Correlation of the rates of solvolysis of the S-Ethyldibenzothiophenium Ion

The specific rates of solvolysis of S-ethyldibenzothiophenium trifluoromethanesulfonate have been found to give a good linear correlation with the previously determined specific rates of solvolysis of the triethyloxonium ion, and hence with the solvent nucleophilicity scale established from the triethyloxonium ion study. A value for the sensitivity to changes in solvent nucleophilicity ( 1 ) of greater than unity (1.15) is in accord with reactivity-selectivity considerations.     

Determination of benzoyl bisulphide by desulphurisation and solvolysis

Benzoyl disulphide is rapidly and quantitatively desulphurised to the monosulphide by reaction with triphenylphosphine in aqueous methanol at 25 degrees . The monosulphide reacts with methanol to form thiobenzoic acid and methyl benzoate. Benzoyl disulphide also reacts with methanol in solutions containing ammonium acetate or sodium acetate yielding thiobenzoic acid, methyl benzoate and elemental sulphur. Benzoyl disulphide and sulphur were detected by their polarographic reduction waves. Thiobenzoic acid was detected by its polarographic oxidation wave and yields were determined by amperometric silver nitrate titrations.     

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.     

Cyclodextrin effect on solvolysis of substituted benzoyl chlorides

A kinetic study was carried out on the solvolysis of substituted benzoyl chlorides in the presence of alpha-, beta- and gamma-CD. Combination of the substituent dependent mechanism for solvolysis of benzoyl chlorides and the complexation ability of the cyclodextrin yields the following experimental behavior: (i) catalysis by beta- and gamma-CD for solvolysis of electron-attracting substituted benzoyl chlorides due to the reaction with its hydroxyl group C(6); (ii) absence of alpha-CD influence on solvolysis of benzoyl chlorides with electron withdrawing substituents; (iii) inhibition of solvolysis of benzoyl chlorides with electron-donating groups. This behavior is observed for solvolysis of meta/para substituted substrates in the presence of beta-CD, solvolysis of meta-substituted benzoyl chlorides in the presence of alpha-CD and solvolysis of para-substituted benzoyl chlorides in the presence of gamma-CD. This decrease in the rate constant is a consequence of the complexation of the substrate in the cyclodextrin cavity and its low solvation ability, causing the rate of solvolysis in its interior to be negligible. (iv) The solvolysis of meta-substituted benzoyl chlorides in the presence of gamma-CD yields a new behavior where the reaction of the complexed substrate is not negligible in the interior of the cyclodextrin cavity, which has been interpreted as a consequence of incomplete expulsion of hydration water from its cavity when the complexation takes place. (v) The experimental results obtained in the presence of alpha-CD show that meta-substituted benzoyl chlorides give rise to host : guest complexes with 1 : 1 stoichiometries, whereas those which are para-substituted cause a 2 : 1 stoichiometry to be formed. This difference in behavior has been interpreted taking into account the size of the different benzoyl chlorides and their accommodation in the alpha-CD cavity.     

Cyclodextrin effect on solvolysis of ortho benzoyl chlorides

A kinetic study was carried out on the solvolysis of ortho benzoyl chlorides in the presence of α-, β- and γ-Cyclodextrin (CD). The solvolysis mechanism of benzoyl chlorides is sensitive to the substituents, and to the solvent in which the reaction takes place. In water, the behaviour exhibited by benzoyl chlorides which have electron-attracting groups, is consistent with an associative mechanism whilst electron-donating substituents induce a dissociative mechanism. The results obtained in the presence of CD show a decrease in the observed rate constant, k _obs, as the CD concentration increases. This behaviour can be explained if these substrates undergo solvolysis through a dissociative path in the presence of α-, β- and γ-CD.     

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.     

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 $\text{1-H}$.     

Hydrogen bonding and catalysis of solvolysis of 4-methoxybenzyl fluoride

Values of k(o) = 8.0 x 10(-3) s(-1) and k(H) = 2.5 x 10(-2) M(-1) s(-1), respectively, were determined for the spontaneous and the acid-catalyzed cleavage of 4-methoxybenzyl fluoride (1-F) to form the 4-methoxybenzyl carbocation (1+). Values of k(F) = 1.8 x 10(7) M(-1) s(-1) and k(HF) = 7.2 x 10(4) M(-1) s(-1) were determined for addition of F- and HF to 1+ for reaction in the microscopic reverse direction. Evidence is presented that the reversible addition of HF to 1+ to give 1-F + H+ proceeds by a concerted reaction mechanism. The relatively small 250-fold difference between the reactivities of fluoride ion and neutral HF toward 1+ is attributed to the tendency of the strong aqueous solvation of F- to decrease its nucleophilic reactivity and to the advantage for the concerted compared with the usual stepwise pathway for addition of HF. There is no significant stabilization of the transition state for cleavage of 1-F from general acid catalysis by 0.80 M cyanoacetate buffer at pH 1.7. The estimated 3 kcal/mol larger Marcus intrinsic barrier for heterolytic cleavage of 1-F than for cleavage of 1-Cl is attributed to a lag in the development at the transition state of the ca. 30 kcal/mol greater stabilizing solvation of the product ion F- compared with Cl-. The decrease in the electronegativity of X along the series X = F, OH, Cl is accompanied by a ca. 10(10)-fold increase in the carbon basicity compared with the proton basicity of X-.     

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.     

