1=''R''<sub>a</sub> = +1}}). The parameter <math>\scriptstyle\frac{-\Delta H}{E_a + C_a}</math> reflects the strength of the bonding interaction. The plot shown here allows comparison of three chosen Lewis bases: [[acetonitrile]], [[ammonia]], and [[dimethyl sulfide]]. The Lewis acid iodine ({{math|size=100%|1=''R''<sub>a</sub> = 0.6}}) will interact most strongly with dimethyl sulfide and least strongly with acetonitrile, whereas [[triethylgallium]] ({{math|size=100%|1=''R''<sub>a</sub> = −0.65}}) 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 {{math|size=100%|1=''R''<sub>a</sub> = 0.1}}. {{clarify|text=(NB: guesstimate).|date=June 2021}} 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. |
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⚫ | As a result, ''E'' and ''C'' parameters can be used to glean information about [[pi bond]]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.<ref>{{cite journal|author1=Drago, R. S. |author2=Bilgrien, C. J. |year=1988|title=Inductive transfer and coordination of ligands in metal-metal bonded systems|journal=Polyhedron|volume=7|issue=16–17 |pages=1453–1468|doi=10.1016/S0277-5387(00)81774-X}}</ref><ref>{{cite journal|author=Drago, R. S.|year=1982|title=The question of a synergistic metal-metal interaction leading to .pi.-back-bond stabilization in dirhodium tetrabutyrate adducts|journal=Inorg. Chem.|volume=21|issue=4 |pages=1697–1698|doi=10.1021/ic00134a093}}</ref> |
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⚫ | The ᐃH calculated for the reaction of [[trimethylborane|Me<sub>3</sub>B]] with Me<sub>3</sub>N 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]], a value otherwise not attainable. Steric effects have also been identified with [[trimethyltin chloride|(CH<sub>3</sub>)<sub>3</sub>SnCl]] and with [[hexafluoroacetylacetone|Cu(HFacac)<sub>2</sub>]]. |
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⚫ | The use of ''E'' and ''C'' parameters have been extended to analyze spectroscopic changes occurring during adduct formation.<ref>{{cite journal|author1=Drago, R. S. |author2=Vogel, G. C. |year=1992|title= Interpretation of spectroscopic changes upon adduct formation and their use to determine electrostatic and covalent (E and C) parameters|journal= Journal of the American Chemical Society |volume=114|issue=24 |pages=9527–9532|doi= 10.1021/ja00050a035}}</ref> For example, the shift of the phenol OH stretching frequency, {{math|size=100%|Δ''χ''}}, that occurs upon adduct formation has been analyzed using the following equation: |
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⚫ | As a |
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⚫ | The ᐃH calculated for the reaction of [[trimethylborane |
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⚫ | where asterisks on the ''E''<sub>A</sub> and ''C''<sub>A</sub> 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''<sub>B</sub> and ''C''<sub>B</sub> parameters as a reference scale of donor strengths for frequency shifts. This type analysis has also been applied to other spectroscopic shifts ([[nuclear magnetic resonance|NMR]], [[Electron paramagnetic resonance|EPR]], [[Ultraviolet–visible spectroscopy|UV-vis]], [[Infrared spectroscopy|IR]], etc.) accompanying adduct formation. Any physicochemical property, {{math|size=100%|Δ''χ''}}, that is dominated by σ donor-acceptor interaction can be correlated with the enthalpy-derived ''E'' and ''C'' parameters. |
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⚫ | The use of E and C parameters have been extended to analyze spectroscopic changes occurring during adduct formation.<ref>Drago, R. S. |
