Kinetics and mechanism of the anilinolysis of aryl 4‐nitrophenyl thionocarbonates in aqueous ethanol

The reactions of bis(4‐nitrophenyl), 3‐chlorophenyl 4‐nitrophenyl, and 3‐methoxyphenyl 4‐nitrophenyl thionocarbonates ( 1 , 2 , and 3 , respectively) with a series of anilines are subjected to a kinetic investigation in 44 wt.% ethanol–water, at 25.0 °C and an ionic strength of 0.2 M. Under aniline excess, pseudo‐first‐order rate coefficients (k_obs) are found. Plots of k_obs versus aniline concentration are linear, with the slopes (k_N) pH independent, k_N being the rate coefficient for the anilinolysis of the thionocarbonates. The Brønsted plot (log k_N vs. pK_a of anilinium ions) for thionocarbonate 1 is linear, with slope (β) 0.62, which is consistent with a concerted mechanism. The Brønsted plots for thionocarbonates 2 and 3 are curved, with slopes 0.1 at high pK_a for both reaction series and slopes 0.84 and 0.79 at low pK_a for the reactions of 2 and 3 , respectively. The latter plots are in accordance to stepwise mechanisms, through a zwitterionic tetrahedral intermediate (T^±) and its anionic analogue (T^?), the latter being formed by deprotonation of T^± by the basic form of the buffer (HPO). The Brønsted curves are explained by a change in the rate‐limiting step, from deprotonation of T^± at low pK_a, to its formation at high pK_a. The influence of the amine nature and the non‐leaving and electrophilic groups of the substrate on the kinetics and mechanism is also discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Kinetics of alkaline hydrolysis of p‐substituted benzylidenemalononitriles in 50% aqueous acetonitrile: substituent effects and quantification of the electrophilic reactivity

The rate constants of the reaction of p‐X‐substituted benzylidenemalononitriles 1a , 1b , 1c , 1d , 1e , 1f , 1g , 1h with hydroxide ion were measured in 50% water–50% acetonitrile at 20 °C. The experimental kinetic data reveal that the points pertaining to electron donating substituted compounds (X = Me, OMe and NMe₂) exhibit negative deviations from the Hammett plot. However, the Yukawa–Tsuno plot for the same rate constants resulted in a good straight line with an excellent correlation coefficient (r² = 0.9916) and an r value of 1.15. Possible ground‐state stabilization through resonance interactions has been suggested to explain the origin of the nonlinear Hammett plot. On the basis of the relationship between E and σ_p⁺, the electrophilicity parameter E of some benzylidenemalononitriles 1c and 1e , 1f , 1g , 1h has been evaluated. More importantly, the three compounds 1f (E = ?7.90), 1g (E = ?7.80) and 1h (E = ?7.55) exhibit high electrophilicities that compare well with that of 4,6‐dinitrobenzoselenadiazole (E = ?7.40), a compound which has a general behaviour representative for the superelectrophilic dimension. We have shown that the second‐order rate constants calculated from Mayr's approach for the reaction of 1a , 1b , 1c , 1d , 1e , 1f , 1g , 1h with hydroxide ion do not agree with the available experimental data. On the other hand, a good linear correlation between log k_exp and log k_calc has been observed and discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Kinetics and mechanism of the reaction of alkoxymethylidene malonate and malononitrile with hydrazines and anilines

The kinetics and mechanism of the nucleophilic vinylic substitution of dialkyl (alkoxymethylidene)malonates (alkyl: methyl, ethyl) and (ethoxymethylidene)malononitrile with substituted hydrazines and anilines R¹–NH₂ (R¹: (CH₃)₂N, CH₃NH, NH₂, C₆H₅NH, CH₃CONH, 4‐CH₃C₆H₄SO₂NH, 3‐ and 4‐X‐C₆H₄; X: H, 4‐Br, 4‐CH₃, 4‐CH₃O, 3‐Cl) were studied at 25 °C in methanol. It was found that the reactions with all hydrazines (the only exception was the reaction of (ethoxymethylidene)malononitrile with N,N‐dimethylhydrazine) showed overall second‐order kinetics and k_obs were linearly dependent on the hydrazine concentration which is consistent with the rate‐limiting attack of the hydrazine on the double bond of the substrate. Corresponding Brønsted plots are linear (without deviating N‐methyl and N,N‐dimethylhydrazine), and their slopes (β_Nuc) gradually increase from 0.59 to 0.71 which reflects gradually increasing order of the C–N bond formed in the transition state. The deviation of both methylated hydrazines is probably caused by the different site of nucleophilicity/basicity in these compounds (tertiary/secondary vs. primary nitrogen). A somewhat different situation was observed with the anilines (and once with N,N‐dimethylhydrazine) where parabolic dependences of the kinetics gradually changing to linear dependences as the concentration of nucleophile/base increases. The second‐order term in the nucleophile indicates the presence of a steady‐state intermediate ‐ most probably T^±. Brønsted and Hammett plots gave β_Nuc = 1.08 and ρ = ?3.7 which is consistent with a late transition state whose structure resembles T^±. Copyright © 2013 John Wiley & Sons, Ltd.     

