Correlation of the rates of solvolysis of 2-phenyl-2-ketoethyl bromide and tosylate

[reaction: see text] The extended Grunwald-Winstein equation has been applied to the specific rates of solvolysis of 2-phenyl-2-ketoethyl bromide and tosylate and the correlation parameters are consistent with an S(N)2 mechanism over the full range of solvents. The k(OTs)/k(Br) ratios are close to unity, consistent with this assignment. Comparisons with the specific rates of solvolysis of 2-phenylethyl bromide and methyl tosylate show only a modest influence upon introduction of the carbonyl group.     

Correlation of the rates of solvolysis of benzoyl fluoride and a consideration of leaving-group effects

The specific rates of solvolysis of benzoyl fluoride have been determined at 25.0 degrees C in 37 pure and binary solvents. Together with seven values from the literature, these give a satisfactory correlation over the full range of solvents when the extended Grunwald-Winstein equation is applied. The sensitivities to changes in solvent nucleophilicity and solvent ionizing power are very similar to those for octyl fluoroformate, suggesting that the addition step of an addition-elimination mechanism is rate determining. In the solvent-composition region where benzoyl chloride also shows bimolecular solvolysis, the appreciable k(Cl)/k(F) values are proposed as being primarily due to a more efficient ground-state stabilization for the fluoride.     

Correlation of the rates of solvolysis of cyclopropylcarbinyl and cyclobutyl bromides using the extended Grunwald-Winstein equation

The reactions of cyclopropylcarbinyl bromide (1) and cyclobutyl bromide (2) in hydroxylic solvents proceed with both solvolysis and rearrangement. Depending on the solvent, the reactions of 1 are 10-120 times faster than those of 2, and both are faster than the previously studied allylcarbinyl bromide (3). Specific rates are reported for the reactions of 2 proceeding to solvolysis products and 3. Reactions of 1 proceed to solvolysis products and both 2 and 3; since 2 slowly undergoes further solvolysis, specific rates are obtained by a modified Guggenheim treatment. The two sets of specific rates are analyzed using the extended Grunwald-Winstein equation to give sensitivities toward changes in solvent nucleophilicity of 0.42 for 1 and 0.53 for 2 and corresponding sensitivities toward changes in solvent ionizing power of 0.75 and 0.94. A mechanism is proposed involving a rate-determining ionization with an appreciable nucleophilic solvation of the incipient carbocation.     

Comprehensive study of the methyl effect on the solvolysis rates of bridgehead derivatives

The effect of a bridgehead methyl group on the hydride ion affinity in the gas phase of bicyclo[1.1.1]pent-1-yl (1+), 1-norbornyl (3+), cubyl (5+), 1-adamantyl (7+), bicyclo[2.2.2]oct-1-yl (9+),and bicyclo[3.1.1]hept-1-yl (11+) cations has been studied using density functional theory and ab initio methods. It is concluded that the methyl group always increases the stability of the substituted cations. The effect of the solvent on the stability of methyl-substituted cations in relation to the unsubstituted cations has been studied using the polarizable continuum model of the self-consistent reaction field theory. In the case of rearranging cations, the nucleophilic assistance of the solvent is determined by means of the interaction energy of the corresponding water complexes. It is concluded that the solvent causes the relative stabilization of the parent cations. As a consequence, most of the methyl-substituted bridgehead derivatives show a lower solvolysis rate than the corresponding unsubstituted compounds. A nonqualitative explanation of the methyl effect on the relative stability of bridgehead cations in both gas phase and solution is given for the first time. The ratios of solvolysis products in the case of rearranging bridgehead cations have also been computed from the relative stability of the intermediate water complexes.     

Remote inductive effects on secondary and tertiary 2-norbornyl solvolysis rates

A 5-cyano substituent decelerates the solvolysis rate of exo-2-norbornyl brosylate $\text{(1-H)}$ by a factor of 1790. A much smaller deceleration (24–90 fold) is observed for the secondary $\text{endo}$ and for both $\text{exo}$ and $\text{endo}$ tertiary 2-norbornyl derivatives. These results support the occurrence of σ-participation in the solvolysis of $\text{1-H}$.     

Solvent and stereoelectronic effects on the solvolysis rates of oxaspirocyclopropanated 1-norbornyl triflates and related bridgehead derivatives

The study of the stereochemical outcome of the solvolysis of oxaspirocyclopropanated 1-norbornyl triflates is highly interesting since these reactions do not lead to the usual retention or fragmentation products but only synthetically interesting rearranged products are enantiospecifically formed. There is no correlation between the experimental solvolysis rates (ln k) and the B3LYP/6-31G(d)-computed ionization energies (Delta E) of the corresponding bridgehead hydrocarbons in gas phase. However, this work demonstrates the existence of a fair linear correlation between the experimental reaction rates and the PCM//B3LYP/6-31G(d)-computed free ionization energies in solution (Delta G). This theoretically relevant result reveals that the reason for the lack of linearity in gas phase is not the rearrangement of the intermediate carbocations but unspecific solvent effects on the solvolysis rates, accounted for by the PCM model.     

