Potential energy diagrams with hydroxide

Potential energy diagram for the reaction of methyl bromide with hydroxide ion by the S s 2 mechanism. 

Figure 7-2 Potential-energy diagrams for (/ Sn2 reaction of chioromethane with hydroxide and (B) Sn1 hydroiysis of 2-bromo-2-methyipropane. Whereas the Sn2 process takes piace in a singie step, the SnI mechanism consists of three distinct events rate-determining dissociation of the haioalkane into a halide ion and a carbocation, nucieophiiic attack by water on the carbocation to give an alkyloxonium ion, and proton ioss to furnish the finai product. Note For ciarity, inorganic species have been omitted from the intermediate stages of (B).

Sodium hydroxide has been the most commonly used base in experimental nitroalkane proton transfer reaction studies.However, the computational studies of these reactions have generaUy been with hydroxide ion without the sodium counter ion. Recently a computational study of the proton transfer reactions of the three simple nitroalkanes in the presence of NaOH in water has been carried out and it was found that the presence of Na had an enormous effect on the energetics of the reactions. Double potential energy well diagrams, much like those found for the Sn2 reactions, were recorded for the proton transfer reactions of NM, NE and 2-NP with hydroxide ion in water. The computations included two explicit water molecules in the water cavity. The Gibbs free energies and enthalpies observed for the reactant complex (CPI), the TS and the product complex (CP2) both in the presence and absence of sodium ion and two explicit water molecules are summarized in Table 1.24. 

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