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| [[File:Thermodyamic versus kinetic control.png|thumb|250px|alt=A Generalised energy profile diagram for kinetic versus thermodynamic product reaction.|Energy profile diagram for kinetic versus thermodynamic product reaction.]]
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| '''Thermodynamic reaction control''' or '''kinetic reaction control''' in a [[chemical reaction]] can decide the composition in a reaction product mixture when competing pathways lead to different products and the reaction conditions influence the [[Stereoselectivity|selectivity]]. The distinction is relevant when product '''A''' forms faster than product '''B''' because the [[activation energy]] for product '''A''' is lower than that for product '''B''', yet product '''B''' is more stable. In such a case '''A''' is the kinetic product and is favoured under kinetic control and '''B''' is the thermodynamic product and is favoured under thermodynamic control. <ref>Organic Chemistry, 3rd ed., M. A. Fox & J. K. Whitesell, Jones & Bartlett, 2004 ISBN 0-7637-2197-2</ref><ref>A Guidebook to Mechanism in Organic Chemistry, 6th Edition, Peter Sykes, Pearson Prentice Hall, 1986. ISBN 0-582-44695-3</ref><ref>Introduction to Organic Chemistry I, Seth Robert Elsheimer, Blackwell Publishing, 2000 ISBN 0-632-04417-9</ref>
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| The conditions of the reaction, such as temperature, pressure, or solvent, affect which reaction pathway may be favored: either the kinetically controlled or the thermodynamically controlled one. Note this is only true if the activation energy of the two pathways differ, with one pathway having a lower E<sub>a</sub> ([[energy of activation]]) than the other.
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| Prevalence of thermodynamic or kinetic control determines the final composition of the product when these competing reaction pathways lead to different products. The reaction conditions as mentioned above influence the [[Stereoselectivity|selectivity]] of the reaction - i.e., which pathway is taken.
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| == Scope ==
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| ===In Diels-Alder reactions===
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| The [[Diels-Alder reaction]] of [[cyclopentadiene]] with [[furan]] can produce two [[isomeric]] products. At [[room temperature]], kinetic reaction control prevails and the less stable [[endo isomer]] '''2''' is the main reaction product. At 81°C and after long reaction times, the [[chemical equilibore stable [[exo isomer]] '''1''' is formed. The ''exo'' product is more stable by virtue of a lower degree of [[steric effects|steric congestion]], while the ''endo'' product is favoured by orbital overlap in the [[transition state]].
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| [[File:DielsAlderCyclopentadieneFuran.svg|center|Thermodynamic versus kinetic reaction control in reaction of cyclopanetdiene and furan]]
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| ===In enolate chemistry===
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| In the [[protonation]] of an [[enolate ion]], the kinetic product is the [[enol]] and the thermodynamic product is a [[ketone]] or [[aldehyde]]. [[Carbonyl compounds]] and their enols interchange rapidly by [[proton]] transfers catalyzed by [[acid]]s or [[base (chemistry)|base]]s, even in trace amounts, in this case mediated by the enolate or the proton source.
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| In the [[deprotonation]] of an unsymmetrical [[ketone]], the kinetic product is the [[enolate]] resulting from removal of the most accessible α-H while the thermodynamic product has the more highly substituted enolate moiety.<ref name="thermovskinetic">[http://pharmaxchange.info/press/2011/03/thermodynamic-product-vs-kinetic-product-with-examples/ Thermodynamic Product vs Kinetic Product]</ref><ref>Jean d'Angelo, Tetrahedron report number 25 :'' Ketone enolates: regiospecific preparation and synthetic uses'', Tetrahedron, Volume 32, Issue 24, 1976, Pages 2979-2990, ISSN 0040-4020, {{DOI|10.1016/0040-4020(76)80156-1}}</ref><ref>
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| ''The Chemistry of Carbanions. IX. The Potassium and Lithium Enolates Derived from Cyclic Ketones'' Herbert O. House, [[Barry M. Trost]] J. Org. Chem., 1965, 30 (5), pp 1341–1348 {{DOI|10.1021/jo01016a001}}</ref><ref>Chemistry of carbanions. XV. ''Stereochemistry of alkylation of 4-tert-butylcyclohexanone'' Herbert O. House, Ben A. Tefertiller, Hugh D. Olmstead
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| J. Org. Chem., 1968, 33 (3), pp 935–942 {{DOI|10.1021/jo01267a002}}</ref> Use of low temperatures and sterically demanding [[base (chemistry)|base]]s increases the kinetic selectivity. Here, the difference in [[pKb|p''K''<sub>b</sub>]] between the base and the enolate is so large that the reaction is essentially irreversible, so the equilibration leading to the thermodynamic product is likely a proton exchange occurring during the addition between the kinetic enolate and as-yet-unreacted ketone. An inverse addition (adding ketone to the base) with rapid mixing would minimize this. The position of the equilibrium will depend on the countercation and solvent.