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: {{math|1=Δ''χ'' = ''Ε<sub>A</sub>''{{sup|*}}''E<sub>B</sub>'' + ''C<sub>A</sub>''{{sup|*}}''C<sub>B</sub>'' + ''W''{{sup|*}}}} |
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⚫ | where asterisks on the ''E<sub>A</sub> |
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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.<ref>Drago, R. S. Applications of Electrostatic-Covalent Models in Chemistry, Surfside: Gainesville, FL, 1994.</ref><ref>{{cite journal| |
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.<ref>Drago, R. S. Applications of Electrostatic-Covalent Models in Chemistry, Surfside: Gainesville, FL, 1994.</ref><ref>{{cite journal |author=Drago, R. |author2=Wong, N. S.|year=1996|journal=Journal of Chemical Education|volume=73|pages=123–129|title=The Role of Electron-Density Transfer and Electronegativity in Understanding Chemical Reactivity and Bonding|issue=2 |doi=10.1021/ed073p123|bibcode=1996JChEd..73..123D }}</ref> Others have concluded that the ECW model "is generally found helpful in many fields of solution chemistry and biochemistry".<ref>Laurence, C. and Gal, J-F. Lewis Basicity and Affinity Scales, Data and Measurement, (Wiley 2010) p 51 ISBN 978-0-470-74957-9</ref> |
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|- {{ts|vtp}} |
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{| class="wikitable" |
{| class="wikitable" |
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|+ Selected E and C |
|+ Selected ''E'' and ''C'' parameters for acids{{efn|name=aa}} |
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! Acid |
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! Acid !! E<sub>A</sub>!! C<sub>A</sub> !! W!! C<sub>A</sub>/E<sub>A</sub> |
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! ''E''<sub>A</sub> |
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! ''C''<sub>A</sub> |
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! W |
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| [[Iodine|I<sub>2</sub>]] || 0.50||2.0 || 0|| 4.0 |
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| [[Iodine monobromide|IBr]] || 1.20 || 3.29 || 0 || 2.7 |
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| [[Iodine monochloride|ICl]] || 2.92 || 1.66 || 0 || 0.57 |
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| [[Phenol|C<sub>6</sub>H<sub>5</sub>OH]] || 2.27|| 1.07 || 0 || 0.47 |
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| 4-FC<sub>6</sub>H<sub>4</sub>OH || 2.30 ||1.11|| 0 || 0.48 |
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| 3-CF<sub>3</sub>C<sub>6</sub>H<sub>4</sub>OH || 2.38 ||1.22 || 0 || 0.51 |
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| [[p-Cresol|4-CH<sub>3</sub>C<sub>6</sub>H<sub>4</sub>OH]] || 2.23 ||1.03 || 0 || 0.44 |
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| [[2,2,2-Trifluoroethanol|CF<sub>3</sub>CH<sub>2</sub>OH]] || 2.07 || 1.06 || 0 || 0.51 |
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| [[Hexafluoro-2-propanol|(CF<sub>3</sub>)<sub>2</sub>CHOH]] ||2.89 || 1.33 || −0.16 || 0.46 |
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| [[tert-Butyl alcohol|(CH<sub>3</sub>)<sub>3</sub>COH]] || 1.07|| 0.69 || 0|| 0.65 |
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| [[Nonafluoro-tert-butyl alcohol|(CF<sub>3</sub>)<sub>3</sub>COH]] ||3.06 || 1.88 || −0.87 || 0.61 |
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| [[1-Octanol|C<sub>8</sub>H<sub>17</sub>OH]] || 0.85|| 0.87 || 0 || 1.0 |
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| [[Chloroform|HCCl<sub>3</sub>]] || 1.56 || 0.44 || 0 || 0.28 |