Kinetic study on the aromatic nucleophilic substitution reaction of 3,6‐dichloro‐1,2,4,5‐tetrazine by biothiols

The aromatic nucleophilic substitution reaction of 3,6‐dichloro‐1,2,4,5‐tetrazine (DCT) with a series of biothiols RSH: (cysteine, homocysteine, cysteinyl–glycine, N‐acetylcysteine, and glutathione) is subjected to a kinetic investigation. The reactions are studied by following spectrophotometrically the disappearance of DCT at 370 nm. In the case of an excess of N‐acetylcysteine and glutathione, clean pseudo first‐order rate constants (k_obs1) are found. However, for cysteine, homocysteine and cysteinyl–glycine, two consecutive reactions are observed. The first one is the nucleophilic aromatic substitution of the chlorine by the sulfhydryl group of these biothiols (RSH) and the second one is the intramolecular and intermolecular nucleophilic aromatic substitutions of their alkylthio with the amine group of RSH to give the di‐substituted compound. Therefore, in these cases, two pseudo first‐order rate constants (k_obs1 and k_obs2, respectively) are found under biothiol excess. Plots of k_obs1 versus free thiol concentration at constant pH are linear, with the slope (k_N) independent of pH (from 6.8 to 7.4). The kinetic data analysis (Brønsted‐type plot and activation parameters) is consistent with an addition–elimination mechanism with the nucleophilic attack as the rate‐determining step. Copyright © 2014 John Wiley & Sons, Ltd.     

Effect of substitution of oxygen by sulfur in the nonleaving group of a carbonate: kinetics of the phenolysis and benzenethiolysis of S‐methyl aryl thiocarbonates

The phenolysis and benzenethiolysis of S‐methyl 4‐nitrophenyl thiocarbonate ( 1 ) and S‐methyl 2,4‐dinitrophenyl thiocarbonate ( 2 ) in water are studied kinetically. The Brønsted plots (log k _N versus nucleophile basicity) are linear for all reactions. The Brønsted slopes for 1 and 2 are, 0.51 and 0.66 (phenolysis) and 0.55 and 0.70 (benzenethiolysis), respectively. These values suggest a concerted mechanism for these reactions, as found in the corresponding carbonates. Namely, substitution of OMe by SMe in the nonleaving group does not change the mechanism. Copyright © 2007 John Wiley & Sons, Ltd.     

Kinetics and mechanism of the pyridinolysis of diaryl carbonates

The reactions of 4‐methylphenyl and 4‐chlorophenyl 4‐nitrophenyl carbonates ( 1 and 2 , respectively), phenyl, 4‐methylphenyl, 4‐chlorophenyl, and 4‐nitrophenyl 2,4‐dinitrophenyl carbonates ( 3 , 4 , 5 , and 6 , respectively), and bis(2,4‐dinitrophenyl) carbonate ( 7 ) with a series of pyridines are studied kinetically at 25.0 °C in 44 wt% ethanol–water and an ionic strength of 0.2 M (KCl). The reactions are followed spectrophotometrically and under excess amine pseudo‐first‐order rate coefficients (k_obs) are found. For all these reactions, plots of k_obs versus free amine concentration at constant pH are linear, the slope (k_N) being independent of pH. The Brønsted‐type plots (log k_N vs. pK_a of the conjugate acids of the pyridines) are all biphasic (linear portions at high and low pK_a and a curvature in between). These plots are in accordance with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T^±), and a change in the rate‐determining step from formation of T^± to its breakdown to products, as the pyridine basicity decreases. Also studied are the effects of the leaving, non‐leaving, and electrophilic groups of the substrate, and of the amine nature, on the value (value at the center of curvature of the Brønsted‐type plots). Copyright © 2008 John Wiley & Sons, Ltd.     

