Competing S N2 and carbonyl addition pathways for solvolyses of benzoyl chloride in aqueous media

Abstract

The further development of a convenient, conductimetric method for studying the rates of relatively fast solvolytic reactions in highly aqueous media is described. Rate and product data are reported for solvolyses of benzoyl chloride at 25 "C in water and in binary aqueous mixtures with acetone, ethanol, methanol, trifluoroethanol, and hexafluoropropan-2-01. Comparison of these kinetic data with ratedata for the s N 1 model, 1 -adamantyl chloride, reveals sensitivity to solvent nucleophilicity even for highly aqueous media. In contrast with other nucleophilically solvent-assisted processes, the solvent effects are not satisfactorily correlated by one linear free energy relationship. Gas-phase thermochemical data show the feasibility of a direct heterolytic cleavage of the C-CI bond in benzoyl chloride. Rate-product correlations for hydrolysis and aminolysis in 50% w / w acetone-water are observed, providing that allowance is made for the medium effect of the added o-nitroaniline. These results are consistent with an sN2 mechanism, not SN1 and/or ion-pair mechanisms previously discussed. Another pathway, dominant in less aqueous media, is much less sensitive to changes in solvent ionizing power, consistent with earlier proposals for a carbonyl addition-elimination mechanism.

Kinetic and Theoretical Consideration of 3,4- and 3,5-Dimethoxybenzoyl Chlorides Solvolyses

“…Substrates of 3 and 5 were reported that they are mainly followed by unimolecular pathway (ionization) due to the resonance effect of paramethoxy substituent. [6][7][8][9]15 The l/m ratio of 1 in all range of solvents was 0.47, it is the similar value of 3 and 5, which means that the solvolysis reaction pathway of 1 containing para-methoxy substituent, is consistent with the results of 3 and 5. In case of 2, the l/m ratio was 2.59 in 17 nucleophilic solvents and was 0.64 in 8 electrophilic solvents, respectively.…”

The role of water release from the cyclodextrin cavity in the complexation of benzoyl chlorides by dimethyl-β-cyclodextrin

“…44 The solvolysis of benzoyl chlorides, which involves a dissociative mechanism, is highly sensitive to the solvent polarity. 41,42 Figure 2 illustrates the effect of the DM-b-CD concentration on the observed rate constant for the solvolysis of 4-MeO. As can be seen, k obs clearly decreased with increasing DM-b-CD concentration, whatever the temperature.…”

“…The mechanism behind the solvolysis of benzoyl chlorides (BzCl) in water and various other solvents is well known. [40][41][42] Previous studies in our laboratory have shown the influence of a-, b-and g-cyclodextrins on the solvolysis of substituted benzoyl chlorides. 43 We found that the presence of a-and g-CD causes alterations in behaviour, which are related to the stability of the CD-G complexes, the stoichiometry of the complexes (causing the formation of complexes a-CD-BzCl with 1:1 stoichiometries for those which are substituted in the meta position and 2:1 for those substituted in the para position) and the displacement of water in the interior of the cavity causing the solvolysis reaction to be detected in the interior of the cavity of g-CD when meta-substituted…”

Investigation of Reaction Mechanisms by Product Studies

“…Medium effects of non-electrolytes have also been observed. For solvolyses of benzoyl chloride (59) in 50% acetone-water, 0.1 M o-nitroaniline unexpectedly caused a significant kinetic effect; when an inert model nitro compound (nitrobenzene) was used to maintain a constant solute concentration of nitroarene, Equation 2.15 was successful [45]. This use of nitrobenzene is exactly analogous to the use of an inert electrolyte to maintain a constant ionic strength.…”

“…Variations in pH often lead to mechanistic changes because substrates may be protonated by H 3 O + , or attacked by HO − , much more rapidly than by water (see Chapter 11). Dediazoniation of methylbenzenediazonium ions (44 in Scheme 2.16) in aqueous ethanol is heterolytic at low pH; typical products are ArOH or ArOEt (45) derived from attack on a cationic intermediate by nucleophiles. Above pH 6, however, the process is homolytic and toluene (46) …”