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| [[Methylene chloride|CH<sub>2</sub>Cl<sub>2</sub>]] || 0.86 || 0.11 || 0 || 0.13 |
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| [[Pyrrole|C<sub>4</sub>H<sub>4</sub>NH]] || 1.38 || 0.68 || 0 || 0.49 |
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| [[Isocyanic acid|HNCO]] || 1.60 || 0.69 || 0 || 0.43 |
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| [[Isothiocyanic acid|HNCS]] || 2.85 || 0.70 || 0 || 0.25 |
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| [[Trimethylborane|B(CH<sub>3</sub>)<sub>3</sub>]] || 2.90 || 3.60 || 0 || 1.2 |
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| [[Trimethylaluminium|Al(CH<sub>3</sub>)<sub>3</sub>]] || 8.66 || 3.68 || 0 || 0.43 |
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| [[Triethylgallium|Ga(C<sub>2</sub>H<sub>5</sub>)<sub>3</sub>]] || 6.95 || 1.48 || 0 || 0.21 |
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| [[Trimethylindium|In(CH<sub>3</sub>)<sub>3</sub>]] || 6.60 || 2.15 || 0 || 0.33 |
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| [[Trimethyltin chloride|(CH<sub>3</sub>)<sub>3</sub>SnCl]] || 2.87 || 0.71 || 0 || 0.25 |
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| [[Sulfur dioxide|SO<sub>2</sub>]] || 0.51 || 1.56 || 0 || 3.1 |
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| [[Antimony pentachloride|SbCl<sub>5</sub>]] || 3.64 || 10.42 || 0 || 2.9 |
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| [[Hexafluoroacetylacetone|Cu(HFacac)<sub>2</sub>]]{{efn|name=ab}} || 1.82 || 2.86 || 0 || 1.6 |
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| [[Zinc bis(bis(trimethylsilyl)amide)|Zn[N(Si(CH<sub>3</sub>)<sub>3</sub>]<sub>2</sub>]] || 2.75 || 2.32 || 0 || 0.84 |
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| [[Cadmium bis(bis(trimethylsilyl)amide)|Cd[N(Si(CH<sub>3</sub>)<sub>3</sub>]<sub>2</sub>]] || 2.50 || 1.83 || 0 || 0.73 |
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| Zn[[Tetraphenylporphyrin|TPP]]{{efn|name=ac}} || 2.72 || 1.45 || 0 || 0.53 |
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| {{ |
| Co[[Protoporphyrin IX|PPIX]]DME{{efn|name=ad}} || 2.32 || 1.34 || 0 || 0.58 |
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| {{ |
| {{sfrac|1|2}}[MeCo([[Dimethylglyoxime|Hdmg]])<sub>2</sub>]<sub>2</sub>{{efn|name=ae}} || 4.70 || 3.24 || −5.84 || 0.60 |
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| {{ |
| {{sfrac|1|2}}[[Rhodium carbonyl chloride|[Rh(CO)<sub>2</sub>Cl]<sub>2</sub>]] || 4.32|| 4.13 || −10.39 || 0.96 |
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|{{ |
| {{sfrac|1|2}}[[Cyclooctadiene rhodium chloride dimer|[Rh(COD)<sub>2</sub>Cl]<sub>2</sub>]] || 2.43 || 2.56 || −5.78 ||1.1 |
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| {{sfrac|1|2}}[[Allylpalladium chloride dimer|[Pd(π-ally)Cl]<sub>2</sub>]] || 1.70 || 2.14|| −3.38|| 1.3 |
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{| class="wikitable" |
{| class="wikitable" |
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|+ and for |
|+ and for bases{{efn|name=ba}} |
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! Base |
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! Base !! E<sub>B</sub> !! C<sub>B</sub> !! C<sub>B</sub>/E<sub>B</sub> |
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! ''E''<sub>B</sub> |
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! ''C''<sub>B</sub> |
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! ''C''<sub>B</sub>/''E''<sub>B</sub> |
