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| In [[chemistry]], a '''radical clock''' is a [[chemical compound]] that assists in the indirect methodology to determine the [[kinetics (chemistry)|kinetics]] of a [[free-radical reaction]]. The radical-clock compound itself reacts at a known rate, which provides a calibration for determining the rate of another reaction.
| | If an existing Word - Press code is found vulnerable, Word - Press will immediately issue an update for that. Offshore expert Word - Press developers high level of interactivity, accessibility, functionality and usability of our website can add custom online to using. * A community forum for debate of the product together with some other customers in the comments spot. 2- Ask for the designs and graphics that will be provided along with the Word - Press theme. The number of options offered here is overwhelming, but once I took the time to begin to review the video training, I was amazed at how easy it was to create a squeeze page and a membership site. <br><br>As you know today Word - Press has turn out to be a tremendously popular open source publishing and blogging display place. If you wish to sell your services or products via internet using your website, you have to put together on the website the facility for trouble-free payment transfer between customers and the company. It allows Word - Press users to easily use HTML5 the element enable native video playback within the browser. This is identical to doing a research as in depth above, nevertheless you can see various statistical details like the number of downloads and when the template was not long ago updated. Once you've installed the program you can quickly begin by adding content and editing it with features such as bullet pointing, text alignment and effects without having to do all the coding yourself. <br><br>It is very easy to install Word - Press blog or website. It was also the very first year that the category of Martial Arts was included in the Parents - Connect nationwide online poll, allowing parents to vote for their favorite San Antonio Martial Arts Academy. I hope this short Plugin Dynamo Review will assist you to differentiate whether Plugin Dynamo is Scam or a Genuine. Enough automated blog posts plus a system keeps you and your clients happy. To read more info about [http://scridle.nl/backup_plugin_395058 wordpress backup plugin] stop by our page. There are plenty of tables that are attached to this particular database. <br><br>The disadvantage is it requires a considerable amount of time to set every thing up. I didn't straight consider near it solon than one distance, I got the Popup Ascendancy plugin and it's up and lengthways, likely you make seen it today when you visited our blog, and I yet customize it to fit our Thesis Wound which gives it a rattling uncomparable visage and search than any different popup you know seen before on any added journal, I hump arrogated asset of one of it's quatern themes to make our own. Thus it is difficult to outrank any one of these because of their different usages. If you choose a blog then people will be able to post articles on your site and people will be able to make comments on your posts (unless you turn comments off). If your blog employs the permalink function, This gives your SEO efforts a boost, and your visitors will know firsthand what's in the post when seeing the URL. <br><br>Website security has become a major concern among individuals all over the world. An ease of use which pertains to both internet site back-end and front-end users alike. It's not a secret that a lion share of activity on the internet is takes place on the Facebook. Page speed is an important factor in ranking, especially with Google. Definitely when you wake up from the slumber, you can be sure that you will be lagging behind and getting on track would be a tall order. |
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| == Introduction ==
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| Many reactions in organic mechanisms involve intermediates that have a key role in the reaction. Understanding these intermediates and their [[chemical kinetics]] may be helpful in predicting the outcome of the reaction prior to performing the reaction itself.<ref name="Johnson">{{cite journal |authors= Johnson, C.C.; Lippard, S.J.; Liu, K.E.; Newcomb, M. |journal= J. Am. Chem. Soc. |year= 1993 |volume= 115 |pages= 939–947 |title= Radical Clock Substrate Probes and Kinetic Isotope Effect Studies of the Hydroxylation of Hydrocarbons by Methane Monooxygenase |doi= 10.1021/ja00056a018 }}</ref> Among the different types of [[reactive intermediate]]s that are involved in synthetic organic mechanisms such as [[carbocation]]s, [[carbanion]]s, and [[carbene]]s, interests in radical intermediates reactions have increased. A great deal of attention was drawn to them due to the discovery of their linkage in [[enzymatic pathway]]s and in [[sonodynamic therapy]] in the treatment of cancer.<ref>Misik, V.; Riesz, P. ''Annals of New York Academy of Sciences.'' 2000, 899, 335–348.</ref>
