Identity SN2 reactions

Table 13-4. Complexation energy (AEC) and barrier heights (Al .oh and Al .h. see text) for the gas phase bimolecular Sn2 identity reaction CE + CH3C1 —> C1CH3 + Cl" [kcal/mol]. HF and DFT calculations were done with the 6-311+G(d,p) basis set and include zero-point vibrational contributions. 

Sn2 identity reactions. In ref. 89 SC theory was applied to study the electronic rearrangements taking place during gas-phase Sn2 identity reactions... 

Other measures of nucleophilicity have been proposed. Brauman et al. studied Sn2 reactions in the gas phase and applied Marcus theory to obtain the intrinsic barriers of identity reactions. These quantities were interpreted as intrinsic nucleo-philicities. Streitwieser has shown that the reactivity of anionic nucleophiles toward methyl iodide in dimethylformamide (DMF) is correlated with the overall heat of reaction in the gas phase he concludes that bond strength and electron affinity are the important factors controlling nucleophilicity. The dominant role of the solvent in controlling nucleophilicity was shown by Parker, who found solvent effects on nucleophilic reactivity of many orders of magnitude. For example, most anions are more nucleophilic in DMF than in methanol by factors as large as 10, because they are less effectively shielded by solvation in the aprotic solvent. Liotta et al. have measured rates of substitution by anionic nucleophiles in acetonitrile solution containing a crown ether, which forms an inclusion complex with the cation (K ) of the nucleophile. These rates correlate with gas phase rates of the same nucleophiles, which, in this crown ether-acetonitrile system, are considered to be naked anions. The solvation of anionic nucleophiles is treated in Section 8.3. 

A model has been considered for Sn2 reactions, based on two interacting states. Relevant bond energies, standard electrode potentials, solvent contribntions (nonequi-librinm polarization), and steric effects are included. Applications of the theory are made to the cross-relation between rate constants of cross- and identity reactions, experimental entropies and energies of activation, the relative rates of Sn2 and ET reactions, and the possible expediting of an outer sphere ET reaction by an incipient SN2-type interaction (Marcus, 1997). 

While ot,3-unsaturated aldehydes react exclusively with triotganoaluminums in the 1,2-mode, the identical reaction of the corresponding chiral a,3-unsaturated acetals (5), obtained from (RJi)-(+)-N V V V -tetramethyltartaric acid amides, is solvent dependent." For example, in 1,2-dichloroethane, preferential y-addition (Sn2 ) occurs (5 giving 7 Scheme 2), while in dichloromethane, preferential a-addition occurs (5 giving 8 Scheme 2). Subsequent hydrolysis of (7) affords the -substituted aldehydes in high enantiomeric purity (Scheme 2) thus the acetals (5) serve as effective surrogates for —CH=CH—CHO (+CH2CH2CHO). 

Fig. 8.2 Profile for the SN2 reaction Cl- + CH3C1 in water. Calculations by the author using B3LYP/6-31+G with the continuum solvent method SM8 [22] as implemented in Spartan [31]. Note that r is the distance of the CP from the transition state bond length (2.426 A), not the CP/C distance thus r measures the deviation from the transition state and becomes zero at the transition state. This makes the graph symmetrical about the energy axis, as it should be presented for this identity reaction. The zero of energy is taken as rc cl = 25 A, r = 22.574 A...

The B3LYP/6-31+G(d,p) level of theory has been used to calculate the rotamer populations, the energy barriers, and the reaction path curvature for the SN2 identity... 

The SN2 identity exchange reaction of chloride with chloroacetonitrile (equation 23) was studied in an FT-ICR spectrometer and also theoretically by means of statistical theories (RRKM with the microcanonical variational transition state) and high-level ab initio calculations238. 

Briefly explain why the Sn2 reaction of l-bromo-2-cyclohexylethane with KI has a relatively short half-life in THF, but the identical reaction with l-bromo-2-cyclohexyl-2-methylpropane has a particularly long half-life. 

The Sn2 reaction is one of the fundamental aspects in chemical education, and a most vivid research area in experimental and theoretical gas phase [41, 42], solution [43], and reaction dynamics studies [44]. Our main purpose in this section is to demonstrate the use of the two-curve diagram as a means to conceptualize the barrier and the structure of the transition state. As such, the discussion is limited to the class of identity reactions in the gas phase. The interested reader is directed to recent reviews and a monograph which give detailed accounts of 5 2 reactivity in the gas phase and in solvents for identity and nonidentity reactions as well [4b 5 a, b]. 

McMahon and coworkers considered the study of reaction (Eqn (1.14)) with isotopically labeled reactants, Cl and CH Cl, which would have given very valuable information, but were unable to do so because the cost of reagents was prohibitive. They did carry out an excellent high-pressure mass spectrometric study that provided thermochemical data for reaction (Eqn (1.14)) and related Sn2 reactions. For identity reactions they found that an increase in the well depth increases with increasing size... 

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