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| [[Ammonia|NH<sub>3</sub>]] || 2.31 || 2.04 || 0.88 |
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| [[Methylamine|NH<sub>2</sub>(CH<sub>3</sub>)]] || 2.16 || 3.13 || 1.4 |
||
|- |
|- |
||
| |
| [[Dimethylamine|NH(CH<sub>3</sub>)<sub>2</sub>]] || 1.80 || 4.21 || 2.3 |
||
|- |
|- |
||
| |
| [[Trimethylamine|N(CH<sub>3</sub>)<sub>3</sub>]] || 1.21|| 5.61 ||4.6 |
||
|- |
|- |
||
| |
| [[Quinuclidine|HC(C<sub>2</sub>H<sub>4</sub>)<sub>3</sub>N]] || 0.80 || 6.72 || 8.4 |
||
|- |
|- |
||
| |
| [[Triethylamine|(C<sub>2</sub>H<sub>5</sub>)<sub>3</sub>N]] || 1.32 || 5.73 || 4.3 |
||
|- |
|- |
||
| 1-CH<sub>3</sub>NC<sub>3</sub>N{{efn|name=ah}} || 1.16 || 4.92 || 4.2 |
|||
|- |
|- |
||
| [[Pyridine|C<sub>5</sub>H<sub>5</sub>N]] || 1.78 || 3.54 || 2.0 |
|||
|- |
|- |
||
| |
| [[3-Methylpyridine|3CH<sub>3</sub>C<sub>5</sub>H<sub>4</sub>N]] || 1.81 || 3.67 || 2.0 |
||
|- |
|- |
||
| |
| [[3-Bromopyridine|3BrC<sub>5</sub>H<sub>4</sub>N]] || 1.66 || 3.08 || 1.9 |
||
|- |
|- |
||
| |
| [[4-Methoxypyridine|4CH<sub>3</sub>OC<sub>5</sub>H<sub>4</sub>N]] || 1.83 || 3.83 || 2.1 |
||
|- |
|- |
||
| |
| [[Acetonitrile|CH<sub>3</sub>CN]] || 1.64 || 0.71 || 0.43 |
||
|- |
|- |
||
| |
| (CH<sub>3</sub>)<sub>2</sub>NCN || 1.92||0.92 ||0.48 |
||
|- |
|- |
||
| |
| [[Chloroacetonitrile|ClCH<sub>2</sub>CN]] ||1.67|| 0.33||0.20 |
||
|- |
|- |
||
| |
| [[Acetone|CH<sub>3</sub>C(O)CH<sub>3</sub>]] ||1.74|| 1.26 || 0.72 |
||
|- |
|- |
||
| |
| [[Cyclopentanone|(CH<sub>2</sub>)<sub>4</sub>CO]] || 2.02 || 0.88 || 0.43 |
||
|- |
|- |
||
| [[Ethyl acetate|CH<sub>3</sub>C(O)OC<sub>2</sub>H<sub>5</sub>]] || 1.62 || 0.98 || 0.61 |
|||
|- |
|- |
||
| |
| [[Dimethylacetamide|CH<sub>3</sub>C(O)N(CH<sub>3</sub>)<sub>2</sub>]] || 2.35 || 1.31 || 0.56 |
||
|- |
|- |
||
| |
| [[Diethyl ether|(C<sub>2</sub>H<sub>5</sub>)<sub>2</sub>O]] || 1.80 ||1.63 || 0.91 |
||
|- |
|- |
||
| |
| [[Tetrahydrofuran|(CH<sub>2</sub>)<sub>4</sub>O]] || 1.64|| 2.18||1.3 |
||
|- |
|- |
||
| |
| [[1,4-Dioxane|O(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub>)O]] || 1.86|| 1.29|| 0.69 |
||
|- |
|- |
||
| |
| (CH<sub>2</sub>)<sub>4</sub>(CH)<sub>2</sub>)O{{efn|name=ak}} || 1.45|| 2.14 || 1.5 |
||
|- |
|- |
||
| [[Dimethyl sulfide|(CH<sub>3</sub>)<sub>2</sub>S]] ||0.25 || 3.75 || 15 |
|||
|- |
|- |
||
| |
| [[Tetrahydrothiophene|(CH<sub>2</sub>)<sub>4</sub>S]] || 0.26 || 4.07 || 16 |
||
|- |
|- |
||
| |
| [[Diethyl sulfide|(C<sub>2</sub>H<sub>5</sub>)<sub>2</sub>S]] || 0.24 ||3.92 || 16 |
||
|- |
|- |
||
| |
| [[Dimethyl sulfoxide|(CH<sub>3</sub>)<sub>2</sub>SO]] || 2.40 || 1.47 || 0.61 |
||
|- |
|- |
||
| |
| [[Tetrahydrothiophene 1-oxide|(CH<sub>2</sub>)<sub>4</sub>SO]] || 2.44 || 1.64 || 0.67 |
||
|- |
|- |
||
| |
| [[pyridine-N-oxide|C<sub>5</sub>H<sub>5</sub>NO]] || 2.29 || 2.33 || 1.0 |
||
|- |
|- |
||
| {{ |
| 4{{nbh}}CH<sub>3</sub>OC<sub>5</sub>H<sub>4</sub>NO || 2.34 || 3.02 || 1,3 |
||
|- |
|- |
||
| |
| [[Triphenylphosphine oxide|(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub>PO]] || 2.59 || 1.67 ||0.64 |
||
|- |
|- |
||
| |
| [[Trimethylphosphine|(CH<sub>3</sub>)<sub>3</sub>P]] || 0.25|| 5.81 || 24 |
||
|- |
|- |
||
| |
| [[Trimethyl phosphite|(CH<sub>3</sub>O)<sub>3</sub>P]] || 0.13 || 4.83 || 37 |
||
|- |
|- |
||
| |
| [[Dimethyl selenide|(CH<sub>3</sub>)<sub>2</sub>Se]] || 0.05 || 4.24 || 83 |
||
|- |
|||
| [[Triphenylphosphine sulfide|(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub>PS]] || 0.35|| 3.65 || 10 |
|||
|- |
|||
| [[Benzene|C<sub>6</sub>H<sub>6</sub>]] || 0.70 || 0.45 || 0.64 |
|||
|} |
|} |
||
|} |
|||
==Charge-transfer complexes of I<sub>2</sub>== |
