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In
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
, a reaction mechanism is the step by step
sequence In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is called ...
of elementary reactions by which overall chemical change occurs. 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 In chemistry an activated complex is defined by the International Union of Pure and Applied Chemistry (IUPAC) as "that assembly of atoms which corresponds to an arbitrary infinitesimally small region at or near the col (saddle point) of a potential ...
, and
transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked ...
, and 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 reactants and
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
used, the stereochemistry observed in reactants and products, all products formed and the amount of each. The 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. A reaction mechanism must also account for the order in which molecules react. Often what appears to be a single-step conversion is in fact a multistep reaction.


Reaction intermediates

Reaction intermediates are chemical species, often unstable and short-lived (however sometimes can be isolated), which are not reactants or products of the overall chemical reaction, but are temporary products and/or reactants in the mechanism's reaction steps. Reaction intermediates are often
free radicals In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spon ...
or ions. The kinetics (relative rates of the reaction steps and the
rate equation In chemistry, the rate law or rate equation for a reaction is an equation that links the initial or forward reaction rate with the concentrations or pressures of the reactants and constant parameters (normally rate coefficients and partial reac ...
for the overall reaction) are explained in terms of the energy needed for the conversion of the reactants to the proposed transition states (molecular states that corresponds to maxima on the reaction coordinates, and to saddle points on the potential energy surface for the reaction).


Chemical kinetics

Information about the mechanism of a reaction is often provided by the use of
chemical kinetics Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with chemical thermodynamics, which deals with the direction in ...
to determine the
rate equation In chemistry, the rate law or rate equation for a reaction is an equation that links the initial or forward reaction rate with the concentrations or pressures of the reactants and constant parameters (normally rate coefficients and partial reac ...
and the
reaction order In chemistry, the rate law or rate equation for a reaction is an equation that links the initial or forward reaction rate with the concentrations or pressures of the reactants and constant parameters (normally rate coefficients and partial reac ...
in each reactant. Consider the following reaction for example: :CO + NO2 → CO2 + NO In this case, experiments have determined that this reaction takes place according to the rate law r = k
O_2 O, or o, is the fifteenth letter and the fourth vowel letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''o'' (pronounced ), pl ...
2. This form suggests that the rate-determining step is a reaction between two molecules of NO2. A possible mechanism for the overall reaction that explains the rate law is: :2 NO2 → NO3 + NO (slow) :NO3 + CO → NO2 + CO2 (fast) Each step is called an elementary step, and each has its own rate law and
molecularity In chemistry, molecularity is the number of molecules that come together to react in an elementary (single-step) reactionAtkins, P.; de Paula, J. Physical Chemistry. Oxford University Press, 2014 and is equal to the sum of stoichiometric coeffic ...
. The elementary steps should add up to the original reaction. (Meaning, if we were to cancel out all the molecules that appear on both sides of the reaction, we would be left with the original 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 NO2 molecules, it is a bimolecular reaction with a rate r which obeys the rate law r = k O_(t)2. Other reactions may have mechanisms of several consecutive steps. In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J ...
, the reaction mechanism for the benzoin condensation, put forward in 1903 by A. J. Lapworth, was one of the first proposed reaction mechanisms. A chain reaction is an example of a complex mechanism, in which the
propagation Propagation can refer to: *Chain propagation in a chemical reaction mechanism *Crack propagation, the growth of a crack during the fracture of materials * Propaganda, non-objective information used to further an agenda * Reproduction, and other for ...
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: # Chain initiation: this can be by
thermolysis Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is re ...
(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 Acetaldehyde (IUPAC systematic name ethanal) is an organic chemical compound with the formula CH3 CHO, sometimes abbreviated by chemists as MeCHO (Me = methyl). It is a colorless liquid or gas, boiling near room temperature. It is one of the ...
(CH3CHO) to
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
(CH4) and
carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...
(CO). The experimental reaction order is 3/2, which can be explained by a ''Rice-Herzfeld mechanism''. This reaction mechanism for acetaldehyde has 4 steps with rate equations for each step : # Initiation : CH3CHO → •CH3 + •CHO (Rate=k1 H3CHO # Propagation: CH3CHO + •CH3 → CH4 + CH3CO• (Rate=k2 H3CHO•CH3]) # Propagation: CH3CO• → •CH3 + CO (Rate=k3 H3CO• # Termination: •CH3 + •CH3 → CH3CH3 (Rate=k4 ��CH3sup>2) For the overall reaction, the rates of change of the concentration of the intermediates •CH3 and CH3CO• are zero, according to the steady-state approximation, which is used to account for the rate laws of chain reactions. d ��CH3dt = k1 H3CHO– k2 ��CH3H3CHO+ k3 H3CO•- 2k4 ��CH3sup>2 = 0 and d H3CO•dt = k2 ��CH3H3CHO– k3 H3CO•= 0 The sum of these two equations is k1 H3CHO– 2 k4 ��CH3sup>2 = 0. This may be solved to find the steady-state concentration of •CH3 radicals as ��CH3= (k1 / 2k4)1/2 H3CHOsup>1/2. It follows that the rate of formation of CH4 is d H4dt = k2 ��CH3H3CHO= k2 (k1 / 2k4)1/2 H3CHOsup>3/2 Thus the mechanism explains the observed rate expression, for the principal products CH4 and CO. The exact rate law may be even more complicated, there are also minor products such as
acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscibl ...
(CH3COCH3) and
propanal Propionaldehyde or propanal is the organic compound with the formula CH3CH2CHO. It is the 3-carbon aldehyde. It is a colourless, flammable liquid with a slightly fruity odour. It is produced on a large scale industrially. Production Propionalde ...
(CH3CH2CHO).