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| [[File:Thermodynamic deprotonation methylcyclohexanone.svg|center|The kinetic and thermodynamic deprotonation of 2-methylcyclohexanone.]]
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| :If a much weaker base is used, the deprotonation will be incomplete, and there will be an equilibrium between reactants and products. Thermodynamic control is obtained, however the reaction remains incomplete unless the product enolate is trapped, as in the example below. Since H transfers are very fast, the trapping reaction being slower, the ratio of trapped products largely mirrors the deprotonation equilibrium.
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| [[File:Thermodynamic deprotonation methylcyclohexanone enolate trapping.svg|center|The thermodynamic deprotonation of 2-methylcyclohexanone, with trapping of the enolate.]] | |
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| === In electrophilic additions ===
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| The [[electrophilic addition]] reaction of [[hydrogen bromide]] to [[1,3-butadiene]] above room temperature leads predominantly to the thermodynamically more stable 1,4 adduct, 1-bromo-2-butene, but decreasing the reaction temperature to below room temperature favours the kinetic 1,2 adduct, 3-bromo-1-butene.<ref>''Introduction to Organic Chemistry I'' Seth Robert Elsheimer 2000</ref>
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| [[File:Butadiene hydrogen bromide addition.svg|center|The addition of HBr to butadiene in ether. Data from Elsheimer (2000).]] | |
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| :The rationale for the differing selectivities is as follows: Both products result from [[Markovnikov's rule|Markovnikov]] protonation at position 1, resulting in a [[resonance (chemistry)|resonance]]-stabilized [[allylic]] cation. The 1,4 adduct places the larger Br atom at a less congested site and includes a more highly substituted alkene moiety, while the 1,2 adduct is the result of the attack by the nucleophile (Br<sup>-</sup>) at the [[carbon]] of the allylic cation bearing the greatest positive charge (the more highly substituted carbon is the most likely place for the positive charge).
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| [[File:Thermodynamic kinetic control butadiene.svg|center|butadiene hydrobromination mechanism]]
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| == Characteristics ==
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| * In every reaction, the first product formed is that which is most easily formed. Thus, every reaction ''a priori'' starts under kinetic control.<ref>Only if a subsequent equilibration is as fast or faster is this not true.</ref>
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| * A necessary condition for thermodynamic control is reversibility or a mechanism permitting the equilibration between products. Reactions are considered to take place under thermodynamic reaction control when the reverse reaction is sufficiently rapid that the [[Chemical equilibrium|equilibrium]] establishes itself within the alloted reaction time. In this way, the thermodynamically more stable product is always favoured.
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| * Under kinetic reaction control, the forward reaction is faster than the reverse reaction. After reaction time ''t'', the product ratio is the ratio of rate constants<!-- <-constant? --> ''k'' and thus a function of the difference in activation energies ''E''<sub>a</sub> or Δ''G''<sup>‡</sup>:
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| :<math>\ln \left(\frac {[A]_t}{[B]_t}\right) = \ln\left(\frac {k_A}{k_B}\right) = -\frac {\Delta E_a}{RT}</math> (equation 1)
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|
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| :Unless equilibration is prevented, pure kinetic control is practically impossible, because equilibration will have started before the reactants will have been entirely consumed.
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| * Under pure thermodynamic reaction control, when the equilibrium has been reached, the product distribution will be a function of the stabilities ''G''°. After an infinite amount of reaction time, the ratio of product concentrations will equal the [[equilibrium constant]] ''K''<sub>eq</sub> and therefore be a function of the difference in [[Gibbs free energy|Gibbs free energies]],
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| :<math>\ln\left(\frac {[A]_{\infty}}{[B]_{\infty}}\right) = \ln\ K_{eq} = -\frac {\Delta G^\circ}{RT}</math> (equation 2) | |
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| * In general, short reaction times favour kinetic control, whereas longer reaction times favour thermodynamic reaction control. Low temperatures will enhance the selectivity under both sets of conditions, since ''T'' is in the denominator in both cases. The ideal temperature to optimise the yield of the fastest-forming product will be the lowest temperature that will ensure reaction completion in a reasonable amount of time.<ref>Unless one is content with an incomplete reaction, whence a separation of product from unreacted starting material may be necessary.</ref> The ideal temperature for a reaction under thermodynamic control is the lowest temperature at which equilibrium will be reached in a reasonable amount of time.<ref>At worst, ''K''<sub>eq</sub> will approach 1 as ''T'' rises and the proportion of the most stable product will tend toward 50% of the reaction mixture.</ref> If needed, the selectivity can be increased by then slowly cooling the reaction mixture to shift the equilibrium further toward the most stable product. When the difference in product stability is very large, the thermodynamically controlled product can dominate even under relatively vigorous reaction conditions.
