Concerted reaction

S_N2 reaction of a bromide ion with chloroethane showing the concerted nature of the reaction, the transition state and the predictable stereochemistry through Walden inversion.

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.^[1] ^[2] 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 S_N2 reaction, and some rearrangements - such as the Claisen rearrangement - are concerted reactions.

The rate of the S_N2 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.

References

^ 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.
^ "IUPAC Gold Book - concerted reaction". IUPAC. 24 February 2014. doi:10.1351/goldbook.CT07011. Retrieved 12 April 2014.

[1] 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.

[2] "IUPAC Gold Book - concerted reaction". IUPAC. 24 February 2014. doi:10.1351/goldbook.CT07011. Retrieved 12 April 2014.

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