Nucleophilic Substitution (SN1) Reactions
This Demonstration describes the progress of a generic alkyl halides nucleophilic substitution reaction, also designated as SN1. This reaction occurs in two stages: the first stage involves the formation of a carbocation (this process can be stabilized with benzylic, allylic, tertiary and secondary halides thanks to inductive and resonance effects ). Select "carbocation formation" to see how the concentration of the chloride ion (green sphere) affects the reaction kinetics. In general, the higher the concentration, the lower the reaction rate. It is also possible to see the allylic carbon changing its hybridization from to .[more]
The molecule chosen is an allylic halide (R)-3-Chloro-1-butene, where halogen Cl, is represented with the green sphere bonded with the secondary (allylic) C, represented as a black sphere, also bonded with a: (1) hydrogen (gray sphere); (2) methyl group: (blue sphere); and (3) vinyl group: (dark yellow sphere). Note the inhibition of C-Cl bond-breaking by increased concentration of the chloride ion, . The concentration can be hidden with the "hide/show halide" button.
Select "nucleophilic attack" to see the second stage of the process; a generic nucleophile (lighter purple sphere) appears, which will bind to the carbocation to produce the substituted product.
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Since attack from above or below is equally probable, the two enantiomers will be formed in equal quantities, resulting in a racemic mixture.
Select "reaction coordinate" to observe the reaction coordinates as a function of potential energy. It is possible to distinguish the two stages of the reaction: formation of carbocation (bottleneck stage) and the subsequent (faster) formation of the bond with the nucleophile. The full course of the reaction can be followed with the "time" slider.
By increasing the concentration of the halide with "[Cl-]", the mass effect  first inhibits the formation of carbocation, later it reverses the chemical equilibrium. To display this effect, the graph was repeated twice, so in this way three cases can happen:
1. "low ion concentration" → The molecules of alkyl halide react in their entirety exceeding the two activation energies and obtaining the desired product (the replaced halide).
2. "intermediate ion concentration → Even if the molecules form the carbocation, the chemical equilibrium of the first stage recedes for nearly half of the molecules, regaining the alkyl halide.
3. "high ion concentration" → The chemical equilibrium of the first stage is shifted to the left for all molecules.
Select "reaction rate" to show the slope of the nucleophilic substitution reaction rate as a function of the alkyl halide (RX), nucleophile (Nu), halide () concentrations:
rate of formation of RNu =
= kinetic constant of the first stage of the forward reaction
= kinetic constant relative of the first stage of the reverse reaction
= kinetic constant relative to the second stage of the forward reaction
[RCl] = molarity of alkyl halide
 = molarity of the ion
= molarity of the nucleophile
If the concentration of the ion was particularly high compared to that of the nucleophile ("high ion concentration"), the following condition applies:
The reaction rate would take the following simplified form:
rate of formation of RNu = .
If, on the other hand, the concentration of was lower than that of the nucleophile ("low ion concentration"), this would be the simplified form of the reaction rate:
rate of formation of RNu = .
In both cases, there is a linear dependence. In the first case, the slope decreases as  increases; in the second case the slope remains the same as the concentration varies.[less]
Snapshot 1: C-Cl bond breaking and simultaneous carbocation formation in the first stage of the reaction
Snapshot 2: breaking of the C-Cl bond inhibited by the high concentration of the ion
Snapshot 3: nucleophile attack to the carbocation and racemization of the solution
Snapshot 4: some of the molecules after the first stage of the reaction (bottleneck stage) reach the second stage, but a high concentration of ions causes the equilibrium to reverse
Snapshot 5: slope of the reaction rate depending only on the [R-Cl]
 H. Hart, L. E. Craine and D. J. Hart, Organic Chemistry: A Short Course, 10th ed., Boston: Houghton Mifflin, Co., 1999.
 D. A. McQuarrie and J. D. Simon, Chimica Fisica: Un approccio molecolare, Bologna: Zanichelli, 2000.