|||
The enthalpies of formation of some Donor-I<sub>2</sub> adducts are listed below. I<sub>2</sub> is a Lewis acid classified as a [[HSAB theory|soft acid]] and its acceptor properties are discussed in the ECW model. The relative acceptor strength of I<sub>2</sub> toward a series of bases, versus other Lewis acids, can be illustrated by C-B plots.<ref>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</ref> |
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{| class="wikitable" |
|||
|+ Enthalpies of formation of some donor-I<sub>2</sub> adducts |
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|- |
|||
! Base |
|||
! −Δ''H'' (kcal/mol) |
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|- |
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| Benzene || 1.4 |
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|- |
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| Toluene || 1.8 |
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|- |
|||
| Dioxane || 3.5 |
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|- |
|||
| Diethyl ether || 4.3 |
|||
|- |
|||
| Diethyl sulfide || 8.3 |
|||
|- |
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| Dimethylacetamide || 4.7 |
|||
|- |
|||
| Acetonitrile || 1.9 |
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|- |
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| Pyridine || 7.80 |
|||
|- |
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| Triethylamine || 12.0 |
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|} |
|} |
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<!--{{notelist}}--> |
<!--{{notelist}}--> |
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{{notelist|refs= |
{{notelist|refs= |
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{{efn|name=aa| units for ''E<sub>A</sub>'', and ''C<sub>A</sub>'' are (kcal/mol) |
{{efn|name=aa| units for ''E<sub>A</sub>'', and ''C<sub>A</sub>'' are (kcal/mol){{sup|{{1/2}}}} and W is kcal/mol}} |
||
{{efn|name=ab| [[Hexafluoroacetylacetone|bis(hexafloroacetylacetonato)copper(II)]]}} |
{{efn|name=ab| [[Hexafluoroacetylacetone|bis(hexafloroacetylacetonato)copper(II)]]}} |
||
{{efn|name=ac| zinc [[tetraphenylporphine]]}} |
{{efn|name=ac| zinc [[tetraphenylporphine]]}} |
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Line 219: | Line 266: | ||
{{efn|name=af| bis(3-trifluoro-d-camphorate)nickel(II) dimer}} |
{{efn|name=af| bis(3-trifluoro-d-camphorate)nickel(II) dimer}} |
||
{{efn|name=ag| [[molybdenum|molybdenum perfluorobutyrate]]}} |
{{efn|name=ag| [[molybdenum|molybdenum perfluorobutyrate]]}} |
||
{{efn|name=ah| [[1-Methylimidazole |
{{efn|name=ah| [[1-Methylimidazole|N-methylimidazole]]}} |
||
{{efn|name=ba| Units for ''E<sub>B</sub> |
{{efn|name=ak| 7-Oxabicyclo[2.2.1]heptane}} |
||
{{efn|name=ba| Units for ''E''<sub>B</sub> and ''C''<sub>B</sub> are [[Kilocalorie per mole|(kcal/mol)]]<sup>{{1/2}}</sup>}} |
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}} |
}} |
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*[[Acid–base reaction]] |
*[[Acid–base reaction]] |
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*[[Lewis acid]]–[[Lewis base]] |
*[[Lewis acid]]–[[Lewis base]] |
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*[[Acid]] |
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*[[Russell S. Drago]] |
*[[Russell S. Drago]] |
||
Latest revision as of 16:59, 28 May 2024
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.
History of the problem
[edit]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.[1] 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 −ΔH = 5.94 kcal/mole
