Editing SN2 reaction (section)

==Reaction kinetics==
The rate of an S<sub>N</sub>2 reaction is [[second order reaction|second order]], as the [[rate-determining step]] depends on the nucleophile concentration, <nowiki>[</nowiki>Nu<sup>−</sup><nowiki>]</nowiki> as well as the concentration of substrate, <nowiki>[RX]</nowiki>.<ref name="Clayden-2012" />

:[[Reaction rate|r]] = k<nowiki>[RX][</nowiki>Nu<sup>−</sup><nowiki>]</nowiki>

This is a key difference between the S<sub>N</sub>1 and S<sub>N</sub>2 mechanisms. In the S<sub>N</sub>1 reaction the nucleophile attacks after the rate-limiting step is over, whereas in S<sub>N</sub>2 the nucleophile forces off the leaving group in the limiting step. In other words, the rate of S<sub>N</sub>1 reactions depend only on the concentration of the substrate while the S<sub>N</sub>2 reaction rate depends on the concentration of both the substrate and nucleophile.<ref name = "Clayden-2012" />

It has been shown<ref>Absence of S<sub>N</sub>1 Involvement in the Solvolysis of Secondary Alkyl Compounds, T. J. Murphy, J. Chem. Educ.; 2009; 86(4) pp 519-24; (Article) doi: 10.1021/ed041p678</ref> that except in uncommon (but predictable cases) primary and secondary substrates go exclusively by the S<sub>N</sub>2 mechanism while tertiary substrates go via the S<sub>N</sub>1 reaction.  There are two factors which complicate determining the mechanism of nucleophilic substitution reactions at secondary carbons:
# Many reactions studied are solvolysis reactions where a solvent molecule (often an alcohol) is the nucleophile.  While still a second order reaction mechanistically, the reaction is kinetically first order as the concentration of the nucleophile–the solvent molecule, is effectively constant during the reaction. This type of reaction is often called a pseudo first order reaction.
# In reactions where the leaving group is also a good nucleophile (bromide for instance) the leaving group can perform an S<sub>N</sub>2 reaction on a substrate molecule.  If the substrate is chiral, this inverts the configuration of the substrate before solvolysis, leading to a racemized product–the product that would be expected from an S<sub>N</sub>1 mechanism. In the case of a bromide leaving group in alcoholic solvent Cowdrey et al.<ref>{{cite journal|title=Relation of Steric orientation to Mechanism in Substitution Involving Halogen Atoms and Simple or Substituted Hydroxyl Groups|author1=W.A. Cowdrey|author2=E.D. Hughes|author3=C.K. Ingold|author4=S. Masterman|author5=A.D. Scott|journal=J. Chem. Soc.|year=1937|pages=1252–1271|doi=10.1039/JR9370001252}}</ref> have shown that bromide can have an S<sub>N</sub>2 rate constant 100-250 times higher than the rate constant for ethanol.  Thus, after only a few percent solvolysis of an enantiospecific substrate, it becomes racemic.

The examples in textbooks of secondary substrates going by the S<sub>N</sub>1 mechanism invariably involve the use of bromide (or other good nucleophile) as the leaving group have confused the understanding of alkyl nucleophilic substitution reactions at secondary carbons for 80 years<sup>[3]</sup>. Work with the 2-adamantyl system (S<sub>N</sub>2 not possible) by Schleyer and co-workers,<ref>The 2-Adamantyl System, a Standard for Limiting Solvolysis in a Secondary Substrate J. L. Fry, C. J. Lancelot, L. K. M. Lam, J. M Harris, R. C. Bingham, D. J. Raber, R. E. Hill, P. v. R. Schleyer, J. Am. Chem. Soc.; 1970; 92, pp 1240-42 (Article); doi: 10.1021/ja00478a031</ref> the use of azide (an excellent nucleophile but very poor leaving group) by Weiner and Sneen,<ref>A Clarification of the Mechanism of Solvolysis of 2-Octyl Sulfonates. Stereochemical Considerations; H. Weiner, R. A. Sneen, J. Am. Chem. Soc.; 1965; 87 pp 287-91; (Article) doi: 10.1021/ja01080a026</ref><ref>A Clarification of the Mechanism of Solvolysis of 2-Octyl Sulfonates. Kinetic Considerations; H. Weiner, R. A. Sneen, J. Am. Chem. Soc.; 1965; 87 pp 292-96; (Article) doi: 10.1021/ja01080a027</ref> the development of sulfonate leaving groups (non-nucleophilic good leaving groups), and the demonstration of significant experimental problems in the initial claim of an S<sub>N</sub>1 mechanism in the solvolysis of optically active 2-bromooctane by Hughes et al.<ref>Homogeneous Hydrolysis and Alcoholysis of β-n-Octyl halides, E. D. Hughes, C. K. Ingold, S. Masterman, J. Chem. Soc.; 1937; pp 1196–1201; (Article) doi: 10.1039/JR9370001196</ref><sup>[3]</sup> have demonstrated conclusively that secondary substrates go exclusively (except in unusual but predictable cases) by the S<sub>N</sub>2 mechanism.