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{{Short description|Any model explaining a chemical reaction}} {{About|general process analysis|electron flow diagrams|arrow pushing}} {{Original research|date=February 2025}} In [[chemistry]], a '''reaction mechanism''' is the step by step [[sequence]] of [[elementary reaction]]s by which overall [[chemical reaction]] occurs.<ref>{{JerryMarch}}</ref> A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage of an overall chemical reaction. The detailed steps of a reaction are not observable in most cases. The conjectured mechanism is chosen because it is thermodynamically feasible and has experimental support in isolated intermediates (see next section) or other quantitative and qualitative characteristics of the reaction. It also describes each [[reactive intermediate]], [[activated complex]], and [[transition state]], which bonds are broken (and in what order), and which bonds are formed (and in what order). A complete mechanism must also explain the reason for the [[reactant]]s and [[catalyst]] used, the [[stereochemistry]] observed in reactants and products, all [[Product (chemistry)|product]]s formed and the amount of each. [[Image:BromoethaneSN2reaction-small.png|thumb|center|540px|[[SN2 reaction|S<sub>N</sub>2 reaction]] mechanism. Note the negatively charged [[transition state]] in brackets in which the central [[carbon atom]] in question shows five bonds, an unstable condition .]] The [[Arrow pushing|electron or arrow pushing]] method is often used in illustrating a reaction mechanism; for example, see the illustration of the mechanism for [[benzoin condensation]] in the following examples section. [[File:Reaction Mechanism Acetone and Methanol.png|thumb|400px|center|A reaction mechanism shows how [[acetone]] reacts with [[methanol]] in acidic environment using curved arrow (electron or arrow pushing method)]] Mechanisms also are of interest in [[inorganic chemistry]]. A often quoted mechanistic experiment involved the reaction of the labile hexaaquo chromous reductant with the exchange inert [[pentammine cobalt(III) chloride]]. [[File:TaubeETexpCrCo.svg|thumb|490px|center|[[Henry Taube]]'s experiment establishing the role of a [[bridging ligand]] in [[inner sphere electron transfer]].]] ==Reaction intermediates== Reaction intermediates are chemical species, often unstable and short-lived. They can, however, sometimes be isolated. They are neither reactants nor products of the overall chemical reaction, but temporary products and/or reactants in the mechanism's reaction steps. Reaction intermediates are often confused with the [[transition state]]. The [[transition state]]s are, in contrast, fleeting, high-energy species that cannot be isolated. The kinetics (relative rates of the reaction steps and the [[rate equation]] for the overall reaction) are discussed in terms of the energy required for the conversion of the reactants to the proposed transition states (molecular states that correspond to maxima on the [[reaction coordinate]]s, and to [[saddle point]]s on the [[potential energy surface]] for the reaction). ==Chemical kinetics== Information about the mechanism of a reaction is often provided by analyzing [[chemical kinetics]] to determine the [[reaction order]] in each reactant.<ref>Espenson, James H. ''Chemical Kinetics and Reaction Mechanisms'' (2nd ed., McGraw-Hill, 2002) chap.6, ''Deduction of Reaction Mechanisms'' {{ISBN|0-07-288362-6}}</ref> Illustrative is the oxidation of carbon monoxide by nitrogen dioxide: :CO + NO<sub>2</sub> → CO<sub>2</sub> + NO The [[rate law]] for this reaction is: <math>r = k[NO_2]^2</math> This form shows that the [[rate-determining step]] does not involve CO. Instead, the slow step involves two molecules of NO<sub>2</sub>. A possible mechanism for the overall reaction that explains the rate law is: :2 NO<sub>2</sub> → NO<sub>3</sub> + NO (slow) :NO<sub>3</sub> + CO → NO<sub>2</sub> + CO<sub>2</sub> (fast) Each step is called an elementary step, and each has its own [[rate law]] and [[molecularity]]. The sum of the elementary steps gives the net reaction. When determining the overall rate law for a reaction, the slowest step is the step that determines the reaction rate. Because the first step (in the above reaction) is the slowest step, it is the [[rate-determining step]]. Because it involves the collision of two NO<sub>2</sub> molecules, it is a bimolecular reaction with a rate <math>r</math> which obeys the rate law <math>r = k[NO_{2}(t)]^2</math>. Other reactions may have mechanisms of several