Reaction mechanism

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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.

S_N2 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 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 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).

Chemical kinetics

Information about the mechanism of a reaction is often provided by the use of chemical kinetics to determine the rate equation and the reaction order 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 ${\displaystyle r=k[NO_{2}]^{2}}$. 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. 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 ${\displaystyle r=k[NO_{2}]^{2}}$. 

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. 

 

Benzoin condensation reaction mechanism. Cyanide ion (CN⁻) acts as a catalyst here, entering at the first step and leaving in the last step. Proton (H⁺) 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 is an example of a complex mechanism, in which the propagation steps 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.

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.

Molecularity

Molecularity in chemistry 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.