Kinetics and Mechanism of Monomolecular Heterolysis of Cage-Like Compounds: XVIII. Solvent Effect on the Rate of Heterolysis of 3-Bromocyclohexene. Correlation Analysis of Solvation Effects

The kinetics of E1 dehydrobromination of 3-bromocyclohexene in 23 aprotic and 9 protic solvents were studied by the verdazyl technique. The reaction rate is described by the polarity, electrophilicity, and ionizing power parameters of the solvent. Nucleophilicity, polarizability, and cohesion parameters of the solvent do not affect the reaction rate. The effects of equilibrium and nonequilibrium solvation of the transition state are discussed.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXIV. Solvent Effect on the Heterolysis Rate of 1-Chloro-1-methylcyclohexane. Correlation Analysis of Solvation Effects in Heterolysis of 1-Chloro-1-methylcyclohexane and 1-Chloro-1-methylcyclopentane

The kinetics of heterolysis of 1-chloro-1-methylcyclohexane in 9 protic and 25 aprotic solvents at 25°C were studied by the verdazyl method. The kinetic equation is v = k[RCl] (E1 mechanism). The heterolysis rate of 1-chloro-1-methylcyclohexane in protic solvents is two orders of magnitude lower than that of 1-chloro-1-methylcyclopentane, whereas in low-polarity and nonpolar aprotic solvents the rates are close. A correlation analysis was made to reveal the solvation effects in heterolysis of both chlorides in a set of 9 protic and 25 aprotic solvents, and separately in protic and aprotic solvents.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXX.1 Correlation Analysis of Solvation Effects in Heterolysis of tert-Butyl Chloride

Quantitative analysis of the effect of solvent parameters on the rate of heterolysis of tert-butyl chloride was performed; the reaction rate is fairly described by the polarity, polarizability, and electrophilicity parameters or by the ionizing ability parameter, while the nucleophilicity of the solvent has no rate effect. A negative effect of nucleophilic solvation was revealed in protic solvents.     

Kinetics of reaction of triethylammonium carboxylates with ethyl α‐halogenoacetate in various solvents

The kinetics of the reaction of ethyl α‐halogenoacetate with benzoic acid in the presence of triethylamine in aqueous acetone and in various solvents have been investigated. The rate constant of the reaction is 4–260 times higher in aprotic dipolar solvents than in protic solvents. The simple regression and multiple regression of log k₂ with various solvent parameters have led to the conclusion that in addition to the solvent polarity, various other solvent properties, together or individually, influence the reaction rate. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 894–900, 1999     

Kinetics and Mechanism of Unimolecular Heterolysis of Framework Compounds: XVII. Solvation Effects in Dehydrobromination of <Emphasis Type="Italic">tert</Emphasis>-Butyl Bromide, 1-Bromo-1-Methylcyclohexane,and 2-Bromo-2-methyladamantane in Dipolar Aprotic Solvents

Dehydrobromination rate of tert-butyl bromide, 1-bromo-1-methylcyclohexane, and 2-bromo-2-methyladamantane grows with increasing polarity and dipole moment of solvents. No correlation was found between rate constants of the process and electrophilicity or ionizing power of the solvents. The observed solvation effects are due mainly to dispersion interactions.     

Kinetics and mechanism of unimolecular heterolysis of cage-like compounds: XIX. Effect of the nucleofuge nature on the activation parameters of heterolysis of 1-halo-1-methylcyclohexanes in cyclohexane. Heterolysis rate ratio in aprotic and protic solvents

Heterolysis of 1-bromo-1-methylcyclohexane in cyclohexane (E1 reaction) involves solvation of the transition state (ΔS^≡ = ?81 J mol^?1K^?1), while heterolysis of 1-chloro-1-methylcyclohexane is characterized by desolvation of the transition state (ΔS^≡ = 92 J mol^?1K^?1). The probability for the formation of transition state (interaction between cationoid intermediate and solvent cavity) increases in the first case due to enhanced stability of the solvated intermediate, and in the second, due to reduction in its size. The bromide/chloride heterolysis rate ratio decreases as the ionizing power of aprotic solvent decreases and that of protic solvent increases.     