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| Many techniques have been developed to measure the rates of radical reactions. The two possible ways to measure the reaction rates are direct and indirect approaches. Direct approaches such as the [[rotating sector method]] have restrictions that hinder their expansion or application to the wide variety of types of radical reactions of chemical interest.<ref name="Roschek">{{cite journal |authors= Roschek, B. Jr.; Tallman, K.A.; Rector, C.L.; Gillmore, J.G.; Pratt, D.A.; Punta, C.; Porter, N.A. |journal= J. Org. Chem. |year= 2006 |volume= 71 |pages= 3527–3532 |title= Peroxyl Radical Clocks |doi= 10.1021/jo0601462 }}</ref><ref name="Griller">{{cite journal |authors= Griller, D.; Ingold, K.U. |journal= Acc. Chem. Res. |year= 1980 |volume= 13 |pages= 317–323 |title= Free-radical clocks |doi= 10.1021/ar50153a004 }}</ref> In addition, direct methods such as [[flash photolysis]] and [[pulse radiolysis]] require a relatively large amount of time and expensive equipment. With an indirect approach, one can still obtain relative or absolute rate constants without the need for instruments or equipment beyond those normally needed for the reaction being studied.<ref name="Moss">Moss, R.A.; Platz, M.; Jones, M. Reactive Intermediate Chemistry. Wiley, John & Sons, Incorporated, 2004. 127–128.</ref>
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| == Theory and technique ==
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| Radical clock reactions involve a competition between a [[unimolecular]] [[Radical (chemistry)|radical]] reaction with a known [[rate constant]] and a [[bimolecular]] radical reaction with an unknown rate constant to produce unrearranged and rearranged products. The rearrangement of an unrearranged radical, U•, proceeds to form R• (the clock reaction) with a known rate constant (''k''<sub>r</sub>). These radicals react with a [[trapping agent]], AB, to form the unrearranged and rearranged products UA and RA, respectively.<ref name="Yao"/>
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| :<math>\begin{array}{lcl}
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| U\cdot + AB & \xrightarrow{k_R} & UA + B\cdot \\
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| \bigg\downarrow{k_r} \\
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| R\cdot + AB & \xrightarrow{} & RA + B\cdot
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| \end{array}</math>
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| The yield of the two products can be determined by [[gas chromatography]] (GC) or [[nuclear magnetic resonance]] (NMR). From the concentration of the trapping agent, the known rate constant of the radical clock, and the ratio of the products, the unknown rate constant can be indirectly established.
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| If a [[chemical equilibrium]] exists between U• and R•, the rearranged products are dominant.<ref name="Griller" /> Because the [[unimolecular]] rearrangement reaction is first order and the [[bimolecular]] trapping reaction is second order (both irreversible), the unknown rate constant (''k''<sub>R</sub>) can be determined by:<ref>{{cite journal |authors= Newcomb, M. |journal= Tetrahedron |year= 1993 |volume= 49 |issue= 6 |pages= 1151–1176 |title= Competition Methods and Scales for Alkyl Radical Reaction Kinetics |doi= 10.1016/S0040-4020(01)85808-7 }}</ref>
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| :<math>k_R = \frac{k_r [UA]}{[AB][RA]}</math>
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| == Clock rates ==
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| The driving force behind radical clock reactions is their ability to rearrange.<ref name="Johnson" /> Some common radical clocks are radical cyclizations, ring openings, and 1,2-migrations.<ref name="Griller" /> Two popular rearrangements are the cyclization of 5-hexenyl and the ring-opening of cyclopropylmethyl:<ref name="Johnson" />
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| <center>
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| {|class=wikitable
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| |valign=bottom | [[File:Radical Cyclization.svg|200px]]
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| |valign=bottom | [[File:Ring Opening.svg|200px]]
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| |}
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| </center>
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| 5-hexenyl radical undergoes cyclization to produce a five-membered ring because this [[entropy|entropically]] and [[enthalpy|enthalpically]] more favored than the six-membered ring possibility.<ref name="Johnson" /><ref name="Griller" /> The rate-constant for this reaction is 2.3×10<sup>5</sup> s<sup>–1</sup> at 298 K.<ref name="Yao"/>