Other experimental methods to determine mechanism

Many
experiment An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs whe ...
s that suggest the possible sequence of steps in a reaction mechanism have been designed, including: * measurement of the effect of temperature (
Arrhenius equation In physical chemistry, the Arrhenius equation is a formula for the temperature dependence of reaction rates. The equation was proposed by Svante Arrhenius in 1889, based on the work of Dutch chemist Jacobus Henricus van 't Hoff who had noted in ...
) to determine the
activation energy In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...
* spectroscopic observation of reaction intermediates * determination of the stereochemistry of products, for example in nucleophilic substitution reactions * measurement of the effect of isotopic substitution on the reaction rate * 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 * for reactions of ions in solution, measurement of the effect of ionic strength on the reaction rate * direct observation of the
activated complex In chemistry an activated complex is defined by the International Union of Pure and Applied Chemistry (IUPAC) as "that assembly of atoms which corresponds to an arbitrary infinitesimally small region at or near the col (saddle point) of a potential ...
by pump-probe spectroscopy * infrared
chemiluminescence Chemiluminescence (also chemoluminescence) is the emission of light (luminescence) as the result of a chemical reaction. There may also be limited emission of heat. Given reactants A and B, with an excited intermediate â—Š, : + -> lozenge - ...
to detect vibrational excitation in the products * electrospray ionization mass spectrometry. * crossover experiments.


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 methods must be used to obtain the required parameters. Computational chemistry methods can also be used to calculate potential energy surfaces for reactions and determine probable mechanisms.Atkins and de Paula p.887-891


Molecularity

Molecularity in
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
is the number of colliding 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 transition state.


See also

* Organic reactions by mechanism * Nucleophilic acyl substitution * Neighbouring group participation * Finkelstein reaction * Lindemann mechanism * Electrochemical reaction mechanism * Nucleophilic abstraction


References

L.G.WADE,ORGANIC CHEMISTRY 7TH ED,2010


External links


Reaction mechanisms for combustion of hydrocarbons
{{Authority control Mechanism Chemical kinetics Chemical reaction engineering Combustion