concerted reaction
{{short description|Chemical reaction in which all bond reformation occurs in one step}}
File:Sn2EtCl+bromide.png reaction of a bromide ion with chloroethane showing the concerted nature of the reaction, the transition state and the predictable stereochemistry through Walden inversion.]]
In chemistry, a concerted reaction is a chemical reaction in which all bond breaking and bond making occurs in a single step. Reactive intermediates or other unstable high energy intermediates are not involved.Carey, Francis A.; Sundberg, Richard J.; (1984). Advanced Organic Chemistry Part A Structure and Mechanisms (2nd ed.). New York N.Y.: Plenum Press. {{ISBN|0-306-41198-9}}.{{Cite journal|url = http://goldbook.iupac.org/CT07011.html|title = IUPAC Gold Book - concerted reaction|doi = 10.1351/goldbook.CT07011|date = 24 February 2014|accessdate = 12 April 2014|publisher = IUPAC|doi-access = free}} Concerted reaction rates tend not to depend on solvent polarity ruling out large buildup of charge in the transition state. The reaction is said to progress through a concerted mechanism as all bonds are formed and broken in concert. Pericyclic reactions, the SN2 reaction, and some rearrangements - such as the Claisen rearrangement - are concerted reactions.
The rate of the SN2 reaction is second order overall due to the reaction being bimolecular (i.e. there are two molecular species involved in the rate-determining step). The reaction does not have any intermediate steps, only a transition state. This means that all the bond making and bond breaking takes place in a single step. In order for the reaction to occur both molecules must be situated correctly.