Solvent-modified reaction

The model offers insights on several key points regarding the role of the solvent. But in the final analysis it cannot substitute for realistic dynamical simulations. In particular, the model overlooks any role played the internal modes of the solvent molecules or the internal modes of the solute. What the model provides is insight into the motion along the solvent-modified reaction coordinate. Specifically, the diffusion forward and backward across the barrier, that in the Kramers modef occurs along the reaction coordinate of the isolated solute, is shown in the model to be equivalent to a single crossing of the transition state provided that we use a collective reaction coordinate, one that involves both solute and solvent motion. 

The classical procedure for the Wolff-Kishner reduction—i.e. the decomposition of the hydrazone in an autoclave at 200 °C—has been replaced almost completely by the modified procedure after Huang-Minlon The isolation of the intermediate is not necessary with this variant instead the aldehyde or ketone is heated with excess hydrazine hydrate in diethyleneglycol as solvent and in the presence of alkali hydroxide for several hours under reflux. A further improvement of the reaction conditions is the use of potassium tcrt-butoxide as base and dimethyl sulfoxide (DMSO) as solvent the reaction can then proceed already at room temperature. ... 

This theory is associated in its early protonic form with Franklin (1905, 1924). Later it was extended by Germaim (1925a,b) and then by Cady Elsey (1922,1928) to a more general form to include aprotic solvents. Cady Elsey describe an acid as a solute that, either by direct dissociation or by reaction with an ionizing solvent, increases the concentration of the solvent cation. In a similar fashion, a base increases the concentration of the solvent anion. Cady Elsey, in order to emphasize the importance of the solvent, modified the above defining equation to ... 

Supported Co, Ni, Ru, Rh, Pd and Pt as well as Raney Ni and Co catalysts were used for the hydrogenation of dodecanenitrile to amines in stirred SS autoclaves both in cyclohexane and without a solvent. The reaction temperature and the hydrogen pressure were varied between 90-140 °C and 10-80 bar, respectively. Over Ni catalysts NH3 and/or a base modifier suppressed the formation of secondary amine. High selectivity (93-98 %) to primary amine was obtained on Raney nickel, Ni/Al203 and Ru/A1203 catalysts at complete nitrile conversion. With respect to the effect of metal supported on alumina the selectivity of dodecylamine decreased in the order Co Ni Ru>Rh>Pd>Pt. The difference between Group VIII metals in selectivity can be explained by the electronic properties of d-band of metals. High selectivity to primary amine was achieved on base modified Raney Ni even in the absence of NH3. 

The oxaspirocyclization was applied to the synthesis of theaspirone and vitispirane (equations 26 and 27)59. Under slightly modified reaction conditions where water is employed as the major solvent, palladium-catalyzed 1,4-oxidation of 64 afforded 65. Alcohol 65 was oxidized to theaspirone, which was obtained as a 1 1 isomeric mixture of cis and trans isomers. When the analogous reaction was performed at a lower pH by the use of trifluoroacetic acid, vitispirane was formed in high yield, again as a 1 1 isomeric mixture of stereoisomers. 

The benefits from tuning the solvent system can be tremendous. Again, remarkable opportunities exist for the fruitful exploitation of the special properties of supercritical and near-critical fluids as solvents for chemical reactions. Solution properties may be tuned, with thermodynamic conditions or cosolvents, to modify rates, yields, and selectivities, and supercritical fluids offer greatly enhanced mass transfer for heterogeneous reactions. Also, both supercritical fluids and near-critical water can often replace environmentally undesirable solvents or catalysts, or avoid undesirable byproducts. Furthermore, rational design of solvent systems can also modify reactions to facilitate process separations (Eckert and Chandler, 1998). 

Solvent and additives. Several systems have been studied concerning solvent effects. Fig. 6 shows that quite small changes in substrate, modifier or reaction conditions can lead to rather different results. Generally, very good results are obtained in apolar solvents with dielectric constants of 2-6. But in some cases alcohols can give equally high ee s. An important conclusion is that the optimal modifier concentration is dependent on solvent, modifier and substrate type [33]. The addition of amines and weak acids can affect the enantioselectivity [31,33]. 

