Effects of solvents

Increase of electron-withdrawing inductive (through-bond) and field (through-space) effects of OH by acylation 

6-Acetalisation increases electron-withdrawing through space effect of 6-OH by constraining C5-C6 to tg conformation, with positive end of C6-06 dipole pointing towards 05 and 01 

Sawada, Y. Takai, C. Chong, T. Hanafusa, S. Misumi and Y. Tsuno, Tetrahedron Lett., 1985, 26, 5065. 

Deslongchamps, Stereolectronic Effects in Organic Chemistry, Perga-mon Press, Oxford, 1983. 

The Anomeric Effect and Related Stereoelectronic Effects at Oxygen, Springer-Verlag, Berlin, 1983 A. J. Kirby, Acc, Chem. Res. 1984, 17, 305. 

It has been demonstrated that the length of the hydrocarbon solvent molecule can have a significant impact of the stability of sterically stabilized nanoparticles [24, 30]. The solvation of a sterically stabilized nanoparticle depends on the interaction parameter, %, between the solvent and the ligand [25, 30-33], such that 

Different solvents used in the electrolyte have been shown to have an impact on the extent of actuation achievable and this may attributed to two main factors  

These two effects were clearly seen in a study undertaken by Kara et al. [36]. In their work, PPy doped with bis(trifluoromethanesulfonyl)imide (TFSI) was actuated in various water/propylene carbonate (PC) solutions containing LiTFSI. The optimum performance of 23.6 % maximum strain at a strain rate of 10.8 % s was achieved within an actuation solution that consisted of 60 % water and 40 % PC. Improvements in both the strain rate and the maximum strain were seen with actuation in LiTFSI electrolytes of water/PC blended solvents over actuation in electrolytes of either water or PC alone. The improved actuation was attributed to the fact that a greater swelling occurred from the PC solvent (enabling a faster and easier ion transfer) and an improvement in the ionic conductivity from the water solvent (enabling a better charge transfer). As such, the optimised performance for this system was realised at 40 % PC. 

A major advantage obtained from ionic liquid electrolytes, however, is a dramatically increased actuation stability on extended cycling. This effect was clearly illustrated in a 

The solvent is the reaction medium and as such, by solvating the ground and activated states, will influence the energetics of the activation process. In addition it acts as a nucleophile in the reaction path represented by Ati. A large value of relative to Atj is observed in solvents capable of coordinating strongly to the metal so that generally the order 

There is naturally an overriding interest in the geometry of the five-coordinate intermediate, or activated complex. General considerations of the shape in which there will be least mutual 

The reaction of ketoximes with acetylene is effectively carried out only in high-polar non hydroxylic solvents, which form (as it was already stated) a superbase medium together with strong bases. Notably, among the solvents tested, DMSO was found to be best (Table 1.1) [159,160], 

the solvent neither changes the solubility of acetylene in the reaction mixture (at 15 atm, DMSO dissolves acetylene only by 1.3-1.5 times better than hexam-etapol [172]) nor alters the polarity of medium (the dipole moments of these two solvents are also similar [173]). Apparently, the effect of solvent is caused by distinctions in specific solvation that activates reactants. In this case, this solvation does not essentially differ from catalysis. 

Similar results have been obtained when studying the reaction of acetophenone oxime with acetylene [160]. So, 2-phenyl- and 2-phenyl-N-vinylpyrroles are formed only in DMSO in sulfolane, these pyrroles are detected only as traces, while in dimethylformamide, hexametapole, dioxane, benzene, and methanol. 

As compared to DMSO, hexametapol and sulfolane form the systems, which catalyze the synthesis of NH- and N-vinylpyrroles from ketoximes and acetylene much less actively. At least, protocols involving these systems are underdeveloped and so far have no high preparative value. In such solvents as ether, alcohols, and hydrocarbons, the reaction does not proceed [4]. 

Interestingly, in aqueous medium, acetylene and ketoximes interact in an absolutely different fashion, delivering pyridines instead of pyrroles [4,174] (see Section 1.5.4). The reaction occurs also with acetylene obtained directly in the autoclave from calcium carbide [174]. 

4 Factors Affecting Cuticular Penetration Rates 6.2.4.1 Effect of Solvent 

Important solvent properties are volatility, viscosity, surface tension, and lipid solubility. The first three determine the area over which a given volume of solvent spreads the larger the area of contact between insecticide and outer cuticle layers, the larger its total penetration rate will be. Acetone does not spread very far from the site of application, because it is so volatile. Lipid solubility affects the dissolution of the wax components of the epicuticle. By disrupting this layer, e.g., depositing a drop of acetone, the insecticide could bypass the epicuticular barrier. All these effects together may explain why an optimal balance of solvent properties is necessary to obtain maximal penetration rates (Welling and Patterson, 1985). 

The solvent has several roles to play in an organic reaction. It must dissolve the reagents so that they can come in contact with one another. It must not react with or decompose any of the reagents. In addition, for reactions that involve ionic or polar molecules (as reactants, intermediates, or products), the polarity of the solvent often dramatically affects the reaction rate. 