consecutive steps. In [[organic chemistry]], the reaction mechanism for the [[benzoin condensation]], put forward in 1903 by [[A. J. Lapworth]], was one of the first proposed reaction mechanisms. [[Image:Benzoin condensation2.svg|center|thumb|700px|[[Benzoin condensation]] '''reaction mechanism'''. [[Cyanide]] ion (CN<sup>−</sup>) acts as a [[catalyst]] here, entering at the first step and leaving in the last step. Proton (H<sup>+</sup>) transfers occur at (i) and (ii). The [[arrow pushing]] method is used in some of the steps to show where electron pairs go.]] A [[Chain reaction#Chemical chain reactions|chain reaction]] is an example of a complex mechanism, in which the [[Chain propagation|propagation]] steps form a closed cycle. In a chain reaction, the intermediate produced in one step generates an intermediate in another step. Intermediates are called chain carriers. Sometimes, the chain carriers are radicals, they can be ions as well. In nuclear fission they are neutrons. Chain reactions have several steps, which may include:<ref>{{cite journal |last1=Bäckström |first1=Hans L. J. |title=The chain-reaction theory of negative catalysis |journal=Journal of the American Chemical Society |date=1 June 1927 |volume=49 |issue=6 |pages=1460–1472 |doi=10.1021/ja01405a011 |bibcode=1927JAChS..49.1460B |url=https://pubs.acs.org/doi/abs/10.1021/ja01405a011 |access-date=20 January 2021|url-access=subscription }}</ref> # Chain initiation: this can be by [[thermolysis]] (heating the molecules) or [[photolysis]] (absorption of light) leading to the breakage of a bond. # Propagation: a chain carrier makes another carrier. # Branching: one carrier makes more than one carrier. # Retardation: a chain carrier may react with a product reducing the rate of formation of the product. It makes another chain carrier, but the product concentration is reduced. # Chain termination: radicals combine and the chain carriers are lost. # Inhibition: chain carriers are removed by processes other than termination, such as by forming radicals. Even though all these steps can appear in one chain reaction, the minimum necessary ones are Initiation, propagation, and termination. An example of a simple chain reaction is the thermal decomposition of [[acetaldehyde]] (CH<sub>3</sub>CHO) to [[methane]] (CH<sub>4</sub>) and [[carbon monoxide]] (CO). The experimental reaction order is 3/2,<ref>[[Keith J. Laidler|Laidler K.J.]] and Meiser J.H., ''Physical Chemistry'' (Benjamin/Cummings 1982) p.416-417 {{ISBN|0-8053-5682-7}}</ref> which can be explained by a ''Rice-Herzfeld mechanism''.<ref>Atkins and de Paula p.830-1</ref> This reaction mechanism for acetaldehyde has 4 steps with rate equations for each step : # Initiation : CH<sub>3</sub>CHO → •CH<sub>3</sub> + •CHO (Rate=k<sub>1</sub> [CH<sub>3</sub>CHO]) # Propagation: CH<sub>3</sub>CHO + •CH<sub>3</sub> → CH<sub>4</sub> + CH<sub>3</sub>CO• (Rate=k<sub>2</sub> [CH<sub>3</sub>CHO][•CH<sub>3</sub>]) # Propagation: CH<sub>3</sub>CO• → •CH<sub>3</sub> + CO (Rate=k<sub>3</sub> [CH<sub>3</sub>CO•]) # Termination: •CH<sub>3</sub> + •CH<sub>3</sub> → CH<sub>3</sub>CH<sub>3</sub> (Rate=k<sub>4</sub> [•CH<sub>3</sub>]<sup>2</sup>) For the overall reaction, the rates of change of the concentration of the intermediates •CH<sub>3</sub> and CH<sub>3</sub>CO• are zero, according to the [[Steady state (chemistry)|steady-state approximation]], which is used to account for the rate laws of chain reactions.<ref>[[Peter Atkins|Atkins P]] and de Paula J, ''Physical Chemistry'' (8th ed., W.H. Freeman 2006) p.812 {{ISBN|0-7167-8759-8}}</ref> d[•CH<sub>3</sub>]/dt = k<sub>1</sub>[CH<sub>3</sub>CHO] – k<sub>2</sub>[•CH<sub>3</sub>][CH<sub>3</sub>CHO] + k<sub>3</sub>[CH<sub>3</sub>CO•] - 2k<sub>4</sub>[•CH<sub>3</sub>]<sup>2</sup> = 0 and d[CH<sub>3</sub>CO•]/dt = k<sub>2</sub>[•CH<sub>3</sub>][CH<sub>3</sub>CHO] – k<sub>3</sub>[CH<sub>3</sub>CO•] = 0 The sum of these two equations is k<sub>1</sub>[CH<sub>3</sub>CHO] – 2 k<sub>4</sub>[•CH<sub>3</sub>]<sup>2</sup> = 0. This may be solved to find the steady-state concentration of •CH<sub>3</sub> radicals as [•CH<sub>3</sub>] = (k<sub>1</sub> / 2k<sub>4</sub>)<sup>1/2</sup> [CH<sub>3</sub>CHO]<sup>1/2</sup>. It follows that the rate of formation of CH<sub>4</sub> is d[CH<sub>4</sub>]/dt = k<sub>2</sub>[•CH<sub>3</sub>][CH<sub>3</sub>CHO] = k<sub>2</sub> (k<sub>1</sub> / 2k<sub>4</sub>)<sup>1/2</sup> [CH<sub>3</sub>CHO]<sup>3/2</sup> Thus the mechanism explains the observed rate expression, for the principal products CH<sub>4</sub> and CO. The exact rate law may be even more