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Nucleophilic Substitution SN1-SN2

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The nucleophilic substitution SN1/SN2 typically occur in a competitive regime. There are various conditions that define the predominant reaction mechanism taking place. Since SN1 leads to the racemic mixture, SN2 is more popular in asymmetric organic synthesis. So, detailed computational studies of model SN2 reactions have been carried out during the last three decades[2-6, 9]. The influence of solvation of the nucleophile with several common solvents on the rate constant of the reactions F-(Sn) + CH3Cl → CH3F + Cl-(Sn) where S is a solvent molecule and n=0-3, was studied experimentally (flowing plasma mass spectroscopy) by Bohm and Raksit[2] . The results of their work are summarized in Table 1:

Table 1. Rate constants measured for reactions of solvated fluoride ions at room temperature in the gas-phase. Values of kr are given in units of 10-9 cm3mol-1s-1.
F-Sn
kr at different n

0
1
2
3
F-(D2O)n
1.9
0.015
0.0003
0.003
F-(CH3OH)n
1.9
0.0006
0.0003
0.0003
F-(CH3CH2OH)n
1.9
0.0003
0.0003
-

It is clear that the solvation slowers the reaction at least 100 times. This work suggests the existence of higher barriers on the potential energy surface for the solvated nucleophile. Morokuma[3], using HF/3-21G level of theory, showed that the solvation in protic polar solvents (such as water or alcohols) increases the activation energy accordingly to the number of solvent molecules, which form hydrogen bonds with the nucleophile. (see Figure 1)
Doi et al.[4] studied
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