Polar solvents help to stabilize ions and polar molecules. To understand the effect of the solvent polarity on reaction rates, the polarity of the reactant must be compared with the polarity of the transition state. The one (reactant or transition state) that is more polar (has more charge separation) will be stabilized more by an increase in the polarity of the solvent. If the transition state is more polar than the reactants, increasing the solvent polarity will stabilize the transition state more than the reactants. This will decrease AG, resulting in a faster reaction. In contrast, if the reactants are more polar than the transition state, increasing the solvent polarity will stabilize the reactants more, resulting in a larger JG and a slower reaction. 

SNI reaction. Because the transition state is more polar (has more charge separation) than the reactant, the change to a more polar solvent stabilizes the transition state more than it stabilizes the reactant. This results in. AG, the activation energy in the less polar solvent, being larger than JG2. the activation energy in the more polar solvent.Therefore, the reaction is faster in the more polar solvent. This diagram applies for all SNI reactions. 

The effect of the solvent polarity on the rate of the SN2 reaction depends on the charge that is initially present on the nucleophile. If the nucleophile has a negative charge, the reaction can be represented as 

The reactants are neutral, whereas charges have partially formed in the transition state. In this situation the transition state is stabilized more than the reactants as the solvent is changed to a more polar one. Therefore, the rate of an SN2 reaction involving a neutral nucleophile is faster in a more polar solvent. 

The solvent influences initiation, propagation, transfer and termination reactions in cationic polymerizations initiated with alkyl-aluminum/coinitiator systems. The solvent affects the dielectric constant, is involved in solvation (particularly of ions) and can act as a transfer agent, terminating agent, and in some instances even as coinitiator. 

Fundamental research into the nature cS the simplest member of the alkylaluminum/coinitiatcv systems, Al(CH3)3/t-C4H9Cl, led to the discovery of an interesting solvent effect which may have an important bearing on the mode of action of these initiator/coinitiator systems in general and on the mechanism of initiation in particular. It was found that the NMR spectrum of AlfCHjlj, which displays two peaks in 

With the Al(CH3)3/t-C4H9Q system, polymerization of isobutylene occurs in methyl chloride solution but not in n-pentane. Methyl chloride may be considered as being a co-coinitiator in profoundly 

On the basis of these facts a theory for the initiation of isobutylene polymerization with trialkylaluminum compounds has been proposed 

The (CH3)3CC1—Al2(CH3)g bond (complex) should be stronger (more stable) than the CH3CI— Al2(CH3)g bond (complex) because the tert-butyl group is a better electron donor than the methyl group. It is conceivable that formation of the (013)3CCl—Al2(CH3)e bond weakens 

Reaction Medium -AH0 (kcal mole-1) AS0 (e.u.) — AF° (kcal mole-1) 

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The relation between the microscopic friction acting on a molecule during its motion in a solvent enviromnent and macroscopic bulk solvent viscosity is a key problem affecting the rates of many reactions in condensed phase. The sequence of steps leading from friction to diflfiision coefficient to viscosity is based on the general validity of the Stokes-Einstein relation and the concept of describing friction by hydrodynamic as opposed to microscopic models involving local solvent structure. In the hydrodynamic limit the effect of solvent friction on, for example, rotational relaxation times of a solute molecule is [ ]... 

Gamarnik A, Johnson B A and Garcia-Garibay M A 1998 Effect of solvents on the photoenolization of omicron-methylanthrone at low temperatures. Evidence for H-atom tunneling from nonequilibrating triplets J. Rhys. Chem. A102 5491... 

Before run ti in g a molecu lar dyn am ics sim ulatioti with solvent and a m olccular median ics meth od, choose the appropriate dielectric con Stan i. You specify th e type an d value of th c dielectric con slari t in thehorce hield Option s dialog box. ITi e dielectric con star t defines the screen irig effect of solvent molecules on nonbonded (electrostalic) in teraction s. 

A til Stan cc-dcpM don 1 diolacLric con sLtiii L is com in on ly used to mimic ihe effect of solvent in moleciiltir mechanics ctilciikilioiis, in the absence ol explicit water molecules. 

Many molecular mechanics potentials were developed at a time when it was computationally impractical to add large numbers of discrete water m olecules to ih e calcu la Lion to sim ulate th e effect of ac ueous media. As such, tech n iq ties cam e into place that were intended to Lake into account the effect of solvent in some fashion. These tech niqiieswcre difficult to justify physically but they were used n cvcrth eless. 

In Chapter 1 mechanistic aspects of Are Diels-Alder reaction are discussed. The literature on the effects of solvents and Lewis-acid catalysts on this reaction is surveyed. The special properties of water are reviewed and the effects of water on the Diels-Alder reaction is discussed. Finally, the effect of water on Lewis acid - Lewis base interactions is described. 

We have investigated the effect of solvents on the enantioselectivity. It turned out that water (74% ee) favours the enantioselectivity of the Cu (L-abrine) catalysed Diels-Alder reaction between Ic and 2 as compared to chloroform (44% ee), ethanol (39% ee), THF (24% ee) and acetonitrile (17% ee). The... 