Aromatic aldehydes (10 mmol) and trimethylorthoformate (20 mmol) was added to a mixture of sulfonamide (10 mmol), finely powdered calcium carbonate (9 g) and K-10 clay (2 g). The solid homogenized mixture was placed in a modified reaction tube which was connected to a removable cold finger and sample collector to trap the ensuing methanol and methyl formate. The reaction tube is inserted into Maxidigest MX 350 (Prolabo) microwave reactor equipped with a rotational mixing system. After irradiation for a specified period, the contents were cooled to room temperature and mixed thoroughly with ethyl acetate (2 x 20 mL). The solid inorganic material was filtered off and solvent was evaporated to afford tlie residue which was crystallized from the mixture of hexane and ethyl acetate. 

The reaction has been found unpredictable in practice, 2 out of 3 attempts leading to eruption or explosion of the flask contents [1]. However, using a modified version of a published procedure [2], with methanol as solvent, the reaction can be performed without incident, provided that the working scale is restricted to one third of that published (and initially one twelfth until one gains experience of the reaction) that a flanged reaction flask is used and that the rate of addition of chloroform to the methanolic potassium sulfide is carefully controlled to A—5... 

Thus, copolymers of the same composition can have qualitatively different sequence distributions depending on the solvent in which the chemical transformation is performed. In a solvent selectively poor for modifying agent, hydrophobically-modified copolymers were found to have the sequence distribution with LRCs, whereas in a nonselective (good) solvent, the reaction always leads to the formation of random (Bernoullian) copolymers [52]. In the former case, the chemical microstructure cannot be described by any Markov process, contrary to the majority of conventional synthetic copolymers [ 10]. 

In this chapter we consider chemical reactions in solution first, how solvents modify the potential energy surface of the reacting molecules, and second the role of diffusion. The reactants of bimolecular reactions are brought into contact by diffusion, and there will therefore be an interplay between diffusion and chemical reaction that determines the overall reaction rate. The results are as follows. 

With regard to the use of protease in the synthetic mode, the reaction can be carried out using a kinetic or thermodynamic approach. The kinetic approach requires a serine or cysteine protease that forms an acyl-enzyme intermediate, such as trypsin (E.C. 3.4.21.4), a-chymotrypsin (E.C. 3.4.21.1), subtilisin (E.C. 3.4.21.62), or papain (E.C. 3.4.22.2), and the amino donor substrate must be activated as the ester (Scheme 19.27) or amide (not shown). Here the nucleophile R3-NH2 competes with water to form the peptide bond. Besides amines, other nucleophiles such as alcohols or thiols can be used to compete with water to form new esters or thioesters. Reaction conditions such as pH, temperature, and organic solvent modifiers are manipulated to maximize synthesis. Examples of this approach using carboxypeptidase Y (E.C. 3.4.16.5) from baker s yeast have been described.219... 

The most common sc-fluid for industrial processing and benchtop research is supercritical carbon dioxide, chosen because of its moderate and easily attained critical temperature and pressure and its non-toxicity. Reactions in SC-CO2 produce similar results as reactions in nonpolar organic solvents. Its solvent polarity, empirically determined using solvatochromic dyes as polarity indicators (see Section 7.4), corresponds to that of hydrocarbons such as cyclohexane [221, 222]. Carbon dioxide has no dipole moment and only a small quadrupole moment, a small polarizabihty volume, and a low relative permittivity (er = 1.4-1.6 at 40 °C and 108-300 bar) [221, 223]. Thus, SC-CO2 is only suitable as a solvent for nonpolar substances, which unfortunately imposes considerable limitations on its practical applications. To overcome this limitation, more polar co-solvents (modifiers) such as methanol can be added to SC-CO2. 

Recently, some modified reaction conditions have been proposed to improve yields, particularly for aromatic carboxylic acids. A phosphonium anhydride reagent (11) made from triphenylphosphine oxide and triflic anhydride appears to induce reaction at low temperatures in common solvents giving high yields of 2-arylbenzi midazoles in 30-60 min at room temperature (Scheme 3.1.12) [78]. 

When SAS are present in solution, pesticide compounds partition between the bulk aqueous solution and the (sub)micellar phase (Figure 17). This partitioning may affect the overall rates and products of transformation of these compounds if their rates of reaction in the (sub)micellar phase are significantly different from those in the aqueous phase (Barbash, 1987 Macalady and Wolfe, 1987 Barbash and Resek, 1996). In some cases, relatively minor variations in SAS stmcture can have substantial impacts on pesticide transformation rates (Kamiya et ai, 1994). Even if reaction rates are not substantially different in the (sub)mi-cellar phase, however, the presence of SAS may modify reaction rates in solution for sparingly soluble pesticide compounds by simply increasing their dissolved concentrations, as may occur in the presence of polar solvents (e.g., Barbash and Reinhard, 1989a Schwarzenbach et al. 

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