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SN2 reaction mechanism

In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.[1]

Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in a reaction mechanism.



A mechanism describes in detail exactly what takes place at each stage of a chemical transformation. It describes each transition state, which bonds are broken (and in what order), which bonds are formed (and in what order) and what the relative rates of the steps are. A complete mechanism must also account for all reactants used, the function of a catalyst, stereochemistry, 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.


Consider the following reaction:

CO + NO2 → CO2 + NO

In this case, it has been experimentally determined that this reaction takes place according to the rate law R = k[NO2]2. Therefore, a possible mechanism by which this reaction takes place 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. All of the elementary steps must add up to the original reaction, by means of organic reactions (e.g.: rearrangement reactions).

When determining the overall rate law for a reaction, the slowest step is the step that determines the reaction rate. Because the first step 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 R = k[NO2]2. 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.

In organic chemistry, one of the first reaction mechanisms proposed was that for the benzoin condensation, put forward in 1903 by A. J. Lapworth.


A correct reaction mechanism is an important part of accurate predictive modelling. 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.

At the different stages of a reaction mechanism's elaboration, appropriate methods must be used.


Molecularity in chemistry is the number of colliding molecular entities that are involved in a single reaction step.

  • A reaction involving one molecular entity is called unimolecular.
  • A reaction involving two molecular entities is called bimolecular.
  • A reaction involving three molecular entities is called termolecular.

See also


  1. ^ March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN 0-471-85472-7 


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