Comments on the putative ion pair mechanism for hydrolysis of methyl halides and methyl perchlorate

SN1‐SN2 and SN2‐SN3 mechanistic changes revealed by transition states of the hydrolyses of benzyl chlorides and benzenesulfonyl chlorides

“…The distance of the newly formed O(5)⋯C(1) one is 2.009 Å < 2.014 Å >, and those of O(5)H(7) and O(5)H(8) are 0.995 Å < 0.995 Å >. Calculated activation energies and entropy are in good agreement with experimental data . Although TS is of molecularity = 2 (i.e., S N 2), the elongated O(5)H(7) bond (0.995 Å) of the attacking OH 2 molecule is incipient for the O(5)‐H(7)‐O(12) intermediate state between covalent and hydrogen bonds.…”

Solvent effects on the intramolecular conversion of trimethylsulfonium chloride to dimethyl sulfide and methyl chloride

“…Tabulated thermodynamic data 58,61 can be used to estimate the change in Gibbs free energy D 19 G + D 20 G for the hydrolysis of CH 3 Cl to be À12 kJ mol À1 which compares favourably with À9 kJ mol À1 obtained from our PCM calculations using a thermodynamic cycle similar to that shown in eqn (16) and (17). Tabulated thermodynamic data 58,61 can be used to estimate the change in Gibbs free energy D 19 G + D 20 G for the hydrolysis of CH 3 Cl to be À12 kJ mol À1 which compares favourably with À9 kJ mol À1 obtained from our PCM calculations using a thermodynamic cycle similar to that shown in eqn (16) and (17).…”

Determining transition state structure using kinetic isotope effects

“…This explanation for these unexpectedly large KIEs is reasonable because the fairly small Hammett 47 It has even been suggested that the lone pairs of electrons on the oxygens of perchlorate-and arenesulfonate-leaving groups affect the C a -H(D) bending vibrations in the ground state and are responsible for the larger adeuterium KIEs found in these S N 2 reactions. 48 Although we do not know how to interpret secondary a-deuterium KIEs for S N 2 reactions with different leaving groups, overwhelming evidence from theoretical calculations has shown that the KIE is primarily determined by contributions from an inverse C a -H(D) stretching vibration, which is effectively constant for each leaving group, and an inverse, or normal, C a -H(D) bending vibration contribution that increases with transition state looseness (the steric crowding around C a in the transition state). The stretching contribution to the total KIE is usually greater than the bending contribution for the S N 2 reactions of methyl substrates; hence, the KIEs for these S N 2 reactions are inverse or small and normal.…”