Organotin chemistry

2013-08-09T03:23:44Z

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'''Molecularity''' in [[chemistry]] is the number of colliding [[molecular entity|molecular entities]] that are involved in a single [[reaction step]].<ref> {{GoldBookRef|title=molecularity|url=http://goldbook.iupac.org/M03989.html|year=1996}}</ref> While the [[order (chemistry)|order]] of a reaction is derived experimentally, the molecularity is a theoretical concept and can only be applied to [[elementary reaction]]s. In elementary reactions, the reaction [[order (chemistry)|order]], the molecularity and the [[stoichiometric coefficient]] are the same, although only numerically, because they are different concepts.

* 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 or trimolecular. Termolecular reactions in [[solution]]s or gas mixtures are very rare, because of the improbability of three molecular entities simultaneously [[collision frequency|colliding]].<ref>[http://www.intute.ac.uk/sciences/reference/plambeck/chem2/p02156.htm Discussion on the improbability of termolecular reactions]</ref> However the term ''termolecular'' is also used to refer to three body association reactions of the type:
:<math>A + B \overset{M}{\to} C</math>
Where the M over the arrow denotes that to conserve [[energy]] and [[momentum]] a second reaction with a third body is required. After the initial bimolecular collision of A and B an energetically excited [[reaction intermediate]] is formed, then, it collides with a M body, in a second bimolecular reaction, transferring the excess energy to it.<ref>Text discussing [[rate constant]]s for termolecular reactions [http://jpldataeval.jpl.nasa.gov/pdf/Jpl15_Sectn2_TermolecRxs.pdf]</ref>

The reaction can be explained as two consecutive reactions:
:<math>A + B \to AB^*</math>
:<math>AB^* + M \to C + M</math>

These reactions frequently have a pressure and temperature dependence region of transition between second and third order kinetics.<ref>IUPAC definition of ''Troe expression'', a semiempirical expression for the rate constant of termolecular reactions [http://goldbook.iupac.org/T06517.html ]</ref>

Catalytic reactions are often three-component, but in practice a complex of the starting materials is first formed and the rate-determining step is the reaction of this complex into products, not an adventitious collision between the two species and the catalyst. For example, in hydrogenation with a metal catalyst, molecular dihydrogen first dissociates onto the metal surface into hydrogen atoms bound to the surface, and it is these monatomic hydrogens that react with the starting material, also previously adsorbed onto the surface.

==Notes and references==
{{Reflist}}

==See also==
* [[Reaction rate]]
* [[Order of reaction]]

{{Reaction mechanisms}}

[[Category:Chemical kinetics]]

{{physical-chemistry-stub}}

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Organotin chemistry