Transition states methyl halides

Rate IS governed by stability of car bocation that is formed in loniza tion step Tertiary alkyl halides can react only by the SnI mechanism they never react by the Sn2 mecha nism (Section 8 9) Rate IS governed by steric effects (crowding in transition state) Methyl and primary alkyl halides can react only by the Sn2 mecha nism they never react by the SnI mechanism (Section 8 6)... 

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 ... 

Hydrolyses of alkyl halides and arenesulfonates have long been known to be micelle-inhibited (Gani et al., 1973 Lapinte and Viout, 1973, 1979) but now k+/k < 1, except for hydrolysis of methyl benzenesulfonate which involves extensive bond making in the transition state (Al-Lohedan et al., 1982b Bunton and Ljunggren, 1984). Thus values of k+/k < 1 seem to be characteristic of hydrolyses in the SN1-SN2 mechanistic spectrum which involve considerable bond breaking in the transition state, and k+/k is very low for hydrolyses of diphenylmethyl halides where the transition state has considerable carbocation character (Table 8). 

It is reasonable to relate the values of k+fk at least qualitatively to the extents of bond making and breaking in the transition state. Bond making is all important in hydrolyses of carboxylic anhydrides, diaryl carbonates and methyl arenesulfonates. Bond breaking will be important in hydrolyses of alkyl halides and sulfonates, except for methyl derivatives, and especially so in water which can effectively solvate the leaving anion. 

It is worthwhile to emphasize that the intermediate Hamiltonian Hc(ij) defines a geometry that can be used to construct a model of an activated complex. A portrait of it can be obtained at the BO level of theory. For thermally activativated processes, the transition state is the analogous of the intermediate Hamiltonian, while for processes without thermal activation (a number of reactions taking place in gas phase, such as for example, the SN2 reaction between methyl halides and halides ions [168-171] ) the quantum states of this Hamiltonian mediate the chemical interconversion. For particular... 

The most recent contribution to this controversy is Glad and Jensen s (1997) theoretical investigation at the MP2/6-31 + + G(d, p) ab initio level of the secondary a-deuterium KIEs for the identity SN2 reactions of three methyl halides (reaction (15)). The KIEs were calculated for various x where AR%-x = (Rc-x Rc-x) and Re x and R°cx represent the length of the C—X bond in the transition state and the reactant, respectively. 

Fig. 10 The secondary a-deuterium KIE for the identity SN2 reactions between halide ions and methyl halides versus the elongation of the C—X bond on going from the reactant to the transition state. Data from Glad and Jensen (1997), modified, with...

Swain, C.G. and Thornton, E.R. Initial-state and transition-state isotope effects of methyl halides in light and heavy water, J. Am. Chem. Soc., 84(5) 822-826, 1962. 

The authors assume that the halides react with stereoinversion, whereas the tosylates prefer suprafacial attack due to binding interaction with the lithium ion in the transition state . A comparable dependence of the stereochemical course from the leaving group has been observed in other stereodefined benzyllithiums, too . The addition of 268 to A-methyl-benzylideneimine proceeds with only low yield . ... 

Swain and Scott56 have measured the knC lkHF ratio in neutral or slightly acidic solutions for the hydrolysis of acetyl and benzoyl halides and compared them with those for triphenylmethyl halides. Values for the triphenyl-methyl halides are — 104 greater than those for acyl halides (Table 10) reflecting the tendency for C-X bond breaking to be more complete than O-C formation at the transition state of trityl hydrolysis and the opposite tendency with the benzoyl halides. The C-F bond is harder to break but the carbon atom is made more electropositive. 

The reaction between ammonia and methyl halides has been studied by using ab initio quantum-chemical methods.90 An examination of the stationary points in the reaction potential surface leads to a possible new interpretation of the detailed mechanism of this reaction in different media, hr the gas phase, the product is predicted to be a strongly hydrogen-bonded complex of alkylammonium and halide ions, in contrast to the observed formation of the free ions from reaction hr a polar solvent. Another research group has also studied the reaction between ammonia and methyl chloride.91 A quantitative analysis was made of the changes induced on the potential-energy surface by solvation and static uniform electric fields, with the help of different indexes. The indexes reveal that external perturbations yield transition states which are both electronically and structurally advanced as compared to the transition state in the gas phase. 

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