Methyl chloride reactions

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... 

Fig. 4 The total secondary a-deuterium KIE versus the bending vibration contribution to the KIE for the SN2 reactions of methyl fluorides and chlorides with different nucleophiles at 25°C. The open circles are for the methyl chloride reactions and the solid circles are for the methyl fluoride reactions. Data from Poirier et al. (1994), with...

The transition structures for the hydrolysis reactions of methyl, /-butyl, and ada-mantyl chlorides in the gas phase and in water were calculated using the B3LYP/6-31-G(d) level of theory and the PCM solvation model.82 In the gas phase, backside attack is strongly favoured for the methyl chloride reaction and slightly favoured for the t -butyl chloride reaction. Frontside attack is favoured for the adamantyl chloride... 

Calculations have been carried out on the. S n2 reactions between chloride ion and methyl chloride or chlorosilane at several DFT levels of theory up to the OLYP/TZ2P level.102 The OLYP/TZ2P method gave much better (excellent compared with the CCDD(T)/aug-cc-pVQZ level) values than the other density functionals for both the geometry and the energies of the reactant complex and transition state for the methyl chloride reaction and the stable transition complex that forms in the chlorosilane reaction. 

Additional catalysts for the methyl chloride reaction are described in patents. For example, aluminum metal 271>, ammonia-water and ammonia-alcohol H4,ii5)> amines 37>116>, aluminum hydrides 313>, and various ethers 36,38-41,132,188,190) are discussed. 

The formation of the expected 2,4,6-trisilaalkanes 2 can be explained by the 2 1 reaction of (chloromethyl)silanes 1 and elemental silicon. The byproducts, 1,3-disilaalkanes 3, were obtained from the reaction of (chloromethyl)silane reactant 1 and hydrogen chloride (or chlorine) with the same silicon atom, as methyldichlorosilane (or methyltrichlorosilane) was obtained from the methyl chloride reaction. This result indicates that some of the (chloromethyl)silane reactant decomposed under the reaction conditions and acted as a chlorine or hydrogen chloride source. In the direct reaction of (chloromethyl)dimethylchlorosilane (lc), a significant amount of starting material was recovered at temperatures below 300 °C, but the reaction went to completion above 320 °C. 2,6-Dimethyl-2,4,4,6-tetra-chloro-2,4,6-trisilaheptane (2c) was obtained in much higher yields than 3-methyl-l,l,l,3-tetrachloro-l,3-disilabutane (3c) at reaction temperatures ranging from 280 to 320 °C, but the ratio of both compounds reversed above 340 °C. 

Dialkylchloronium ions (Scheme 7) The existence of halogen substituted cations (34) is well documented in the literature as is the existence of dialkylhaIonium ions (35). Support for this possibility in our studies comes from low temperature (-35 C) 60 MHz pmr spectra of the acid layer of the methyl chloride reaction in HF-TaF5 which shows a sharp... 

The results in Table 5 show the (ArH/ D)sireich is effectively constant for each leaving group i.e., it varies from 0.66 to 0.69 (by only 0.03) when the nucleophile changes from F to SH in the methyl fluoride reactions and from 0.68 to 0.72 (by only 0.04) when the nucleophile changes from F to SH in the methyl chloride reactions. (/cn/A r ))bend, on the other hand, changes from 0.98 to 1.22, i.e., by 0.24 in... 

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