Transition states methyl chloride

Hydroxide Methyl Transition state Methyl Chloride... 

Among the cases in which this type of kinetics have been observed are the addition of hydrogen chloride to 2-methyl-1-butene, 2-methyl-2-butene, 1-mefliylcyclopentene, and cyclohexene. The addition of hydrogen bromide to cyclopentene also follows a third-order rate expression. The transition state associated with the third-order rate expression involves proton transfer to the alkene from one hydrogen halide molecule and capture of the halide ion from the second ... 

The relative configuration of the diastcrcomers obtained on reaction of A-benzoyl a-methoxy-glycine methyl ester and various activated cyclohexenes is dependent on the cyclohexene substituent88. Whereas the boron trifiuoride catalyzed reaction with l-(4-morpholinyl)cyclohexene gives predominantly the awt/ -isomer, the. vrn-isomcr is predominantly formed in the titan-ium(IV) chloride catalyzed reaction with trimethylsilyloxycyclohexene. These results arc explained by a cyclic and an acyclic transition state, respectively. As expected, acetoxycyclohex-ene is less reactive. 

Levy (Chapter 6) has also explored the use of supercomputers to study detailed properties of biological macromolecule that are only Indirectly accessible to experiment, with particular emphasis on solvent effects and on the Interplay between computer simulations and experimental techniques such as NMR, X-ray structures, and vltratlonal spectra. The chapter by Jorgensen (Chapter 12) summarizes recent work on the kinetics of simple reactions In solutions. This kind of calculation provides examples of how simulations can address questions that are hard to address experimentally. For example Jorgensen s simulations predicted the existence of an Intermediate for the reaction of chloride Ion with methyl chloride In DMF which had not been anticipated experimentally, and they Indicate that the weaker solvation of the transition state as compared to reactants for this reaction In aqueous solution Is not due to a decrease In the number of hydrogen bonds, but rather due to a weakening of the hydrogen bonds. 

Fig. 20 Reaction pathways in the reduction of methyl (a) and /-butyl chloride (b) by NO". , reactant and products , transition states. In (a) and (b), the full line is the mass-weighted IRC path from the reactant to the product states the dashed line is a ridge separating the Sn2 and ET valleys and the dotted-dashed line is the mass-weighted IRC path from the Sn2 product state to the ET product state (homolytic dissociation). The dotted line in (a) represents the col separating the reactant and the SN2 product valleys. The dotted line in (b) represents the steepest descent path from the bifurcation point, B, to the Sn2 product. In (a), B is the point of the col separating the reactant and the SN2 product valleys where the ridge separating the SN2 and ET valleys starts.

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... 

Several ab initio quantum chemical studies have been carried out on much simpler systems that may nevertheless provide valuable qualitative trends.50 51 For example, the reaction of NO as a nucleophile with methyl ethyl isopropyl and tertiobutyl chlorides offers a thorough spectrum of passage between an S 2 and an ET situation as a function of steric hindrance.510 As shown in Figure 3.31, there are two transition states in all cases. The SN2 transition state is lower in free energy than the ET transition state with methyl chloride, and the situation progressively reverser when going to ethyl, isopropyl, and tertiobutyl chlorides. As expected, the... 

Fig. 2 The temperature dependence of the vibrational contributions to the secondary a-deuterium KIE for the SN2 reaction between chloride ion and methyl bromide by (a) the high energy C —H(D) stretching vibrations, (b) the Ca—H(D) bending vibrations and (c) the low-energy transition state vibrations. Modified, with permission, from Hu...

Wolfe and Kim s view of the origin of secondary a-deuterium KIEs has been challenged by two different groups. Barnes and Williams (1993) calculated the transition state structures and the secondary a-deuterium KIEs for the identity SN2 reactions between chloride ion and several substituted methyl chlorides (reaction (11)). 

The calculations were performed at the semiempirical level using AMI parametrization. The results for the methyl chloride reaction (Table 8) supported Williams earlier findings for the methylammonium ion-ammonia reaction (p. 147) and the results by Wolfe and Kim in that the inverse secondary a-deuterium KIE arose from an increase in the C —H stretching force constants which accompanied the change from sp3 hybridization at the a-carbon in the reactant to the spMike hybridization in the transition state. More important, however, were the observations that (i) the total KIE is dominated by the vibrational (ZPE) component of the KIE with which it correlates linearly, and (ii) that the inverse contribution from the C —H(D) stretching vibrations is almost constant for all the reactions. Ibis suggests that the contribution from the other vibrations, i.e. the rest in Table 8, determines the magnitude of the KIE. In fact, Barnes and Williams stated that the... 

Table 8 The AMI calculated semiclassical secondary a-deuterium KIEs, the stretching and other contribution to the KIEs and the C—Cl transition state bond lengths for the identity SN2 reactions between chloride ion and substituted methyl chlorides.0...

Poirier, Wang and Westaway (1994) also investigated the relationship between transition state structure and the magnitude of the secondary a-deuterium KIE in a theoretical study of the SN2 reactions between methyl and ethyl chlorides and fluorides with several different nucleophiles (reaction (12)). 

It used to be postulated that the magnitude of the alpha-carbon KIE in an Sn2 reaction is an indication of the transition state symmetry. The expectation was that this KIE reaches maximal value for a symmetric transition state and drops back to unity for extremely early or late transition states. That expected behavior is marked by the solid line in Fig. 10.10. However, recent calculations of the nucleophilic substitution of chloride from methyl chloride by a broad variety of nucleophiles... 

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