Kinetics and mechanism of monomolecular heterolysis of commercial organohalogen compounds: XLIII. Solvent effect on activation parameters of dehydrochlorination of 3-chloro-3-methylbut-1-ene. Correlation analysis of solvation effects

The influence of temperature on the rate of dehydrochlorination of 3-chloro-3-methylbut-1-ene in 17 aprotic and 13 protic solvents, ν = k[C₅H₉Cl], was studied by the verdazyl method. In aprotic solvents, the electrophilicity, ionizing power, and cohesion of solvents decrease ΔG ^≠ by increasing ΔS ^≠. The nucleophilicity and polarizability increase both ΔH ^≠ and ΔS ^≠ to equal extent and therefore do not affect ΔG ^≠. In protic solvents, the solvent nucleophilicity increases ΔH ^≠ to a greater extent than ΔS ^≠, and the overall effect of the nucleophilic solvation is small and negative.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXII.1 Solvent Effects on Activation Parameters of Heterolysis of 1-Chloro-1-methylcyclopentane. Correlation Analysis of Solvation Effects

Kinetics of heterolysis of 1-chloro-1-methylcyclopentane in MeOH, BuOH, cyclohexane, i-PrOH, t-BuOH, tert-C₅H₁₁OH, -butyrolactone, MeCN, PhCN, PhNO₂, acetone, PhCOMe, cyclohexanone, and 1,2-dichloroethane at 25-50°C were studied by the verdazyl method. Correlation analysis of solvent effects on activation parameters of the reaction in 8 protic (additionally, AcOH and CF₃CH₂OH) and 8 aprotic solvents together and separately in either group of solvents was performed. In all the solvents studied, two H -S compensation effects were revealed.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXVI.1 Solvent Effect on the Activation Parameters of Heterolysis of 1-Methyl-1-chlorocyclohexane. Correlation Analysis of Solvation Effects in Heterolysis of 1-Methyl-1-chlorocyclohexane and 1-Methyl-1-chlorocyclopentane

The kinetics of heterolysis of 1-methyl-1-chlorocyclohexane in six protic and eight aprotic solvents at 25-50°C was studied by the verdazyl method; v = k[RCl], E1 mechanism. The correlation analysis of the solvent effects on the activation free energy G , enthalpy H , and entropy S of heterolysis of 1-methyl-1-chlorocyclohexane and 1-methyl-1-chlorocyclopentane was performed for the same sets of solvents.     

Photochemistry of 5-aminoquinoline in protic and aprotic solvents

Photophysical properties of 5-aminoquinoline (5AQ) have been investigated in various non-polar and polar (protic and aprotic) solvents using steady state and time resolved fluorescence. In aprotic solvents, the spectral maxima depend on the polarity. However, in protic solvents both the fluorescence intensity as well decay time show decrease depending on the hydrogen bonding ability of the solvent. The results suggest that photochemistry 5AQ is quite sensitive towards the polarity as well as protic character of the solvent.     

Solvophobic Acceleration of a Diels–Alder Reaction in True Solutions in Organic Solvents

The rate of Diels–Alder reaction of diene 9,10‐bis(hydroxymethyl)anthracene with dienophile N‐ethylmaleimide was studied in a series of solvents with different polarity and hydrogen‐bonding ability. Enthalpies and entropies of activation were determined from the temperature dependences of the rate constants. Rate acceleration in nonaqueous protic solvents such as glycerol, propylene, and ethylene glycols was observed. In addition, enthalpy versus entropy of activation plots show a compensation pattern different from the other considered solvents, which can be linked with the solvophobic effects observed in polyhydric alcohols. However, the solvophobic acceleration was not as strong as the hydrophobic acceleration in water. Hydrogen bonding of the reactants and transition state with solvent also influences the reaction rate. The studied reaction is slightly promoted in hydrocarbon solvents in comparison with aprotic polar solvents. This was explained by hydrogen bonding of the hydroxyl groups of diene with dienophile in transition state, which requires prior breaking of the hydrogen bonds of these groups with polar solvent molecules.     