Base-catalysed solvolysis of acylsilanes

Kinetic studies have been made of the hydrolysis of XC₆H₄COSiR₃ compounds (R = Me, Et, Ph) in a mixture of THF (3 vol.) and aqueous sodium hydroxide (1 vol.). The rates of cleavage of p-NO₂C₆H₄CH₂SiMe₃, C₆H₅COSiMe₂C₆H₅ and C₆H₅COSiMe₂C₆H₄Cl-p compounds under the same conditions have also been measured. Electron-attracting substituents on either side of the bond being broken lead to an increase in the rate. For the XC₆H₄COSiR₃ compounds, the effects of the substituents X correlate excellently with their Hammett σ constants (? = 3.5 and 2.5 for R = Me and Ph, respectively). The data are consistent with a mechanism in which the attack of the hydroxide ion occurs at the carbonyl C atom.     

Behaviour of 1-phenylethyl chloride in aqueous ethanol: α-deuterium isotope effect on solvolysis rates

α-Deuterium effects on the voltolysis rates of 1-phenylethyl chloride in ethanol-water mixtures within the 80–100% etOH range are reported. According to Shiner's investigations,⁹ a monotonical decrease of K_H/k_D with decreasing water content was expected. Results show the existence of a minimum for k_H/k_D in approximately 90 vol.% EtOH. This behaviour might be accounted for in the framework of Shiner's mechanistic scheme if (i) H-bonded intermolecular associations, in which ethanol water should have their reactivity changed (ii) the structure of such clusters should depend upon the solvent composition.     

Kinetics of the solvolysis of the trans-dichlorobis(1,2-diaminoethane)-cobalt(III) Ion in water + urea mixtures

Following the kinetic investigation of the solvolysis of a range of cobalt(III) complexes in mixtures of water + cosolvent wherethe cosolvent enhances the solvent structure and decreases the dielectric constant, kinetic data are now reported for such a solvolysis in water + urea where urea acts as a structure breaker and enhances the dielectric constant. A plot of log (rate constant) against reciprocal of the dielectric constant shows that differential effects of changes in solvent structure occur between the initial and the transition states and, as in theinvestigations using structure-enhancing cosolvents, the principal effect of change in solvent structure on the cobalt(III) cation occurs on the penta-coordinated ion in the transition state.     

Hammet correlations for the solvolysis of silylpyrroles

The siliconnitrogen bond in (XC₆H₄)Me₂SiNC₄H₄ (X = p-MeO, p-Me, m-Me, H, p-Cl) is cleaved in absolute ethanol containing sulphuric acid or amine buffers. The logarithms of the rate constants correlate with σ^o-constants, the p-values being -0.97 for the acid-catalysed reaction, and +1.8 and +2.6 for the reactions catalysed by amine and ethoxide anion, respectively. Mechanisms involving pentacoordinate silicon are suggested.     

Fluorimetric determination of the fungicide benomyl after solvolysis

A simple spectrofluorimetric method for the determination of the fungicide benomyl in grapes is described. Benomyl is rapidly solvolysed in organic solvents to give the highly fluorescent metabolite carbendazim. Solvent effects on the spectral shape and their influence on analytical performance are studied. DMF was found to be the best solvent. The method is sensitive to 12 ng/ml benomyl, and has an RSD at the 0.5 g/ml level of 2.2%. The method is suitable for determining benomyl in spiked grapes, with recoveries in the range 97 to 104%.     

Nucleophilic participation in the solvolysis of the t-butyldimethylsulfonium ion

Studies with the $\text{t}$-butyldimethylsulfonium ion give evidence for nucleophilic participation in the solvolyses of $\text{t}$-butyl compounds but electrophilic assistance would be the dominant factor for $\text{t}$-butyl chloride solvolysis.     

Thermal properties of solvolysis lignin-derived polyurethanes

Thermal properties of polyurethane (PU) films prepared from solvolysis lignin (SL), polyethylene glycol (PEG) and diphenylmethane diisocyanate (MDI) were examined by differential scanning calorimetry and thermogravimetry. In the SL—PEG—MDI system, the SL content, the molecular weight (M_n) of PEG and the NCO/OH ratio were changed in order to control the thermal properties. Glass transition temperatures (T_g's) of the prepared PU's were dependent on the SL content, the M_n of PEG and the NCO/OH ratio. However, the T_g of PU was significantly influenced by the SL content: the increment of T_g was ca. 150 K when the SL content in PEG increased from 0 to ca. 50%. The decomposition of the PU's was markedly dependent on the content of SL. Other factors, such as the NCO/OH ratio and the M_n of PEG, are less dominant compared with the SL content. This fact suggests that the dissociation between the isocyanate groups and the phenolic OH groups in SL may be the major factor in the whole process of the thermal decomposition of the PU containing SL     

Mechanical properties of solvolysis lignin-derived polyurethanes

In order to study the tensile properties of solvolysis lignin polyurethanes, polyurethane (PU) films were prepared from the solvolysis lignin (SL)–polyethylene glycol (PEG)–diphenylmethane diisocyanate (MDI) system, and in addition to this further PU films were also prepared from the SL–MDI system. In the SL–PEG-MDI system, the SL content, the molecular weight of PEG and the NCO:OH ratio were varied in order to control the physical properties. The tensile stress and Young's modulus of the PUs of the SL–PEG–MDI type increased with increasing SL content and NCO:OH ratio. The tensile properties of the PUs from the SL–MDI system showed no NCO:OH ratio dependency; i.e. the mechanical properties of PUs are not influenced by the change in crosslinking. It is possible to control the tensile properties of PUs of the SL–PEG–MDI type by changing the content of PEG and SL at a constant NCO:OH ratio.