Structural Effects on the Solvolytic Reactivity of Carboxylic and Sulfonic Acid Chlorides. Comparisons with Gas-Phase Data for Cation Formation

Abstract

Kinetic data for solvolyses of 28 acid chlorides in 97% w/w trifluoroethanol (TFE)-water spanning over 10 (9) in rate constant at 25 degrees C are obtained directly or by short extrapolation from published values. G3 calculations of the energy required for cation formation in the gas phase are validated from proton affinities and from other experimental data. G3 calculations of heterolytic bond dissociation enthalpies (HBDEs) for formation of cations from acid chlorides in the gas phase show the following trends when compared with the solvolysis rate constants: (i) electron-rich sulfonyl chlorides and most carboxylic acid chlorides, including thione derivatives, give a satisfactory linear correlation with a significant negative slope; (ii) most sulfonyl chlorides and some chloroformates and thio derivatives have higher HBDEs and fit another correlation with a small, negative slope. A significant deviation is observed for the acyl series (RCOCl), for which both solvolysis rates and HBDEs increase in the order R = Bu ( t ) < Pr ( i ) < Et < Me. The deviation may be explained either by a prior hydration mechanism or preferably by electrostatic effects on the formation of small cations. The above results of structural effects support independent evidence from solvent effects that cationic ionization reaction pathways (with nucleophilic solvent assistance or S N2 character) are involved in the solvolyses of acid chlorides.

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

“…This strongly supports the thesis that the alkylthiocarbonyl cation is more stable than the corresponding alkylcarbonyl cation. Further support comes from a calculation showing that bond dissociation energies are consistent with a simple cleavage of the C–Cl bond of a chlorothioformate being a relatively favorable process [54]. …”

“…Bentley [54] has carried out calculations which combine these effects of cation stabilization with the ground-state stabilization effects discussed at the end of the immediately preceding section. In this way he arrived at heterolytic bond dissociation energies for a series of phenyl esters.…”

On the Importance of the Aromatic Ring Parameter in Studies of the Solvolyses of Cinnamyl and Cinnamoyl Halides

“…The 3-D views for cinnamoyl chloride ( 3’ ) and p -nitrocinnamoyl chloride ( 5’ ) are shown in Figure 3 incorporating prior results [35] for the position of the halogen in the ground-state structure of acid chlorides. The 3-D images of 3’ and 5’ clearly attests to the planar conformation between the aromatic ring, the vinylic double bond, and the carbonyl group.…”

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

“…[10][11][12][13] In this work, solvents having ionizing power values (Y Cl ) > 2.7 were regarded as highly ionizing power media, such as aqueous fluorinated alcohols, pure water and the solvent ranges 40% MeOH → H 2 O, 40% EtOH → H 2 O and 30% acetone → H 2 O, respectively. Within this range of solvents, there are differences in solvent nucleophilicity (N T ) value (e.g., N T ; −0.74 for 40% EtOH and −2.97 for 97 w/w % HFIP).…”