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| * If a reaction is under thermodynamic control at a given temperature, it will also be under thermodynamic control at a higher temperature for the same reaction time.
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| * In the same manner, if a reaction is under kinetic control at a given temperature, it will also be under kinetic control at any lower temperature for the same reaction time.
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| * If one presumes that a new reaction will be ''a priori'' under kinetic control, one can detect the presence of an equilibration mechanism (and therefore the possibility of thermodynamic control) if the product distribution:
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| ** changes over time,
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| ** shows one product to be dominant at one temperature while another dominates at a different temperature (inversion of dominance), or
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| ** changes with temperature but is not consistent with equation 1, that is a change in temperature (without changing the reaction time) causes a change in the product ratio <math>{[A]_t}/{[B]_t}</math> that is larger or smaller than would be expected from the change in temperature alone, assuming that <math>\Delta E_a</math> is largely invariant with temperature over a modest temperature range.<ref><math>\Delta E_a</math> will be temperature-independent or nearly so if <math>\Delta S^\ddagger</math> is small, which would be the case if the rate-determining steps leading to each product were of the same [[molecularity]], for instance if both involved collisions with the same reactant.</ref>
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| * In the same way, one can detect the possibility of kinetic control if a temperature change causes a change in the product ratio that is inconsistent with equation 2, assuming that <math>\Delta G^\circ</math> is largely invariant with temperature over a modest temperature range.<ref><math>\Delta G^\circ</math> will be temperature-independent or nearly so if <math>\Delta S^\circ</math> is small, which would be the case if the overall transformations to each product were of the same [[molecularity]], for instance if both were fragmentations of a molecule to produce a pair of molecules or if both were condensations of two molecules to give a single molecule.</ref>
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| ==History==
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| The first to report on the relationship between kinetic and thermodynamic control were [[R.B. Woodward]] and Harold Baer in 1944.<ref>''Studies on Diene-addition Reactions. II.1 The Reaction of 6,6-Pentamethylenefulvene with Maleic Anhydride'' R. B. Woodward, Harold Baer J. Am. Chem. Soc., 1944, 66 (4), pp 645–649 {{DOI|10.1021/ja01232a042}}</ref> They were re-investigating a reaction between [[maleic anhydride]] and a [[fulvene]] first reported in 1929 by [[Otto Diels]] and [[Kurt Alder]].<ref>Diels, O. and Alder, K. (1929), ''Synthesen in der hydroaromatischen Reihe, IV. Mitteilung: Über die Anlagerung von Maleinsäure-anhydrid an arylierte Diene, Triene und Fulvene'' (Mitbearbeitet von Paul Pries). Berichte der deutschen chemischen Gesellschaft (A and B Series), 62: 2081–2087. {{doi|10.1002/cber.19290620829}}</ref> They observed that ''while the endo isomer is formed more rapidly, longer reaction times, as well as relatively elevated temperatures, result in higher exo / endo ratios'' which had to be '' considered in the light of the remarkable stability of the exo-compound on the one hand and the very facile dissociation of the endo isomer on the other.''
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| :[[File:Fulvene maleic anhydride woordward 1944 3.svg|600px|Woodward/Baer 1944]]
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| [[C. K. Ingold]] with [[E. D. Hughes]] and [[G. Catchpole]] independently described a thermodynamic and kinetic reaction control model in 1948.<ref>''Rearrangement and substitution in anionotropic systems. Part III. Mechanism of, and equilibrium in, anionotropic change'' A. G. Catchpole, E. D. Hughes and C. K. Ingold J. Chem. Soc., 1948, 8-17 {{DOI|10.1039/JR9480000008}}</ref> They were reinvestigating a certain [[allylic rearrangement]] reported in 1930 by [[Jakob Meisenheimer]].<ref>Meisenheimer, J. and Link, J. (1930), ''Über die Verschiebung in der Allyl-Gruppe. 3. Mitteilung über Substitution und Addition''. Justus Liebigs Annalen der Chemie, 479: 211–277. {{doi|10.1002/jlac.19304790114}}</ref> Solvolysis of gamma-phenylallyl chloride with [[Potassium acetate|AcOK]] in acetic acid was found to give a mixture of the gamma and the alpha acetate with the latter converting to the first by equilibration. This was interpreted as a ''case in the field of anionotropy of the phenomenon, familiar in prototropy, of the distinction between kinetic and thermodynamic control in ion-recombination''.
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| :[[File:Thermo kinetic control ingold 1948.svg|400px|Ingold/Hughes/Catchpole 1948]]
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| == References ==
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| {{reflist}}
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| [[Category:Chemical reactions]]
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| [[Category:Thermodynamics]]
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| [[Category:Chemical thermodynamics]]
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