A DFT study of transition structures and reactivity in solvolyses of tert‐butyl chloride, cumyl chlorides, and benzyl chlorides

Abstract

A DFT study of transition structures and reactivity in solvolyses of tert-butyl chloride, cumyl chlorides, and benzyl chlorides Ferenc Ruff a * and Ö dö n Farkas a DFT computations were performed on the S N 1 and S N 2 solvolyses of substituted cumyl chlorides and benzyl chlorides in ethanol and water, by increasing stepwise the C-Cl distance and by optimization. The total energy increases with the increase in the Cl-C distance in S N 1 reactions, while free energy of activation pass through maximum. To validate the results, the calculated free energies of activation were compared with data obtained by kinetic measurements. The structural parameters of the transition states were correlated with the Hammett substituent constants and compared with the data of hydrolyses of tert-butyl chloride and methyl chloride, which proceed with known mechanisms. Conclusions on the mechanisms of the reactions were driven from the effect of substituents on free energies of activation. Cumyl chlorides substituted with electron-donating (e-d) groups solvolyze with S N 1 mechanism, while the reactions of substrates that bear electron-withdrawing groups proceed with weak nucleophilic assistance of the solvent. Benzyl chlorides hydrolyze through an S N 2 pathway except those derivatives that have strongly e-d groups, where the reaction has S N 1 character, but a weak nucleophilic assistance of the water should also be taken into consideration.

Concerted SN2 mechanism for the hydrolysis of acid chlorides: comparisons of reactivities calculated by the density functional theory with experimental data

“…Summing up the results, obtained in this and in our earlier papers35–39 concerning the structure of the TSs in S N 2 nucleophilic substitutions, we can conclude that early TSs are formed if the attacking reagent is better, and late TSs are formed if the attacking reagent is poorer nucleophile than the leaving group. On the other hand, loose TSs are formed if e‐d, and tight TSs are formed if e‐w substituents are bonded to the substrates.…”

“…The conclusions, drawn for the mechanism of the hydrolysis of acid chlorides, have been supported by comparing the computed and experimentally derived Δ G ‡ data and the substituent effect. As we have found earlier,37, 38 the best agreement between computed and experimentally derived Δ G ‡ data can be obtained when computations are performed at the medium (B3LYP)/6‐31G(d) level of theory. This may happen because errors rise from the applied approximations compensate each other at this medium level of calculation.…”

“…6), in this case e‐d substituents also decrease the Δ G ‡ values. Nevertheless, the Δ G ‡ values, computed at this higher level of theory are greater than the experimental data, as it was observed for other reactions, too 37, 38…”

“…The Δ G ‡ = 72.4, 74.0 and 89.8 kJ mol −1 values, computed at (B3LYP)/6‐31G(d) level of theory for TSs 19a and 20a in water and for TS 21a in acetone, respectively, approximate the experimentally derived data well (Scheme , Table 1 and Table S7 in the SI). It must be mentioned, however, that the values of the calculated ΔG ‡ parameters increase with the increasing level of theory, and in the case of MP2 calculations (Table S7 in the SI), as it was observed earlier37, 38 for other reactions, too (see also Section Computational Methods).…”

“…Earlier we have performed DFT calculations on aliphatic S N 2 and S N 1 nucleophilic substitutions 35–39. The medium effect on reactivity was computed using the polarizable continuum model (PCM) of solvents (see Section Computational methods).…”