Transition states methyl chloride/nucleophile

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... 

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... 

The first four compounds have lower j values than methyl bromide because each is quite reactive in itself and therefore is not very dependent on help from the nucleophile /-toluenesulfonate is a good leaving group and does not need much assistance to begin to depart ring strain in propiolactone and in the mustard cation make a ring-opening Sv2 reaction very favorable and the transition state of benzyl chloride is stabilized by resonance and therefore is easily reached. 

The cy-carbon k12/kn KIEs calculated for the. S N2 reactions between methyl chloride and 22 different nucleophiles at the BILYP/aug-cc-pVDZ level of theory64 showed that all the KIEs were near the theoretical maximum and did not change significantly with transition state structure. A review showed that the a-carbon KIEs found experimentally for. S n2 reactions were also all near the theoretical maximum KIE. This suggests that the Melander-Westheimer curve,65,66 relating the magnitude of these... 

For the reactivity parameters Y, n, a+ (but not a) andN+ the lack of curvature is not unexpected. This is because these parameters are defined with respect to the rate of some standard reaction (solvolysis of t-butyl chloride, substitution of methyl iodide, solvolysis of cumyl chlorides, combination reaction of nucleophiles with a standard electrophile). Therefore the resultant plot is of the type log k vs. log k, while the curvature shown in a typical Br nsted plot (Figure 5) results from a plot of log k vs. log K. This curvature is due to a gradual change from a reactant-like transition state, which is insensitive to a perturbation in the reactivity parameter, to a product-like transition state in which equilibrium perturbations are largely reflected in the transition state (and hence the rate). A log k — log k plot is not expected to show this effect and hence is not expected to show curvature. 

Fig. 1 The dependence of the chlorine KIE on the Wiberg Ca-Cl transition state bond order for the Sn2 reactions of methyl chloride with 26 different nucleophiles. The dashed line is the KIE estimated using the BEBOYIB approach. Reprinted with permission from Ref. [11]. Copyright (2004) American Chemical Society.

Table 3 The transition state bond orders for the Ca-Nu and the Ca-Cl bonds and the a-carbon KIEs for the SN2 reactions between methyl chloride and 22 different nucleophiles at 298°C...

These problems led Poirier et al.41 to re-investigate the relationship between the magnitude of the secondary a-deuterium KIE and transition state structure in SN2 reactions. The optimized geometries for the reactants and the transition structures for the Sn2 reactions where the nucleophile was F, Cl, OH, SH, NH2 or SCH3 and the substrate was either methyl fluoride or chloride were calculated at the HF/ 6-31+G level. Then, Sims BEBOVIB-IV program was used to carry out a... 

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