complicated, there are also minor products such as [[acetone]] (CH<sub>3</sub>COCH<sub>3</sub>) and [[propionaldehyde|propanal]] (CH<sub>3</sub>CH<sub>2</sub>CHO). ==Other experimental methods to determine mechanism== Many [[experiment]]s that suggest the possible sequence of steps in a reaction mechanism have been designed, including: * measurement of the effect of temperature ([[Arrhenius equation]]) to determine the [[activation energy]]<ref>Espenson p.156-160</ref> * [[spectroscopy|spectroscopic]] observation of [[reaction intermediates]] * determination of the [[stereochemistry]] of products, for example in [[nucleophilic substitution]] reactions<ref>Morrison R.T. and Boyd R.N. ''Organic Chemistry'' (4th ed., Allyn and Bacon 1983) p.216-9 and p.228-231, {{ISBN|0-205-05838-8}}</ref> * measurement of the effect of [[kinetic isotope effect|isotopic substitution]] on the reaction rate<ref>[[Peter Atkins|Atkins P]] and de Paula J, ''Physical Chemistry'' (8th ed., W.H. Freeman 2006) p.816-8 {{ISBN|0-7167-8759-8}}</ref> * for reactions in solution, measurement of the effect of pressure on the reaction rate to determine the volume change on formation of the activated complex<ref>Moore J.W. and [[Ralph Pearson|Pearson R.G.]] ''Kinetics and Mechanism'' (3rd ed., John Wiley 1981) p.276-8 {{ISBN|0-471-03558-0}}</ref><ref>[[Keith J. Laidler|Laidler K.J.]] and Meiser J.H., ''Physical Chemistry'' (Benjamin/Cummings 1982) p.389-392 {{ISBN|0-8053-5682-7}}</ref> * for reactions of ions in solution, measurement of the effect of [[ionic strength]] on the reaction rate<ref>Atkins and de Paula p.884-5</ref><ref>Laidler and Meiser p.388-9</ref> * direct observation of the [[activated complex]] by [[femtochemistry|pump-probe spectroscopy]]<ref>Atkins and de Paula p.892-3</ref> * infrared [[chemiluminescence]] to detect vibrational excitation in the products<ref>Atkins and de Paula p.886</ref><ref>Laidler and Meiser p.396-7</ref> * [[electrospray ionization mass spectrometry]].<ref>Investigation of chemical reactions in solution using API-MS'' Leonardo Silva Santos, Larissa Knaack, Jurgen O. Metzger [[International Journal of Mass Spectrometry|Int. J. Mass Spectrom.]]; '''2005'''; 246 pp 84 - 104; (Review) {{doi|10.1016/j.ijms.2005.08.016}}</ref> * [[crossover experiment (chemistry)|crossover experiments]].<ref>Espenson p.112</ref> ==Theoretical modeling== A correct reaction mechanism is an important part of accurate [[predictive modeling]]. For many combustion and plasma systems, detailed mechanisms are not available or require development. Even when information is available, identifying and assembling the relevant data from a variety of sources, reconciling discrepant values and extrapolating to different conditions can be a difficult process without expert help. Rate constants or thermochemical data are often not available in the literature, so [[computational chemistry]] techniques or [[group additivity method]]s must be used to obtain the required parameters.{{cn|date=February 2025}} Computational chemistry methods can also be used to calculate [[potential energy surface]]s for reactions and determine probable mechanisms.<ref>Atkins and de Paula p.887-891</ref> ==Molecularity== {{main|molecularity}} '''Molecularity''' in [[chemistry]] is the number of colliding [[molecular entity|molecular entities]] that are involved in a single [[reaction step]]. * A reaction step involving one molecular entity is called unimolecular. * A reaction step involving two molecular entities is called bimolecular. * A reaction step involving three molecular entities is called trimolecular or termolecular. In general, reaction steps involving more than three molecular entities do not occur, because is statistically improbable in terms of Maxwell distribution to find such a transition state. ==See also== *[[Organic reaction#By mechanism|Organic reactions by mechanism]] *[[Nucleophilic acyl substitution]] *[[Neighbouring group participation]] *[[Finkelstein reaction]] *[[Lindemann mechanism]] *[[Electrochemical reaction mechanism]] *[[Nucleophilic abstraction]] ==References== {{Reflist}} L.G.WADE, ORGANIC CHEMISTRY 7TH ED, 2010 ==External links== *[https://www-pls.llnl.gov/?url=science_and_technology-chemistry-combustion-mechanisms Reaction mechanisms for combustion of hydrocarbons] {{Reaction mechanisms}} {{Authority control}} [[Category:Chemical reactions|Mechanism]] [[Category:Reaction mechanisms| ]] [[Category:Chemical kinetics]] [[Category:Chemical reaction engineering]] [[Category:Combustion]]
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