Kinetics and mechanism of the reactions of aryl chlorodithioformates with pyridines and secondary alicyclic amines

The reactions of pyridines and secondary alicyclic (SA) amines with phenyl and 4‐nitrophenyl chlorodithioformates (PClDTF and NPClDTF, respectively) are subjected to a kinetic study in aqueous ethanol (44 wt% ethanol) solution, at 25.0 °C, and an ionic strength of 0.2 M (KCl). The reactions are studied spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients (k_obs) are found. Plots of k_obs versus [amine] are linear and pH independent, with slope k_N. The Brønsted‐type plots (log k_N vs. pK_a of aminium ions) are linear for the reactions of PClDTF with SA amines (slope β of 0.3) and pyridines (β = 0.26) and those of NPClDTF with pyridines (β = 0.30). For the reaction of NPClDTF with SA amines the Brønsted‐type plot is biphasic, with slopes β₁ = 0.2 (at high pKa) and β₂ = 1.1 (at low pKa). The pK_a value at the center of curvature (pK) is 7.7. The magnitude of the slopes indicates that the mechanisms of these reactions are stepwise, with the formation of a zwitterionic tetrahedral intermediate as the rate‐determining step, except for the reaction of NPClDTF with SA amines where there is a change in the rate‐determining step, from formation to breakdown of the tetrahedral intermediate, as the amine basicity decreases. Copyright © 2009 John Wiley & Sons, Ltd.     

Mechanism of nucleophilic substitutions at phenacyl bromides with pyridines. A computational study of intermediate and transition state

DFT computations have been performed on nucleophilic substitutions of phenacyl bromides with pyridines to investigate the mechanism of the reaction. In contrast with earlier suppositions, tetrahedral intermediate is not formed by the addition of pyridine on the C?O group of phenacyl bromide, because the total energy of the reacting species increases continuously, when the distance between the N and C(?O) atoms of reactants is shorter than 2.7 Å. At a greater distance, however, a bridged complex of the reactants is observed, in which the N atom of pyridine is slightly closer to the C atom of the C?O, than to the C atom of the CH₂Br group of phenacyl bromide, the distances are 2.87 and 3.05 Å, respectively. The attractive forces between the oppositely polarized N and C(?O) atoms in the complex decrease the free energy of activation of the S_N2 attack of pyridine at the CH₂Br group. The calculated structural parameters of the S_N2 transition states (TS) indicate, that earlier TSs are formed when the pyridine nucleophile bears electron‐donating (e‐d) groups, while electron‐withdrawing (e‐w) groups on phenacyl bromide substrate increase the tightness of the TS. Free energies of activation computed for the S_N2 substitution agree well with the data calculated from the results of kinetic experiments and correlate with the σ_Py substituent constants, derived for pyridines, and with the Hammett σ constants, when the substituents (4‐MeO‐4‐NO₂) are varied on the pyridine or on the phenacyl bromide reactants. Copyright © 2008 John Wiley & Sons, Ltd.     

Transition from concerted to stepwise processes as a function of leaving group ability: density functional theory and experimental study of lyoxide‐promoted cleavages of phosphorothioate and phosphate triesters in water and methanol

