The Gas Phase SN2 Reaction

Figure 13-3. Reaction path for the gas-phase SN2 reaction of Cl + CH3C1.

Deng, L., V. Branchadell, and T. Ziegler. 1994. Potential Energy Surface of the Gas-Phase Sn2 Reactions X + CH2X = XCH3 + X- (X = F, Cl, Br, I) A Comparative Study by Density Functional Theory and ab initio Methods. 

Hu, W.-P. and Truhlar, D. G. Modeling transition state solvation at the single-molecule level test of correlated ab initio predictions against experiment for the gas-phase SN2 reaction of microhydrated fluoride with methyl chloride, J.Am.Chem.Soc., 116 (1994), 7797-7800... 

Nucleophilicity and leaving group ability 211 Effect of solvation on the gas-phase reaction 212 Mechanism of the gas-phase SN2 reaction 213 Potential energy surfaces for gas-phase SN2 reactions 214 Recent theoretical developments 218 Some examples of gas-phase SN2 reactions involving positive ions 220 Nucleophilic displacement reactions by negative ions in carbonyl systems 222 General features 222... 

An elegant and clever experiment along these lines (Lieder and Brauman, 1974) confirms that the gas phase SN2 reaction proceeds by a predominant backside attack. Analysis of neutral products in the icr experiment shows that for reaction (27), backside attack, and consequently inversion at the reaction centre, occurs to the extent of 92 + 6%. For reaction (28) the same type of experiment indicates that inversion of configuration amounts to 87 + 7% of the reaction. 

Mechanistically, the gas-phase SN2 reaction is described by an energy profile of the type represented in Figure 1, characterized by a double-potential well separated by a central energy barrier which corresponds to a transition state in which the central carbon is pen-tacoordinated. The results of many theoretical and experimental investigations support such an energy profile (cf later in this section for recent evidence of an alternative mechanism). A representative example of the many theoretical investigations on the S 2 reaction... 

In the absence of solvent, the gas-phase SN2 reaction is different, but it still takes place with inversion of stereochemistry. There is a double well in the energy surface the nucleophile and the alkyl halide combine exothermically with no energy barrier to give an ion-molecule complex. In a sense the naked nucleophile is solvated by the only solvent available, the alkyl halide. The Sn2 reaction then takes place with a low barrier, and the product ion-molecule complex dissociates endothermically to give the products. 

The first su estion of the double niinimum potential well energy model for the gas-phase Sn2 reaction resulted from Olmstead and Brauman s work on the reaction of chloride ion with methyl chloride (Eqn (1.14)). °°... 

Fig. 5.2. Potential energy profile for the gas-phase Sn2 reactions at the tetrahedral carbon atom. The...

More complete lists of calculated energies of the gas-phase Sn2 reactions of Y -h CH3X general type are available in Refs. [30,37] (the >i-INDO method) and [26,34] (4-3IG). The data obtained make it possible to check how the well-known relationships among the structural, kinetic and thermodynamic characteristics, such as the Bell-Evans-Polanyi principle, the Hammond postulate, etc., are obeyed in the case of the reactions which proceed without a solvent. 

Several workers " have carried out ab initio molecular-orbital studies of the gas-phase Sn2 reaction... 

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