The solvolysis of benzoyl halides as a chemical probe determining the polarity of the cavity of dimethyl-β-cyclodextrin

The solvolysis of benzoyl halides (BzX) in the presence of dimethyl-β-cyclodextrin (DM-β-CD) was studied. Methylation or hydroxyalkylation of the hydroxyl groups in β-cyclodextrin increases their solubility and the highest possible concentration of DM-β-CD that can be dissolved in water is 0.2 M. The ability to use more readily soluble CDs may allow one to determine the stoichiometry of their complexes and the properties of water held in their cavity with increased precision. Based on the experimental results, this cyclodextrin forms host-guest complexes of variable stoichiometry where two reaction pathways are considered: in water and in the internal cavity of the cyclodextrin. We determined the rate constants for the halides in their reaction inside the internal cavity. This allowed the influence of the substituent and leaving group on the reactions in the bulk water and the internal cavity of DM-β-CD to be compared. Depending on whether the solvolysis reaction is preferentially associative or dissociative, the presence of the cyclodextrin has a catalytic or inhibitory effect, respectively.     

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.     

Polar and anchimeric effects in the solvolysis of 6-endo-substituted 2-norbornyl-p-toluene-sulfonates

A study of the influence of 6-endo substituents on the reactivity of 2-exo- and 2-endo-norbornyl p-toluenesulfonates $\text{3}$ and $\text{4}$, respectively, confirms that polar rather than steric effects control relative rates.     

Substituent effects in the mass spectra of benzoyl hetarenes

The mass spectra of several benzoyl hetarenes, of 2-benzoyl pyridines substituted at the phenyl and pyridyl group, respectively, and of phenyl substituted 2-benzoyl pyrroles, have been studied with respect to the formation of benzoyl and hetaroyl ions. A correlation between the intensity of benzoyl ions, relative to molecular ion intensity, and the π-electron density at the substituted carbon atom of the hetarene has been observed for benzoyl hetarenes. The relative intensities of (substituted) benzoyl ions of substituted 2-benzoyl pyridines and 2-benzoyl pyrroles are not easily related to substituent constants of Hammett equations. The relative ionization energies of phenyl substituted 2-benzoyl pyridines, however, and the relative appearance energies of substituted benzoyl ions derived therefrom follow the σ_IP⁺-constant of Bentley and Johnstone and the σ⁺-constants of Brown, respectively.     

Dramatic acceleration of the Menschutkin reaction and distortion of halide leaving-group order

Compared to structurally related linear trialkylamines, a simple macrocyclic amine with an anion-binding cavity exhibits very large rate enhancements (>10(5)) for stoichiometric N-alkylation with primary alkyl, allyl, and benzyl halides in the weakly polar solvent CDCl3. There is also a major distortion of the halide leaving-group order. For example, with benzyl halides the relative leaving-group order with a control amine is Cl (1) < Br (71) < I (160), whereas the leaving-group order with the macrocyclic amine is I (0.4) < Cl (1) < Br (8.5). Reaction with the macrocyclic amine is inhibited by the addition of DMSO, which is unusual because the Menschutkin reaction is normally enhanced by the presence of a polar aprotic solvent. Competitive inhibition studies indicate that the reaction proceeds through a prereaction complex. Effective molarities for the subsequent unimolecular N-alkylation step with 4-t-butylbenzyl halides are 4-t-BuBnCl (62,000 M) > 4-t-BuBnBr (2200 M) > 4-t-BuBnI (35 M); thus, the free energy of activation is selectively decreased for organohalides having smaller and more charge dense leaving groups. Likely reasons for this selective enhancement effect are: (a) increased transition-state stabilization due to hydrogen bonding in the macrocyclic pocket and (b) reduced entropic penalty in the transition state due to an increased fraction of prereaction complexes that are oriented in a near attack conformation. The study suggests that it should be possible to develop highly reactive macrocyclic amines that selectively sense or scavenge carcinogenic haloalkanes from the atmosphere.     

Polar effects in the solvolysis of 6-exo-substituted endo-2-norbornyl p-toluenesulfonates

The solvolysis rates of endo-2-norbornyl p-toluenesulfonates $\text{1}$ are affected far less by 6-exo-substituents than are the rates of the corresponding exo-2-norbornyl p-toluenesulfonates $\text{2}$¹.     

Isotope effects, dynamics, and the mechanism of solvolysis of aryldiazonium cations in water

The mechanism of the heterolytic solvolysis of p-tolyldiazonium cation in water was studied by a combination of kinetic isotope effects, theoretical calculations, and dynamics trajectories. Significant (13)C kinetic isotope effects were observed at the ipso (k(12)C/k(13)C = 1.024), ortho (1.017), and meta (1.013) carbons, indicative of substantial weakening of the C(2)-C(3) and C(5)-C(6) bonds at the transition state. This is qualitatively consistent with a transition state forming an aryl cation, but on a quantitative basis, simple S(N)1 heterolysis does not account best for the isotope effects. Theoretical S(N)2Ar transition structures for concerted displacement of N(2) by a single water molecule lead to poor predictions of the experimental isotope effects. The best predictions of the (13)C isotope effects arose from transition structures for the heterolytic process solvated by clusters of water molecules. These structures, formally saddle points for concerted displacements on the potential energy surface, may be described as transition structures for solvent reorganization around the aryl cation. Quasiclassical dynamics trajectories starting from these transition structures afforded products very slowly, compared to a similar S(N)2 displacement, and the trajectories often afforded long-lived aryl cation intermediates. Critical prior evidence for aryl cation intermediates is reconsidered with the aid of DFT calculations. Overall, the nucleophilic displacement process for aryldiazonium ions in water is at the boundary between S(N)2Ar and S(N)1 mechanisms, and an accurate view of the reaction mechanism requires consideration of dynamic effects.