The base‐promoted solvolysis of a series of O,O‐dimethyl O‐aryl and O,O‐dimethyl O‐alkyl phosphorothioates (1) as well as O,O‐dimethyl O‐aryl and O,O‐dimethyl O‐alkyl phosphates (2) was studied computationally by density functional theory methods in methanol and water continuum media to determine the transition between concerted and stepwise processes. In addition, an experimental study was undertaken on the solvolysis of these series in basic methanol and water. The computations indicate that the solvolytic mechanism for series 1 involves lyoxide attack anti to the leaving group in a concerted manner with good leaving groups having pK_a^Lg values < 12.3 in methanol and in a stepwise fashion with the formation of a 5‐coordinate thiophosphorane intermediate when the pK_a^Lg > 12.3. A similar transition from concerted to stepwise mechanism occurs with series 2 in methanol as well as with series 1 and 2 in water, although for the aqueous solvolyses with hydroxide nucleophile, the transitions between concerted and stepwise mechanisms occur with better leaving groups than in the case in methanol. The computational data allow the construction of Brønsted plots of log k₂^?OS versus pK_a^Lg in methanol and water, which are compared with the experimental Brønsted plots determined with these series previously and with new data determined in this work. Both the computational and experimental Brønsted data reveal discontinuities in the plots between substrates bearing O‐aryl and O‐alkyl leaving groups, with the gradients of the plots being far steeper than, and non‐collinear with, the O‐aryl leaving groups for solvolysis of the O‐alkyl‐containing substrates. These discontinuities signify that care should be exercised in interpreting breaks in Brønsted plots in terms of changes in rate‐limiting steps that signify the formation of an intermediate during a solvolytic process. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis and electrochemical evaluation of 2‐substituted imidazolium salts

Herein, we report the synthesis, electrochemical, and computational evaluation of six 2‐substituted imidazolium bromides and six 2‐substituted imidazolium triflates. All final compounds were obtained in 2 or fewer synthetic steps from inexpensive starting materials and display a single, irreversible electrochemical reduction. The reduction potentials span a range greater than 1 V depending on the electron withdrawing power of the 2‐substituent. Imidazolium bromides such as Bn₂(H)ImBr reduce with E_1/2 = ?2.70 V vs Fc/Fc⁺, whereas the electron‐withdrawing Br‐containing analog Bn₂(Br)ImBr reduces at only ?1.58 V vs Fc/Fc⁺. The reduction potential of imidazolium bromides obeys a linear free energy relationship to σ_m Hammett constants, whereas imidazolium triflates correlate better with the σ_p Hammett constants. These results indicate that the stabilizing effect of the 2‐substituent is anion‐sensitive, changing from induction to resonance upon exchanging bromide for triflate. Predicted electron affinities from density functional theory–optimized structures of imidazolium cations and reduced species more closely match experimental data for the triflates, suggesting that a triflate anion does not electronically perturb the imidazolium core as much as a bromide. Taken together, these data highlight the dual modularity of imidazolium salts by changing both 2‐substituent and anion.     

Kinetic investigation on carbamate formation from the reaction of carbon dioxide with amino acids in homogeneous aqueous solution

The kinetics of carbamate formation from the reaction of carbon dioxide with α‐amino acids in D₂O was first investigated by means of nuclear magnetic resonance spectroscopy. Potassium carbonate was used as the CO₂ source. For each amino acid, the maximum carbamate yield, the apparent rate constant for the carbamate formation k_app, and the rate constants for the formation k₁ and the breakdown k_?1 of the carbamate were estimated. Plots of log k₁ or log k_?1 versus pK_a of amino acids indicated that the formation rate k₁ increased with the basicity (pK_a) of amino acid, while the decomposition rate k_?1 decreased. A Br?nsted β value of 0.39 was obtained from the former plot, being in good agreement with the previously reported ones (0.26–0.43). The observed negative pK_a dependence of log k_?1 (Br?nsted α = 0.34) is reasonable, because the carbamate decomposition is acid‐catalyzed and the steady‐state concentration of H⁺ should be higher for weaker basic amines. The charge (σ) and the lone‐pair energy (E_N) at the nitrogen atom of the amino group were calculated. Although log k₁ correlated with σ and E_N, log k_?1 was unrelated with both of these parameters. Considering that the carbamate formation (k₁) is not only base‐catalyzed but should also be promoted by the nucleophilicity of the amino nitrogen, its correlation with σ and E_N in addition to pK_a is rational. The irrelevance of log k_?1 to σ and E_N is not surprising, because σ and E_N are not a direct measure of [H⁺] of the solution. Copyright © 2011 John Wiley & Sons, Ltd.     

On the mechanism of the N,N‐dimethyl amination of Grignard reagents: a kinetic study

A direct kinetic study is reported for the electrophilic amination of substituted phenylmagnesium bromides with N,N‐dimethyl O‐(mesitylenesulfonyl)hydroxylamine in THF. Rate data, Hammett relationship, and activation entropy are consistent with a S_N2 displacement involving the attack of carbanions to sp³N in the amination reagent (AR). Copyright © 2007 John Wiley & Sons, Ltd.     