- 4F-C6H4OH + SMe2 −ΔH = 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 −ΔH = 4.16 kcal/mole
- I2 + SMe2 −ΔH = 7.63 kcal/mole
E and C equation
[edit]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.[2] 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.[3]
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.[4] A select set of the improved E and C numbers is found in this article and the complete set is available in the literature.[5] [6] 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.[7]
ECW model
[edit]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:
- 1/2 [Rh(CO)2Cl]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 EA and CA parameters that result are those for uncomplexed BF3.
A graphical presentation of the ECW model
[edit]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.[8]
The EC equation from the ECW Model
can be rearranged into a form which can be plotted as a straight line.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/7/7b/ECW_Model_Graph.png/512px-ECW_Model_Graph.png)
In a Cramer–Bopp plot for Lewis bases, the parameter Ra[a] reflects the mode of bonding of a potential Lewis acid partner, from purely electrostatic interactions (Ra = −1) to purely covalent interactions (Ra = +1). The parameter reflects the strength of the bonding interaction. The plot shown here allows comparison of three chosen Lewis bases: acetonitrile, ammonia, and dimethyl sulfide. The Lewis acid iodine (Ra = 0.6) will interact most strongly with dimethyl sulfide and least strongly with acetonitrile, whereas triethylgallium (Ra = −0.65) 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 Ra = 0.1. (NB: guesstimate).[clarification needed] 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
[edit]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 result, E and C parameters can be used to glean information about pi bonding. 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.[9][10]
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, 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.[11] For example, the shift of the phenol OH stretching frequency, Δχ, that occurs upon adduct formation has been analyzed using the following equation:
- Δχ = ΕA∗EB + CA∗CB + W∗
where asterisks on the EA and CA 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 EB and CB parameters as a reference scale of donor strengths for frequency shifts. This type analysis has also been applied to other spectroscopic shifts (NMR, EPR, UV-vis, IR, etc.) accompanying adduct formation. Any physicochemical property, Δχ, 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.[12][13] Others have concluded that the ECW model "is generally found helpful in many fields of solution chemistry and biochemistry".[14]
|
|
Charge-transfer complexes of I2
[edit]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.[15]
Base | −ΔH (kcal/mol) |
---|---|
Benzene | 1.4 |
Toluene | 1.8 |
Dioxane | 3.5 |
Diethyl ether | 4.3 |
Diethyl sulfide | 8.3 |
Dimethylacetamide | 4.7 |
Acetonitrile | 1.9 |
Pyridine | 7.80 |
Triethylamine | 12.0 |
Notes
[edit]- ^ (CA − EA)/(CA + EA)
- ^ units for EA, and CA are (kcal/mol)1⁄2 and W is kcal/mol
- ^ bis(hexafloroacetylacetonato)copper(II)
- ^ zinc tetraphenylporphine
- ^ cobalt(II) protoporhyrin IX dimethyl ester IX
- ^ methylcobaloxime
- ^ bis(3-trifluoro-d-camphorate)nickel(II) dimer
- ^ molybdenum perfluorobutyrate
- ^ Units for EB and CB are (kcal/mol)1⁄2
- ^ N-methylimidazole
- ^ 7-Oxabicyclo[2.2.1]heptane
References
[edit]- ^ Lewis G. N. (1938) Acids and bases Journal of the Franklin Institute, 226, 293-313.
