In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs. Chemistry (from Greek χημεία khemeia[1] meaning alchemy) is the science of matter at the atomic to molecular scale, dealing primarily with collections of atoms, such as molecules, crystals, and metals. ... In mathematics, a sequence is a list of objects (or events) arranged in a linear fashion, such that the order of the members is well defined and significant. ... A reaction is the following: In physics, a reaction (physics) is defined by Newtons third law: For every action there is an equal and opposite reaction. The idea that any given force has a pair or opposite force. ... A chemical substance is any material substance used in or obtained by a process in chemistry: A chemical compound is a substance consisting of two or more chemical elements that are chemically combined in fixed proportions. ...

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. Observation is an activity of a sapient or sentient living being, which senses and assimiliates the knowledge of a phenomenon in its framework of previous knowledge and ideas. ... In the scientific method, an experiment (Latin: ex-+-periri, of (or from) trying), is a set of actions and observations, performed in the context of solving a particular problem or question, to support or falsify a hypothesis or research concerning phenomena. ...

An overall description of how a reaction occurs. A mechanism describes in detail exactly what takes place at each stage of a chemical transformation. It describes the transition state and 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 each. The transition state of a chemical reaction is a particular configuration along the reaction coordinate. ... It has been suggested that this article or section be merged into Catalysis. ... The different types of isomers. ...

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 example:

CO + NO₂ → CO₂ + NO

In this reaction, 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: A rate law is an equation that relates concentrations of reactants to the reaction rate. ...

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. There are four types of elementary steps: 1) Addition, 2) Elimination, 3) Substitution and 4) Rearrangement. A rate law is an equation that relates concentrations of reactants to the reaction rate. ... Molecularity in chemistry is the number of colliding molecules that are involved in a single reaction step. ...

When determining the overall rate law for a reaction, the slow step is the step that determines the reaction rate. Because the first step is the slow step, it is the rate-determining step. Because it involves the collision of 2 NO₂ molecules, it is a bimolecular reaction with a rate law of R = k[NO₂]². If one were to cancel out all the molecules that appear on both sides of the reaction, you would be left with the original reaction. The rate-determining step is a chemistry term for the slowest step in a chemical reaction. ...

In organic chemistry one of the first reaction mechanisms proposed was that for the benzoin condensation in 1903 by A. J. Lapworth. Organic chemistry is a specific discipline within the subject of chemistry. ... The Benzoin condensation is a condensation reaction between two aromatic aldehydes, especially benzaldehyde that is catalyzed by a cyanide. ... Arthur Lapworth (October 10, 1872 - April 5, 1941) was a Scottish chemist born in Galashiels, Scotland. ...

See also

• SN1 reaction
• SN2 reaction