This page covers the important class of reactions, bimolecular nucleophilic substitutions. (Click here to see mechanism.) The purpose of this page is to point out how various parameters can effect the rate -- or even the success -- of these sorts of reactions. Additionally, comparisons will be made between this mode of reactivity and the related bimolecular elimination (E2) reactions and unimolecular nucleophilic substitution (SN1) reactions.
|polar aprotic solvent||second-order reaction||stereochemistry|
Nucleophilic substitution vs. elimination reactions
Generally speaking, there are two things that can happen when a nucleophile ("Nu:-") encounters an alkyl halide:
The mechanisms shown above illustrate two of the important modes of reactivity for alkyl halides:
If two molecules must come together in order for a reaction to take place, which is the implication of the diagrams above, then the rate at which that reaction occurs will depend on the concentrations of both of these species. If this reaction is the slowest step (i.e., the "rate-determining step") in a series of steps leading to an overall transformation, or if it is the only step in the reaction, then that reaction will exhibit "bimolecular" or "second-order" kinetics.
Both the SN2 and E2 reactions exhibit bimolecular kinetics. That is, these reactions have "rate laws" that show the direct dependence of the reaction rate on the concentrations of both the alkyl halide and the nucleophile:
Of course, this is "putting the cart before the horse" a little bit. When chemists first started studying these reactions, they didn't already know what the mechanisms were. By doing rate studies, and first finding that the rate was proportional to the concentration of both the nucleophile and the alkyl halide, chemists were then able to determine that the slow step in these reactions must involve an encounter between these two species. The reader may want to contrast this kinetic behavior with the "unimolecular" kinetics of the SN1 and E1 reactions, for which the rate depends only on the concentration of the alkyl halide, and not at all on the nucleophile. The different rate law for these reactions implies that they proceed by a different mechanism.
One of the most remarkable features of the SN2 reaction is that it proceeds with complete inversion of configuration at the carbon atom bearing the leaving group. This is illustrated below for the reaction between acetate ion and (S)-2-bromo-1-phenylpropane to give a product with the (R)-configuration at the chiral carbon:
Inversion of stereochemistry at asymmetric carbon atoms during an SN2 displacement almost surely implies a concerted (one-step) reaction mechanism, involving "back side" bond formation between the nucleophile and the alkyl halide, 180 degrees away from the halogen-to-carbon bond that is breaking concomitantly. In the transition state for this process, these bonds are half-formed and half-broken, respectively, the substitutent atoms lie in a plane perpendicular to the forming and breaking bonds, and are halfway through the process of having their configuration inverted:
Self-test question #1
Show the product of the following SN2 reaction, including the absolute stereochemistry at the chiral carbon. If you can, name the starting material and the product, including their stereochemical configurations ("R" or "S").
Factors influencing the rate of SN2 reactions
There are four factors that influence the rate of SN2 reactions:
Substrate (alkyl halide) structure
Self-test question #1
In the page on "Nomenclature," you learned about (or reviewed) the trivial names for twelve alkyl groups having five or fewer carbons. Consider only the bromides derived from each of those alkyl groups. Rank these compounds from "most reactive" to "least reactive" towards SN2 displacement reactions.
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Version 1.3.1, 2/21/97