Salt Effects on the Rates and Mechanisms of Solvolysis Reaction of Organic Halides and Water Structure Distortion in N,N-Dimethylformamide– and N,N-Dimethylacetamide–Water Mixed Solvents

In 50% (v/v) DMF– and DMA–H2O mixed solvents, salt effects on the solvolysis reaction rates of aliphatic halides and related compounds (RX) have been examined for further exploring. Compared with previous results obtained in other organic solvent systems, such as 50% (v/v) acetone–, 1,4-dioxane–, or sulfolane–H2O, no exceptional behavior was observed in the “pseudo” first-order rate constants (k/s−1) of all the typical SN1 and SN2 substrates by the addition of alkali metal and alkaline earth metal perchlorates or many kinds of tetraalkylammonium salts in 50% (v/v) DMF– and DMA–H2O mixed solvents. The detailed examination of Δlog(k/s−1)/Δ[LiClO4] for 1-adamantyl bromide vs. the contents of organic solvents (CH3CN, DMA, and DMSO) suggested that the observed different salt effects were caused by the different solvation abilities of the solvents toward the leaving-group anion as well as the metal cation. As a new highlight in the present paper, we were able to demonstrate a proportionality or correlation between the LiClO4 effects in the solvolysis rates and the carbocation stabilities expressed by the Gibbs free energy values (ΔGo) of RX in the gas phase. Based on the Raman spectra of DMA–H2O and DMA–D2O as well as DMF–D2O mixed solvents, we discuss the distortion of the bulk water structure.