A kinetic investigation of the addition of diazomethane to styrenes. Role of dipolar aprotic solvents in predicting direction of dipole orientation

The addition of diazomethane to a series of substituted styrenes has been kinetically investigated with emphasis on polar, solvent, and solvation effects. Although the reaction is generally insensitive to solvent polarity, a sizeable substituent effect (ρ = +0.90) has been observed. Also, a sizeable increase in rate was generally observed in protic solvents such as water, and a considerable rate enhancement occurred in the case of nitrostyrene in the dipolar aprotic solvent, dimethylformamide. The results are discussed in terms of a concerted mechanism involving a partially-charged transition state II, with negative charge formation on the α-carbon of the styrene. The methoxystyrene offers an exception to the Hammett equation and the unexpectedly high rate constant for this compound could be discussed in terms of a possible participation of the highly nucleophilic resonance form of p-methoxystyrene and an α-addition via the transition state III. However, solvation effects in dimethylformamide (failure to obtain the anticipated rate increase) and the formation of a 3-substituted pyrazoline adduct from the cycloaddition reaction indicate steric control to predominate over electronic factors and β-addition to prevail unequivocally. It is proposed that solvation effects in protic solvents can be utilized to advantage in cycloaddition reactions leading to 1-pyrazolines, for which no other routes are available at the present time.     

Determination of ionic liquids solvent properties using an unusual probe: the electron donor-acceptor complex between 4,4'-bis(dimethylamino)-benzophenone and tetracyanoethene

Michler's ketone (MK) and tetracyanoethene (TCNE) may be used as a UV-vis probe to investigate the solvent properties of ionic liquids (ILs). In molecular solvents, MK and TCNE give an electron donor-acceptor (EDA) complex, a zwitterionic species or a radical ion pair, depending on the aprotic or protic nature of the solvent and on its ionizing power. In agreement with the behavior observed in aprotic solvents, only the EDA complex was detected in ILs bearing low donor anions (beta < 0.7). The formation constant determined in [bmim][Tf(2)N] (K(c) = 5.6 (0.5) M(-1)) is similar to that measured in 1,2-dichloroethane at 25 degrees C. The visible absorption maximum (nu(max,CTC)) of the EDA complex is quantitatively described by a multiple correlation using the Kamlet-Taft pi, beta, and alpha parameters of the solvents. The H-bond donating capacity of ILs is not sufficient to determine the transformation of the EDA complex into the zwitterionic species, but the Kamlet-Taft alpha parameter seems to affect the position of the absorption band. The high ionization power of ILs, moreover, favors the slow dissociation of the EDA complex into its corresponding radical ion pair; this behavior generally characterizes highly polar and highly ionizing protic solvents, such as TFE and HFI. Finally, since the formation of the EDA complex is strongly affected by the presence of impurities, traces of nucleophiles (chloride or amines) or water may be easily detected through the change of the solution color.     

Solvent polarity induced structural changes in 2,6-diamino-9,10-anthraquinone dye

Photophysical properties of 2,6-diamino-9,10-anthraquinone (2,6-DAAQ) dye have been investigated in different solvents and solvent mixtures. The fluorescence quantum yields, fluorescence lifetimes, radiative rate constants, nonradiative rate constants and absorption and fluorescence spectral characteristics show unusual deviations in the lower polarity aprotic solvents in comparison to those in other aprotic solvents of medium to higher polarities. The results indicate that the dye exists in different structural forms in the lower and in the medium to higher polarity solvents. Drawing an analogy with the results reported for other amino-substituted dyes, it is inferred that 2,6-DAAQ dye adopts a planar intramolecular charge transfer (ICT) structure in medium to higher polarity solvents, where the amino lone pairs are in good resonance with the anthraquinone pi-cloud. In the lower polarity solvents, however, the dye is inferred to exist in a nonplanar structure where the amino lone pairs are not in good resonance with the anthraquinone pi-cloud. Due to these structural differences, the dye displays significantly different photophysical behavior in the lower polarity solvents than in the other solvents of medium to higher polarities. Supportive evidence for the above structural changes has been obtained from ab initio quantum chemical calculations on the structures of the dye under different conditions. Unusual deviations in the photophysical properties of 2,6-DAAQ dye in protic solvents in comparison to those in aprotic solvents of similar polarities are attributed to the intermolecular hydrogen bonding effect involving the OH groups of the protic solvents and the quinonoid oxygens of the dye.     

Photophysics of representative ketocyanine dyes: dependence on molecular structure

Spectral properties of a new fluorescent ketocyanine dye have been discussed. The energy of maximum absorption/fluorescence of the dye exhibits bathochromic shift with increasing polarity of the medium. Both dipolarity-polarisability and hydrogen bond donation interaction contribute to solvation of the dye. Study of fluorescence parameters points to existence of different emitting states of the dye for aprotic and protic solvents. While the emitting state is the (1)(π, π*) state for aprotic solvents, fluorescence supposedly take place from a different emitting state involving H-bond formation in the excited state in protic solvents. Fluorescence parameters of the dye have been compared with those for a structurally similar symmetric ketocyanine dye. The faster decay of the dye relative to its symmetric counterpart has been explained as due to an increase of nonradiative decay.     