Multiparameter kinetic analysis of alkaline hydrolysis of a series of aryl diphenylphosphinothioates: models for P=S neurotoxins

Alkaline hydrolysis of a series of X‐substituted‐phenyl diphenylphosphinothioates ( 2a‐i ) in 80 mol%/20 mol% DMSO at 25.0 ± 0.1°C has been studied kinetically and assessed through a multiparameter approach. Substrates 2a to 2i are approximately 12 to 22 times less reactive than their P=O analogues 1a to 1i (ie, the thio effect). The Brønsted‐type plot for the reactions of 2a to 2i is linear with β_lg = ?0.43, consistent with a concerted mechanism. Hammett plots correlated with σ^o and σ^? constants also support a concerted mechanism; the Yukawa‐Tsuno plot results in an excellent linear correlation with ρ_X = 1.26 and r = 0.30, indicating that expulsion of the leaving group occurs in the rate‐determining step (RDS). The ΔH^? value increases from 10.5 to 11.7 and 13.9 kcal/mol as substituent X in the leaving group changes from 3,4‐(NO₂)₂ to 4‐NO₂ and H, in turn, while TΔS^? remains constant at ?6.0 kcal/mol. The strong dependence of ΔH^? on the electronic nature of substituent X also indicates that the leaving group departs in the RDS. The reaction mechanism and origin of the thio effect are discussed by comparison of the current kinetic results with those reported for the reactions of 1a to 1i . The results suggest that for useful OP neurotoxins the mechanism of abiotic hydrolysis is concerted (with varying degrees of asynchronicity) when the substrate bears good leaving groups.     

Reactions of O‐aryl S‐aryl dithiocarbonates with secondary alicyclic amines in aqueous ethanol. Kinetics and mechanism

The reactions of O‐(4‐methylphenyl) S‐(4‐nitrophenyl), O‐(4‐chlorophenyl) (4‐nitrophenyl), O‐(4‐chlorophenyl) S‐phenyl, and O‐(4‐methylphenyl) S‐phenyl dithiocarbonates ( 1 , 2 , 3 , and 4 , respectively) with a series of secondary alicyclic (SA) amines are subjected to a kinetic investigation in 44 wt% ethanol‐water, at 25.0 °C and an ionic strength of 0.2 M. The reactions are followed spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients (k_obs) are found. For some of the reactions, plots of k_obs vs. free amine concentration at constant pH are linear but others are nonlinear upwards. This kinetic behavior is in accordance with a stepwise mechanism with two tetrahedral intermediates, one zwitterionic (T^±) and the other anionic (T^?). In some cases, there is a kinetically significant proton transfer from T^± to an amine to yield T^?. Values of the rate micro constants k₁ (amine attack to form T^±), k_?1 (its back step), k₂ (nucleofuge expulsion from T^±), and k₃ (proton transfer from T^± to the amine) are determined for some reactions. The Brønsted plots for k₁ are linear with slopes β₁ = 0.2–0.4 in accordance with the slope values found when T^± formation is the rate‐determining step. The sensitivity of log k₁ and log k_?1 to the pK_a of the amine, leaving and non‐leaving groups are determined by a multiparametric equation. For the reactions of 1 – 4 with 1‐formylpiperazine and those of 3 and 4 with morpholine the k₂ and k₃ steps are rate determining. Copyright © 2010 John Wiley & Sons, Ltd.     

Reactivity of sodium arenesulfinates in the substitution reaction to γ‐functionalized allyl bromides

The kinetics of nucleophilic bimolecular substitution reactions of γ‐functionalized allyl bromides with non‐substituted and p‐substituted sodium arenesulfinates has been studied. Both the structure of allyl bromides and nucleophilicity of arenesulfinate ions exerted a significant effect on the values of the kinetic parameters such as the second‐order rate constants k, activation energy E_A, and changes in the entropy ΔS^≠, enthalpy ΔH^≠, and free energy ΔG^≠ of the formation of the activated complex from reactants. Based on the evaluation of kinetic parameters, the reactants could be arranged, according to their decreasing reactivity in the S_N2‐reactions as follows: p‐toluenesulfinate ion > benzenesulfinate ion > p‐chlorobenzenesulfinate ion and 4‐bromo‐2‐butenenitrile > 1,3‐ dibromopropene, respectively. Comparison was also made between the kinetic data obtained and some delocalization reactivity indexes for both the substrates and nucleophiles. The enthalpy–entropy compensation effect was observed for the reactions of sodium arenesulfinates with γ‐functionalized allyl bromides. Copyright © 2009 John Wiley & Sons, Ltd.     