Some recent general reviews deal with the mechanism of N-nitrosation in aqueous solution (345), the nitrosation of secondary amines (346). the effect of solvent acidity On diazotization (347) and the reactivity of diazonium salts (1691). Therefore, a complete rationalization of the reactivity of amino azaaromatics would be timelv. 

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

Before running a molecular dynamics simulation with solvent and a molecular mechanics method, choose the appropriate dielectric constant. You specify the type and value of the dielectric constant in the Force Field Options dialog box. The dielectric constant defines the screening effect of solvent molecules on nonbonded (electrostatic) interactions. 

A distance-dependent dielectric constant is commonly used to mimic the effect of solvent in molecular mechanics calculations, in the absence of explicit water molecules. 

This kind of perfect flexibility means that C3 may lie anywhere on the surface of the sphere. According to the model, it is not even excluded from Cj. This model of a perfectly flexible chain is not a realistic representation of an actual polymer molecule. The latter is subject to fixed bond angles and experiences some degree of hindrance to rotation around bonds. We shall consider the effect of these constraints, as well as the effect of solvent-polymer interactions, after we explore the properties of the perfectly flexible chain. Even in this revised model, we shall not correct for the volume excluded by the polymer chain itself. 

Effect of Solvents. The most commonly used solvents in poly(amic acid) preparation are dipolar amide solvents such as DMAc and NMP. 

Studies of reaction mechanisms ia O-enriched water show the foUowiag cleavage of dialkyl sulfates is primarily at the C—O bond under alkaline and acid conditions, and monoalkyl sulfates cleave at the C—O bond under alkaline conditions and at the S—O bond under acid conditions (45,54). An optically active half ester (j -butyl sulfate [3004-76-0]) hydroly2es at 100°C with iaversion under alkaline conditions and with retention plus some racemization under acid conditions (55). Effects of solvent and substituted stmcture have been studied, with moist dioxane giving marked rate enhancement (44,56,57). Hydrolysis of monophenyl sulfate [4074-56-0] has been similarly examined (58). 

The effects of a solvent on growth rates have been attributed to two sets of factors (28) one has to do with the effects of solvent on mass transfer of the solute through adjustments in viscosity, density, and diffusivity the second is concerned with the stmcture of the interface between crystal and solvent. The analysis (28) concludes that a solute-solvent system that has a high solubiUty is likely to produce a rough interface and, concomitandy, large crystal growth rates. 

Fig. 2. Effect of solvent on 4-phenyla2o-1-naphthol absorption. A, pyridine B, methanol C, acetic acid.

The effect of solvent concentration on the activity coefficients of the key components is shown in Fig. 13-72 for the system methanol-acetone with either water or methylisopropylketone (MIPK) as solvent. For an initial-feed mixture of 50 mol % methanol and 50 mol % acetone (no solvent present), the ratio of activity coefficients of methanol and acetone is close to unity. With water as the solvent, the activity coefficient of the similar key (methanol) rises slightly as the solvent concentration increases, while the coefficient of acetone approaches the relatively large infinite-dilution value. With methylisopropylketone as the solvent, acetone is the similar key and its activity coefficient drops toward unity as the solvent concentration increases, while the activity coefficient of the methanol increases. 

FIG. 13-72 Effect of solvent concentration on activity coefficients for acetone-methanol system, (a) water solvent, (h) MIPK solvent. 

It was found that the effect of solvents and various surfactants Triton X-100, Twin-80, Brij-35 sodium laurylsulfate, sodium cetylsulfate, cetylpyridinium chloride, cetyltrimethylammonium bromide on the luminescence intensity is insignificant. 

A number of studies have compared normal mode analysis predictions with results from more realistic simulation techniques or experiments. These studies shed light on the nature of the conformational energy surface and the effect of solvent. 

The magnitude of the anomeric effect depends on the nature of the substituent and decreases with increasing dielectric constant of the medium. The effect of the substituent can be seen by comparing the related 2-chloro- and 2-methoxy-substituted tetrahydropy-rans in entries 2 apd 3. The 2-chloro compound exhibits a significantly greater preference for the axial orientation than the 2-methoxy compound. Entry 3 also provides data relative to the effect of solvent polarity it is observed that the equilibrium constant is larger in carbon tetrachloride (e = 2.2) than in acetonitrile (e = 37.5). 

Many other measures of solvent polarity have been developed. One of the most useful is based on shifts in the absorption spectrum of a reference dye. The positions of absorption bands are, in general, sensitive to solvent polarity because the electronic distribution, and therefore the polarity, of the excited state is different from that of the ground state. The shift in the absorption maximum reflects the effect of solvent on the energy gap between the ground-state and excited-state molecules. An empirical solvent polarity measure called y(30) is based on this concept. Some values of this measure for common solvents are given in Table 4.12 along with the dielectric constants for the solvents. It can be seen that there is a rather different order of polarity given by these two quantities. 

Scheme 4.4. Effect of Solvent Polarity on Reactions of Various Charge Types...

The authors also investigated the effect of solvent composition on the retention of a series of solutes including a dispersion of silica smoke (mean particle diameter 0.002 pm). The silica smoke was used to simulate a solute of very large molecular size... 

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