Nucleophilic substitution Sn2 reactions

Alternatively, the Sn2 nucleophilic substitution reaction between alcohols (phenols) and organic halides under basic conditions is the classical Williamson ether synthesis. Recently, it was found that water-soluble calix[n]arenes (n = 4, 6, 8) containing trimethylammonium groups on the upper rim (e.g., calix[4]arene 5.2) were inverse phase-transfer catalysts for alkylation of alcohols and phenols with alkyl halides in aqueous NaOH solution to give the corresponding alkylated products in good-to-high yields.56... 

Gas-phase SN2 nucleophilic substitution reactions are particularly interesting because they have attributes of both bimolecular and unimolecular reactions.1 As discovered from experimental studies by Brauman and coworkers2 and electronic structure theory calculations,3 potential energy surfaces for gas-phase SN2 reactions of the type,... 

Classical trajectory calculations, performed on the PES1 and PESl(Br) potential energy surfaces described above, have provided a detailed picture of the microscopic dynamics of the Cl- + CH3Clb and Cl" + CH3Br SN2 nucleophilic substitution reactions.6,8,35-38 In the sections below, different aspects of these trajectory studies and their relation to experimental results and statistical theories are reviewed. 

Additional experimental, theoretical, and computational work is needed to acquire a complete understanding of the microscopic dynamics of gas-phase SN2 nucleophilic substitution reactions. Experimental measurements of the SN2 reaction rate versus excitation of specific vibrational modes of RY (equation 1) are needed, as are experimental studies of the dissociation and isomerization rates of the X--RY complex versus specific excitations of the complex s intermolecular and intramolecular modes. Experimental studies that probe the molecular dynamics of the [X-. r - Y]- central barrier region would also be extremely useful. 

DYNAMICS OF GAS-PHASE Sn2 NUCLEOPHILIC SUBSTITUTION REACTIONS William L. Hase, Haobin Wang, and... 

The fundamental reaction types suitable for metal-mediated syntheses of allenes (Scheme 2.1) comprise SN2 nucleophilic substitution reactions of propargylic elec-... 

In a second-order (SN2)nucleophilic substitution reaction, the carbon atom simultaneously experiences the effects of the attacking nucleophile (N) and leaving group or exiphile (E). These mutual effects (shown in Fig. 3) serve to diminish energy diferences between each isotopic substrate and its corresponding transition state. 

It should be noted from the point of view of synthesis that the building up of tert-butyl perester groups in polymethacrylyl chloride proceeds with great difficulties, on account of the high steric hindrance which impedes the SN2 nucleophilic substitution reaction. 

Another important example of hypervalent bonding is the transition state in the Sn2 nucleophilic substitution reaction, which has the same orbital configuration and electron count as a 10-electron hypervalent system. For nucleophilic substitution, the hypervalent species is a transition state rather than a stable species, as shown schematically in Figure 5. Experimental and computational studies on transition states are generally difficult. Stable hypervalent systems can serve as more tractable benchmarks to test the accuracy of computational techniques used on 10-electron systems. 

How does this reaction take place Although it appears superficially similar to the SN1 and SN2 nucleophilic substitution reactions of alkyl halides discussed in Chapter 11, it must be different because aryl halides are inert to both SN1 and SN2 conditions. Aryl halides don t undergo SN-1 reactions because aryl cations are relatively unstable. The dissociation of an aryl halide is energetically unfavorable and does not occur easily. 

Both experiments and simulations have shown that the chemical dynamics of gas-phase X + CHjY XCH + Y Sn2 nucleophilic substitution reactions are non-statistical. Reactions, snch as C/ + CH Br CICH3 + Br, have X — CH3Y and XCHj — Y ion-dipole complexes separated by a central barrier (Figure 20.5) and the unimolecular dynamics of these complexes are intrinsically non-RRKM. These dynamics arise in part from weak couplings between the three low freqnency intermolecular modes of the complex and the complex s much higher frequency nine intramolecular modes. 

Trajectories are being used in a series of studies to investigate the dynamics of gas-phase Sn2 nucleophilic substitution reactions ... 

Karplus employed this level of theory to study the photoisomerization of 2-butene. The first ab initio direct dynamics study is that by Leforestier for the + CH4 CH4 + Sn2 nucleophilic substitution reaction. 

The alkyl ammonium used to render the clay organophilic is known to decompose with the Hofmann elimination or with an SN2 nucleophilic substitution reaction at a temperature as low as 155°C." As result a protonic site is created on the clay surface (Reaction 10.1). 

---