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, 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 used, the stereochemistry observed in reactants and products, all products formed and the amount of each.
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 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 or ions.
The kinetics (relative rates of the reaction steps and the rate equation 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).
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 . 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. 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 which obeys the rate law .
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.
A chain reaction is an example of a complex mechanism, in which the propagation steps form a closed cycle. In a chain reaction, the intermediate produced in one step generates anintermediate 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 has several steps, these are:
1. Chain initiation: this can be by thermolysis or photolysis leading to the breakage of a bond. Thermolysis means heating the molecules and photolysis absorption of light.
2. Propagation: In this chain carrier makes another carrier.
3. Branching: One carrier makes more than one carrier.
4. Retardation: 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.
5. Chain termination: Radicals combine and the chain carriers are lost.
6. Inhibition: Chain carriers are removed by other processes, other than termination, say by foreing 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 reduction from ethanol to ethane. This one may occur in 4 fases:
1. Iniciation : CH3CHO → ·CH3 + ·CHO (R=k1 [CH3CHO])
2. Propagation: CH3CHO + ·CH3 → CH4 + CH3CO· (R=k2 [CH3CHO][·CH3])
3. Propagation: CH3CO· → ·CH3 + CO (R=k3 [CH3CO·])
4. Termination: ·CH3 + ·CH3 → CH3CH3 (R=k4 [·CH3]2)
We must emphasize that at the end of the global reaction all the intermediates must sum zero. This condition is explained as the possibility of derivation of the rate equation on the basis of steady-state approximation.
How do we account for the rate of laws of chain reactions?
We can take as an example the thermal decomposition of acetaldehyde. This appears to follow three-halves order in acetaldehyde.
Overall reaction, CH3CHO(g)→ CH4(g) + CO(g) d[CH4]/dt = k[CH3CHO]3/2
The mechanism for this reaction known as Rice-Herzfeld mechanism is as follows.
(a) Initiation: CH3CHO → ⋅CH3 + ⋅CHO R = ka [CH3CHO]
(b) Propagation: CH3CHO + ⋅CH3→ CH4 + CH3CO⋅R = kb [CH3CHO] [⋅CH3]
(c) Propagation: CH3CO⋅→⋅CH3 + CO R = kc [CH3CO⋅]
(d) Termination: ⋅CH3 + ⋅CH3→ CH3CH3 R = kd [⋅CH3]2
Although the mechanism explains the principal products, there are several minor products such as acetone (CH3COCH3) and propanal(CH3CH2CHO).
The rate equation can be derived on the basis of steady-state approximation. The rate of change of intermediates may be set equal to zero.
d[⋅CH3]/dt = ka[CH3CHO] – kb[⋅CH3][CH3CHO] + kc[CH3CO⋅]-2kd[⋅CH3]2 = 0
d[CH3CO⋅]/dt = kb[⋅CH3][CH3CHO] – kc[CH3CO⋅] = 0
The sum of the two equation is, ka[CH3CHO] – 2kd[⋅CH3]2 = 0
The steady-state concentration of ⋅CH3 radicals is, [⋅CH3] = (ka/2kd)1/2 [CH3CHO]1/2
If follows that the rate of formation of CH4 is d[CH4]/dt = kb[⋅CH3][CH3CHO] = kb (ka/2kd)1/2 [CH3CHO]3/2
Thus the mechanism explains the observed rate expression. It is sure that the true rate law is more complicated than that observed experimentally.
Other experimental methods to determine mechanismEdit
Many experiments 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
- 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 by pump-probe spectroscopy
- infrared chemiluminescence to detect vibrational excitation in the products
- electrospray ionization mass spectrometry.
- crossover experiments.
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.
- 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.
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L.G.WADE,ORGANIC CHEMISTRY 7TH ED,2010
16. The chain-reaction theory of negative catalysis, HLJ Bäckström - Journal of the American Chemical Society, 1927 - ACS Publications