Prediction of ortho substituent effect in alkaline hydrolysis of substituted phenyl benzoates in aqueous acetonitrile

The second‐order rate constants k (dm³mol^?1s^?1) for alkaline hydrolysis of meta‐, para‐ and ortho‐substituted phenyl esters of benzoic acid, C₆H₅CO₂C₆H₄‐X, in aqueous 50.9% (v/v) acetonitrile have been measured spectrophotometrically at 25 °C. In substituted phenyl benzoates, C₆H₅CO₂C₆H₄‐X, the substituent effects log k_X ? log k_H in aqueous 50.9% acetonitrile at 25 °C for para, meta and ortho derivatives showed good correlations with the Taft and Charton equations, respectively. Using the log k values for various media at 25 °C, the variation of the ortho substituent effect with solvent was found to be precisely described with the following equation: Δlog k_ortho = log k_ortho ? log k_H = 1.57σ_I + 0.93σ°_R + 1.08E_s^B ? 0.030ΔEσ_I ? 0.069ΔEσ°_R, where ΔE is the solvent electrophilicity, ΔE = E_S ? E_H20, characterizing the hydrogen‐bond donating power of the solvent. We found that the experimental log k values for ortho‐, para‐ and meta‐substituted phenyl benzoates in aqueous 50.9% acetonitrile at 25 °C, determined in the present work, precisely coincided with the log k values predicted with the equation (log k^X)_calc = (log k^H_AN)_exp + (Δlog k^X)_calc where the substituent effect (Δlog k^X)_calc was calculated from equation describing the variation of the substituent effect with the solvent electrophilicity parameter, using for aqueous 50.9% CH₃CN the solvent electrophilicity parameter, ΔE = ?5.84. In going from water to aqueous 50.9% CH₃CN, the ortho inductive term grows twice less as compared with the para polar effect. Copyright © 2013 John Wiley & Sons, Ltd.     

Nucleophilic reactivity of thiolate, hydroxide and phenolate ions towards a model O-arylated diazeniumdiolate prodrug in aqueous and cationic surfactant media

The kinetics of aromatic nucleophilic substitution of the nitric oxide‐generating diazeniumdiolate ion, DEA/NO, by thiols (L ‐glutathione, L ‐cysteine, DL ‐homocysteine, 1‐propanethiol, 2‐mercaptoethanol, and sodium thioglycolate) from the prodrug, DNP‐DEA/NO, has been examined in aqueous solution and in solutions of cationic DOTAP vesicles. Second‐order rate constants in buffered aqueous solutions (k_RS‐ = 3.48–30.9 M^?1 s^?1; 30 °C) gave a linear Brønsted plot (β_nuc = 0.414 ± 0.068) consistent with the rate‐limiting S_NAr nucleophilic attack by thiolate ions. Cationic DOTAP vesicles catalyze the thiolysis reactions with rate enhancements between 11 and 486‐fold in Tris‐HCl buffered solutions at pH 7.4. The maximum rate increase was obtained with thioglycolate ion. Thiolysis data are compared to data for nucleophilic displacement by phenolate (k_PhO‐ = 0.114 M^?1 s^?1) and hydroxide (k_OH‐ = 1.82 × 10^?2 M^?1 s^?1, 37 °C) ions. The base hydrolysis reaction is accelerated by CTAB micelles and DODAC vesicles, with the vesicles being ca 3‐fold more effective as catalysts. Analysis of the data using pseudo‐phase ion‐exchange (PIE) formalism implies that the rate enhancement of the thiolysis and base hydrolysis reactions is primarily due to reactant concentration in the surfactant pseudo‐phase. Copyright © 2009 John Wiley & Sons, Ltd.     