Effects of Hydrogen-bonding Interaction and Polarity on Emission Spectrum of Naphthalene-Triethylamine in Mixed Solvent

The effects of the protic and aprotic polar solvents on the emission spectrum of the naphthalene-triethyl-amine system in THF were studied under conditions of steady-state illumination. The fluorescence spectrum of the naphthalene-triethylamine system consists of two emission bands, the fluorescence band of naphthalene (band A, 329 nm) and the emission band of the exciplex(band B, 468 nm). The intensities of both the emission bands decrease with increasing the solvent polarity. The intensity of band B also decreases due to the hy-drogen-bonding interaction between triethylamine and protic solvent, while that of band A increases. It is thus suggested that the quenching of naphthalene fluorescence by triethylamine in THF occurs through the charge transfer and electron transfer reactions. The spectral changes upon the increase of solvent polarity can be explained by the dependences of the equilibrium constant between exciplex and ion-pair and the rate constant for the electron transfer reaction from triethyl.amine to the excited naphthalene on the rel.ative permittivity of solvent. It is shown that the formation of intermolecular hydrogen-bonding between triethylamine and protic solvent suppresses the quenching reaction by the decrease in free amine. Acetonitrile has only a polar effect and trichloroacetic acid only a hydrogen-bonding(or protonation) effect, while alcohols have both the effects. The effects of alcohols could be separated into the effects of solvent polarity and intermolecular hy-drogen-bonding interaction quantitatively.     

Solvent effects in the reaction of 2-thiophenesulfonyl chloride with aniline

The second order rate constants k₂ and the activation parameters for the reaction of 2-thiophenesulfonyl chloride with aniline together with solution enthalpies of the reactants have been measured in methanol, ethanol, 2-propanol, acetonitrile and acetone. The reaction rates are slower in dipolar aprotic solvents than in protic ones due to a remarkable activation negative entropy. The rate constants k₂ are correlated with empirical solvent polarity parameters. The data seem in accord with a S_AN reaction mechanism.     

Studies directed toward the synthesis of conformationally restricted neurotransmitter analogs: The addition of N-hydroxyimides to ethyl propiolate

N-Hydroxyimides were found to add readily to ethyl propiolate to yield the imidooxyacrylates in both protic and aprotic solvents. The trans isomer only was formed in aprotic solvents while both isomers were formed in protic solvents.     

Solvent effects and mechanism for a nucleophilic aromatic substitution from QM/MM simulations

The nucleophilic aromatic substitution (SNAr) reaction between azide ion and 4-fluoronitrobenzene has been investigated using QM/MM and DFT/PCM calculations in protic and dipolar aprotic solvents. The effects of solvation on the transition structures, the intermediate Meisenheimer complex, and the rate of reaction are elucidated. The large rate increases in proceeding from protic to dipolar aprotic solvents are only reproduced by the QM/MM methodology.     

The styrylpyridine dye for the silane sol–gel transition studies by time-dependent fluorescence

A trans-4-(p-N,N-dimethylaminostyryl)-N-vinylbenzylpyridinium chloride (vbDMASP) fluorescence probe was optimized in ground and excited state as a function of change in the microenvironment polarity, using the Amsol HyperChem program package. In the calculations, protic and aprotic solvents were used. On this basis a change in the molecule geometry after excitation, depending on the surrounding solvent, was determined.Absorption and steady-state fluorescence spectra of vbDMASP in the solvent of different polarity and in the model water–glycerol solutions were also recorded. On the basis of Stokes’ shift change with the Onsager polarity scale a change in the dipole moment of the probe during transition from ground to excited state, in protic and aprotic solvents was determined.Since during the sol–gel transition of tetraethylorthosilane in the acidic environment both polarity and viscosity of the microenvironment change the vbDMASP probe was applied and fluorescence time-resolved measurements were done. On this basis the correlations between the results of time-resolved measurements for the multichromophoric probe applied in the gelation process and molecular optimization data are discussed.