- ^ Drago, R. S.; Wayland, B. B. (1965). "A Double-Scale Equation for Correlating Enthalpies of Lewis Acid-Base Interactions". Journal of the American Chemical Society. 87 (16): 3571–3577. doi:10.1021/ja01094a008.
- ^ Drago, R. S.; Vogel,G. C.; Needham, T. E. (1971). "A Four-Parameter Equation for Predicting Enthalpies of Adduct Formation". Journal of the American Chemical Society. 93 (23): 6014–6026. doi:10.1021/ja00752a010.
- ^ Drago, R. S.; Ferris, D. C..P.; Wong, N..M. (1990). "A method for the analysis and prediction of gas-phase ion-molecule enthalpies". Journal of the American Chemical Society. 112 (24): 8953–8961–2479. doi:10.1021/ja00180a047.
- ^ Vogel G. C.; Drago, R. S. (1996). "The ECW Model". Journal of Chemical Education. 73 (8): 701–707. Bibcode:1996JChEd..73..701V. doi:10.1021/ed073p701.
- ^ Drago, R. S.; Dadmun, A. P.; Vogel, G. C. (1992). "Addition of new donors to the E and C model". Inorganic Chemistry. 32 (11): 2473–2479. doi:10.1021/ic00063a045.
- ^ Drago, R. S.; Joerg,S. (1996). "Phosphine EB and CB Values". Journal of the American Chemical Society. 118 (11): 2654–2663. doi:10.1021/ja953581e.
- ^ Cramer, R. E.; Bopp, T. T. (1977). "Graphical display of the enthalpies of adduct formation for Lewis acids and bases". Journal of Chemical Education. 54: 612–613. doi:10.1021/ed054p612. The plots shown in this paper used older parameters. Improved E&C parameters are listed in the tables and in references 5 and 6.
- ^ Drago, R. S.; Bilgrien, C. J. (1988). "Inductive transfer and coordination of ligands in metal-metal bonded systems". Polyhedron. 7 (16–17): 1453–1468. doi:10.1016/S0277-5387(00)81774-X.
- ^ Drago, R. S. (1982). "The question of a synergistic metal-metal interaction leading to .pi.-back-bond stabilization in dirhodium tetrabutyrate adducts". Inorg. Chem. 21 (4): 1697–1698. doi:10.1021/ic00134a093.
- ^ Drago, R. S.; Vogel, G. C. (1992). "Interpretation of spectroscopic changes upon adduct formation and their use to determine electrostatic and covalent (E and C) parameters". Journal of the American Chemical Society. 114 (24): 9527–9532. doi:10.1021/ja00050a035.
- ^ Drago, R. S. Applications of Electrostatic-Covalent Models in Chemistry, Surfside: Gainesville, FL, 1994.
- ^ Drago, R.; Wong, N. S. (1996). "The Role of Electron-Density Transfer and Electronegativity in Understanding Chemical Reactivity and Bonding". Journal of Chemical Education. 73 (2): 123–129. Bibcode:1996JChEd..73..123D. doi:10.1021/ed073p123.
- ^ Laurence, C. and Gal, J-F. Lewis Basicity and Affinity Scales, Data and Measurement, (Wiley 2010) p 51 ISBN 978-0-470-74957-9
- ^ 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