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| Cyclopropylmethyl radical undergoes a very rapid ring opening rearrangement that relieves the [[ring strain]] and is enthalpically favorable.<ref name="Johnson" /><ref name="Griller" /> The rate-constant for this reaction is 8.6×10<sup>7</sup> s<sup>–1</sup> at 298 K.<ref>{{cite journal |authors= Bowry, V.W.; Lusztyk, J.; Ingold, K.U. |journal= J. Am. Chem. Soc. |year= 1991 |volume= 113 |pages= 5687–5698 |title= Calibration of a new horologery of fast radical "clocks". Ring-opening rates for ring- and α-alkyl-substituted cyclopropylcarbinyl radicals and for the bicyclo[2.1.0]pent-2-yl radical |doi= 10.1021/ja00015a024 }}</ref>
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| In order to determine absolute rate constants for radical reactions, unimolecular clock reactions need to be calibrated for each group of radicals such as [[primary carbon|primary alkyls]] over a range of time.<ref name="Griller" /> Through the usage of [[EPR spectroscopy]], the absolute rate constants for [[unimolecular]] reactions can be measured with a variety of temperatures.<ref name="Griller" /><ref name="Moss" /> The [[Arrhenius equation]] can then be applied to calculate the rate constant for a specific temperature at which the radical clock reactions are conducted.
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| When using a radical clock to study a reaction, there is an implicit assumption that the rearrangement rate of the radical clock is the same as when the rate of that rearrangement reaction rate is determined. A [[computational chemistry|theoretical study]] of the rearrangement reactions of cyclobutylmethyl and of 5-hexenyl in a variety of solvents found that their reaction rates were only very slightly affected by the nature of the solvent.<ref name="Yao">{{cite journal |authors= Fu, Y.; Li, R.-Q.; Liu, L.; Guo, Q.-X. |journal= Res. Chem. Intermed. |year= 2004 |volume= 30 |issue= 3 |pages= 279–286 |title= Solvent effect is not significant for the speed of a radical clock |doi= 10.1163/156856704323034012 }}</ref>
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| The rates of radical clocks can be adjusted to increase or decrease by what types of substituents are attached to the radical clock. In the figure below, the rates of the radical clocks are shown with a variety of substituents attached to the clock.<ref name="Johnson" />{{Failed verification|date=February 2011}}
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| {|class=wikitable
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| |colspan=3 |[[Image:Radical cyclization substituted.png|315px]]
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| |-
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| !X
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| !Y
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| !''k'' (s<sup>-1</sup>)
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| |-
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| ||Ph ||Ph ||5x10<sup>7</sup>
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| |-
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| ||OCH<sub>3</sub> ||H ||1.4x10<sup>5</sup>
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| |-
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| ||OCH<sub>3</sub> ||CN ||2.5x10<sup>8</sup>
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| |-
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| ||CN ||H ||1.6x10<sup>8</sup>
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| |}
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| By selecting among the general classes of radical clocks and the specific substituents on them, one can be chosen with a rate-constant suitable for studying reactions having a wide range of rates. Reactions having rates ranging from 10<sup>–1</sup> to 10<sup>12</sup> M<sup>–1</sup> s<sup>–1</sup> have been studied using radical clocks.<ref name="Roschek"/>
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| == Examples of use ==
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| Radical clocks are used in [[redox|reduction]] of [[alkyl halide]]s with [[sodium napthalenide]], reaction of [[enones]], the [[Wittig rearrangement]], [[reductive elimination]] reactions of dialkylmercury compounds, [[dioxirane]] [[dihydroxylation]]s, and [[electrophilic fluorinations]].<ref name="Griller" />
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| == References ==
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| <references />
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| == External links ==
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| * [http://www.scs.uiuc.edu/chem/research/organic/seminar_extracts/2005_2006/06_Wang.pdf RADICAL CLOCKS: MOLECULAR STOPWATCHES FOR TIMING RADICAL REACTIONS]
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| * [http://euch6f.chem.emory.edu/radical.html Radical Clock Reactions]
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| {{DEFAULTSORT:Radical Clock}}
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| [[Category:Free radicals]]
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| [[Category:Chemical kinetics]]
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