A chemical mechanism describes in detail exactly what takes place at each stage of an overall chemical reaction (transformation). 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 account for all reactants used, the function of a catalyst, stereochemistry, all products formed and the amount of each. It must also describe the relative rates of the reaction steps and the rate equation for the overall reaction. Reaction intermediates are chemical species, often unstable and short-lived, which are not reactants or products of the overall chemical reaction, but are temporary products and reactants in the mechanism's reaction steps. Reaction intermediates are often free radicals or ions. Transition states can be unstable intermediate molecular states even in the elementary reactions. Transition states are commonly molecular entities involving an unstable number of bonds and/or unstable geometry. They correspond to maxima on the reaction coordinate, and to saddle points on the potential energy surface for the reaction.
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.
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 <math>r = k[NO_2]^2</math>. 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 law of <math>r = k[NO_2]^2</math>.
Other reactions may have mechanisms of several consecutive steps. In organic chemistry, one of the first reaction mechanisms proposed was that for the benzoin condensation, put forward in 1903 by A. J. Lapworth.
There are also more complex mechanisms such as chain reactions, in which the propagation steps of the chain form a closed cycle.
Other experimental methods to determine mechanism
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 termolecular.
In general, reaction steps involving more than three molecular entities do not occur.
- Organic reactions by mechanism under Organic reaction
- Nucleophilic acyl substitution
- Neighbouring group participation
- Finkelstein reaction
- Lindemann mechanism
- Electrochemical reaction mechanism
- Nucleophilic abstraction
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