Further Kinetic Studies of Solvolytic Reactions of Isobutyl Chloroformate in Solvents of High Ionizing Power Under Conductometric Conditions

Abstract

Solvolyses of isobutyl chloroformate (4) in 43 binary solvent mixtures including highly aqueous media, water, D 2 O, CH 3 OD, 2,2,2-trifluoroethanol (TFE) as well as aqueous 1,1,1,3,3,3-hexafluoro-isopropanol (HFIP) solvents were performed at 45 solvolyses of 4 are found to be consistent with the proposed mechanism (Ad E ). The variety of solvent systems was extended to comprise highly ionizing power solvent media (Y Cl > 2.7 excepted for aqueous fluorinated solvents and pure TFE solvent) to investigate whether a mechanistic change occurs as solvent compositions are varied. However, in case of 18-solvent ranges having aqueous fluorinated solvent systems (TFE-H 2 O and HFIP-H 2 O) and/or having Y Cl > 2.7 solvent systems, the solvent effect on reactivity for those of 4 are evaluated by the multiple regression analysis as competition with S N 2 -type mechanism. And in pure TFE and 97 w/w % HFIP solvents with high Y Cl and weak N T , these solvolyses are understood as reactions which proceed through an ionization (S N 1) pathway.

Influence of Sulfur for Oxygen Substitution in the Solvolytic Reactions of Chloroformate Esters and Related Compounds

“…In the trichloro-derivative of tert -butyl chloride, 2,2,2-trichloro-1,1-dimethylethyl chloroformate, the electron-withdrawing chlorides lead to an A-E mechanism [55,56] and an increased value of 2.14 ± 0.03 is obtained [56]. Similarly, values of 2.17 ± 0.03 have been obtained for n -propyl chloroformate [57] and of 2.03 ± 0.01 for isobutyl chloroformate [58] methanolyses. The higher A-E values reflect, in part, the involvement of a second methanol [or methan (ol- d )] molecule as a general-base [37,59].…”

Calculated Third Order Rate Constants for Interpreting the Mechanisms of Hydrolyses of Chloroformates, Carboxylic Acid Halides, Sulfonyl Chlorides and Phosphorochloridates

“…The plot (red in Figure 7 ) of log k 3 vs. Y for solvolyses of isobutyl chloroformate ( 5 , R = CH 2 CH(CH 3 ) 2 ) show very similar values to those for 5 , R = i Pr in 90%–60% acetone-water (first 4 data points); note that the temperatures are not identical (40 and 45 °C). Compared with the line drawn for 5 , R = i Pr, additional data in 50%–10% acetone for 5 , R = CH 2 CH(CH 3 ) 2 show a smaller change in slope; also the KSIE in water of 1.54 [ 73 ] is larger. The KSIE in methanol of 2.03 [ 73 ] and the low slope of the plot for 90%–60% acetone ( Y values from −2 to 1) are consistent with third order reactions, and a change to a bimolecular mechanism was suggested for reaction in water [ 73 ].…”

“…Compared with the line drawn for 5 , R = i Pr, additional data in 50%–10% acetone for 5 , R = CH 2 CH(CH 3 ) 2 show a smaller change in slope; also the KSIE in water of 1.54 [ 73 ] is larger. The KSIE in methanol of 2.03 [ 73 ] and the low slope of the plot for 90%–60% acetone ( Y values from −2 to 1) are consistent with third order reactions, and a change to a bimolecular mechanism was suggested for reaction in water [ 73 ]. Comparing slopes and KSIE values in water, it appears that the change in mechanism begins slightly later and is more gradual for 5 , R = CH 2 CH(CH 3 ) 2 than for 5 , R = i Pr.…”

“… Changes in mechanism based on plots of log k 3 vs. Y Equation (3) for solvolyses in acetone-water of 5 , R = i Pr at 40 °C and 5 , R = CH 2 CH(CH 3 ) 2 at 45 °C; kinetic data from References [ 71 , 72 , 73 ]. …”

LFER Studies Evaluating Solvent Effects on an α-Chloro- and two β,β,β-Trichloro-Ethyl Chloroformate Esters

“…Application of equations 1 and 2 to solvolytic rate data for 2 results in l values of 0.69 and 0.80, and m values of 0.95 and 1.02 [ 47 , 48 ], respectively. The l/m ratios (0.73 and 0.78) can be considered [ 26 , 33 ] as good indicators for ionization (S N 1 type) mechanisms with significant solvation at the developing thioacylium ion. (or acylium ion in the case of the chloroformate analog) The accompanying h value of 0.42 obtained [ 47 , 48 ] for 2 (using equation 2 ), suggests that there is a minimal charge delocalization into the aromatic ring.…”