Reactivities of acridine compounds in hydride transfer reactions

Reactivities of acridine derivatives (10‐benzylacridinium ion, 1a ⁺, 10‐methylacridinium ion, 1b ⁺, and 10‐methyl‐9‐phenylacridinium ion, 1c ⁺) have been compared quantitatively for hydride transfer reactions with 1,3‐dimethyl‐2‐substituted phenylbenzimidazoline compounds, 2Ha–h . Reactions were monitored spectrophotometrically in a solvent consisting of four parts of 2‐propanol to one part of water by volume at 25 ± 0.1 °C. Reduction potentials have been estimated for acridine derivatives by assuming that the equilibrium constants for the reductions of 1a ⁺ –c ⁺ by 2Hb would be the same in aqueous solution and accepting ?361 mV as the reduction potential of the 1‐benzyl‐3‐carbamoylpyridinium ion. The resulting reduction potentials, E, are ?47 mV for 1a ⁺, ?79 mV for 1b ⁺, and ?86 mV for 1c ⁺. Each of acridine derivatives gives a linear Brønsted plot for hydride transfer reactions. The experimental slopes were compared with those obtained by Marcus theory. This comparison shows that the kinetic data are consistent with a one‐step mechanism involving no high‐energy intermediates. Copyright © 2007 John Wiley & Sons, Ltd.     

A high‐level theoretical study into the atmospheric phase hydration,bond dissociation enthalpies,and acidity of aldehydes

CBS‐Q//B3, G4(MP2), and G4 composite method calculations were used to estimate atmospheric phase standard state (298.15 K, 1 atm) free energies of hydration (Δ_hydrG°_(g)), hydration equilibrium constants (log K_hydr,(g)), bond dissociation enthalpies (BDEs), and enthalpies (Δ_dH°_(g)) and free energies (Δ_dG°_(g)) of aldehydic proton acid dissociation for various substituted aldehydes with electron withdrawing and electron releasing groups. Good quality log K_hydr,(g) correlations with the Swain–Lupton resonance effect parameters R and R⁺ were found, allowing extension of the model to predict log K_hydr,(g) values for 487 substituted aldehydes having available R‐values and 108 substituted aldehydes having available R⁺ values. Good correlations were also found between experimental aqueous phase hydration equilibrium constants (log K_hydr,(aq)) and summative R/R⁺ values for peripheral substituents on a range of carbonyl derivatives (aldehydes, ketones, esters, and amides), suggesting that the structure–reactivity modeling approach can be extended to include all possible combinations of R₁C(O)R₂ carbonyl substitution in both gas and aqueous systems. Computationally derived BDEs and Δ_dH°_(g)/Δ_dG°_(g) were in good agreement with the limited experimental and theoretical datasets. BDEs did not generally correlate with any of the Hammett substituent constants or Swain–Lupton parameters considered. Gas phase acidities exhibited high correlation coefficients with Hammett inductive substituent constants (σ_I) and field effect parameters (F), allowing these to be employed as surrogates for estimating the gas phase aldehydic proton acidities of a larger potential compound range. The resulting models will be of use in predicting the environmental behavior for a broad range of environmentally relevant compounds containing carbonyl functionalities. Copyright © 2016 John Wiley & Sons, Ltd.     

Reactions of O‐aryl S‐aryl dithiocarbonates with pyridines in aqueous ethanol: kinetics and mechanism

The reactions of O‐(4‐methylphenyl) S‐(4‐nitrophenyl) dithiocarbonate ( 1 ), O‐(4‐chlorophenyl) S‐(4‐nitrophenyl) dithiocarbonate ( 2 ), and O‐(4‐chlorophenyl) S‐phenyl) dithiocarbonate ( 3 ) with a series of pyridines were subjected to a kinetic investigation in 44 wt% ethanol–water, at 25.0 °C and an ionic strength of 0.2 M. The reactions were followed spectrophotometrically. Under amine excess, pseudo‐first‐order rate coefficients (k_obs) were determined. For the studied reactions, plots of k_obs versus free pyridine concentration at constant pH were linear, with the slope (k_N) independent of pH. The Brønsted‐type plots for ( 1 ) and ( 2 ) were biphasic, suggesting a stepwise mechanism with a change in the rate‐determining step, from breakdown to the formation of a tetrahedral intermediate (T^±), as the basicity of the pyridines increases. For the reactions of ( 3 ), at the pK_a range of the pyridines studied, only the breakdown to products of T^± was observed. Copyright © 2009 John Wiley & Sons, Ltd.