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Reaction mechanism
 
 
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In chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, a reaction mechanism is the step by step sequence
Sequence

In mathematics, a sequence is an ordered list of objects . Like a Set , it contains Element , and the number of terms is called the length of the sequence....
 of elementary reaction
Elementary reaction

An elementary reaction is a chemical reaction in which one or more of the chemical species react directly to form products in a single reaction step and with a single transition state....
s by which overall chemical change occurs .

Although only the net chemical change is directly observable
Observation

Observation is either an activity of a living being , consisting of receiving knowledge of the outside world through the senses, or the recording of data using scientific instruments....
 for most chemical reactions, experiment
Experiment

In scientific inquiry, an experiment is a method of investigating causal relationships among variables. An experiment is a cornerstone of the empiricism approach to acquiring data about the world and is used in both natural sciences and social sciences....
s 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.






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Bromoethanesn2reaction Small
In chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, a reaction mechanism is the step by step sequence
Sequence

In mathematics, a sequence is an ordered list of objects . Like a Set , it contains Element , and the number of terms is called the length of the sequence....
 of elementary reaction
Elementary reaction

An elementary reaction is a chemical reaction in which one or more of the chemical species react directly to form products in a single reaction step and with a single transition state....
s by which overall chemical change occurs .

Although only the net chemical change is directly observable
Observation

Observation is either an activity of a living being , consisting of receiving knowledge of the outside world through the senses, or the recording of data using scientific instruments....
 for most chemical reactions, experiment
Experiment

In scientific inquiry, an experiment is a method of investigating causal relationships among variables. An experiment is a cornerstone of the empiricism approach to acquiring data about the world and is used in both natural sciences and social sciences....
s 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 the transition state
Transition state

The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate....
 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
Stereochemistry

Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chirality molecules ....
, all products formed and the amount 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 example:

CO + NO2 → CO2 + NO


In this reaction, it has been experimentally determined that this reaction takes place according to the rate law . 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
Molecularity

Molecularity in chemistry is the number of colliding molecular entity that are involved in a single reaction step. While the order of a reaction is derived experimentally, the molecularity is a theoretical concept and can only be applied to elementary reactions....
. 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.

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
Rate-determining step

The rate-determining step is a chemistry term for the slowest reaction step in a chemical reaction. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed at which water is poured in....
. Because it involves the collision of 2 NO2 molecules, it is a bimolecular reaction with a rate law of . 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.

In organic chemistry
Organic chemistry

Organic chemistry is a discipline within chemistry which involves the science study of the structure, properties, composition, chemical reaction, and preparation of chemical compounds that contain carbon....
 one of the first reaction mechanisms proposed was that for the benzoin condensation
Benzoin condensation

The benzoin condensation is a condensation reaction between two aromatic aldehydes, particularly benzaldehyde. The reaction is catalysis by a nucleophile such as the cyanide anion or an persistent carbene....
 in 1903 by A. J. Lapworth.

See also

  • Organic reactions by mechanism
    Organic reaction

    Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions and organic redox reaction....
     under Organic reaction
    Organic reaction

    Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions and organic redox reaction....
  • Nucleophilic acyl substitution
    Nucleophilic acyl substitution

    Nucleophilic acyl substitution describes the substitution reaction involving nucleophiles and acyl compounds. Acyl compounds are carboxylic acid derivatives including esters, amides and acid halides....
  • Neighbouring group participation
    Neighbouring group participation

    Neighbouring group participation or NGP in organic chemistry has been defined by IUPAC as the interaction of a reaction centre with a lone pair of electrons in an atom or the electrons present in a sigma bond or pi bond ....
  • Finkelstein reaction
    Finkelstein reaction

    The Finkelstein reaction, named for the German chemist Hans Finkelstein, is an SN2 reaction that involves the exchange of one halogen atom for another....
  • Lindemann mechanism
    Lindemann mechanism

    In chemical kinetics, the Lindemann mechanism, sometimes called the Lindemann-Hinshelwood mechanism, is a schematic reaction mechanism. Frederick Lindemann discovered the concept in 1921 and Cyril Hinshelwood developed it....
  • Electrochemical reaction mechanism
    Electrochemical reaction mechanism

    In chemistry, an electrochemical reaction mechanism is the step by step sequence of elementary steps, involving at least one outer sphere electron transfer, by which an overall chemical change occurs ....