Reactions Using Solvents

An approach to studying transition states in enzyme-catalyzed reactions using solvent isotope effects. In this treatment, very useful in isotope effect experiments, the relative rates of contributing steps in a multistep reaction are grouped into a fraction referred to as the commitment factor ... 

With the aid of the Bronsted equation [Eq. (15)] the reactivities of simple azoles or benzazoles may be predicted from a knowledge of their pKa values. While this equation was derived for N-methylation reactions using solvents such as DMSO and dimethyl sulfate, it can be applied to estimate nucleophilicity toward other alkylating agents in various solvents provided that (a) the Bronsted / values for the new systems are similar to the old one, and (b) that steric factors are unimportant. The foregoing discussion serves as a guide to indicate when such limitations apply. 

Batch reactions using solvents carried out on a laboratory scale are directly amenable to scale up, since the conditions are defined. 

In recent years, interest has increased sharply in inorganic reactions using solvents other than water. Reactions in liquid ammonia are dts ... 

B. Metal Extraction Reactions Using Solvent-Impregnated Resins... 

New technologies have been applied to this chemistry. Rate acceleration induced by microwave irradiation occurs in examples of difficult reactions. Using solvent-free conditions, ylide 81 was treated with acetophenone 80 to afford 82. This process occurred more efficiently than traditional heating. 

Scheme 7.27 Synthesis of star-iike dibiock poiymer chain the bottom panel depicts the copolymer via CuAAC reaction using solvent- aggregation of PDMA-block-PNIPAM in water assisted collocation of the azide and an at 50°C, which is above the LCST of PNIPAM.

Unfortunately, the number of mechanistic studies in this field stands in no proportion to its versatility" . Thermodynamic analysis revealed that the beneficial effect of Lewis-acids on the rate of the Diels-Alder reaction can be primarily ascribed to a reduction of the enthalpy of activation ( AAH = 30-50 kJ/mole) leaving the activation entropy essentially unchanged (TAAS = 0-10 kJ/mol)" . Solvent effects on Lewis-acid catalysed Diels-Alder reactions have received very little attention. A change in solvent affects mainly the coordination step rather than the actual Diels-Alder reaction. Donating solvents severely impede catalysis . This observation justifies the widespread use of inert solvents such as dichloromethane and chloroform for synthetic applications of Lewis-acid catalysed Diels-Alder reactions. 

Numerous applications have been reported. A derivative of the (alkyn-1-yl)nucleosides 295. which have anticancer and antiviral activities, has been synthesized by this reaction. They are also used as chain-terminating nucleosides for DN.A. sequencing[l98,199]. In this reaction, use of DMF as the solvent is most important for successful operation[200]. Only the alkenyl bromide moiety in 2-bromo-3-aceto.xycycloheptene (296) reacts with alkynes without attacking the allylic acetate moiety[201]. 

Section 16 7 Dialkyl ethers are useful solvents for organic reactions but must be used cautiously due to their tendency to form explosive hydroperoxides by air oxidation in opened bottles... 

A process developed in Israel (263) uses solvent extraction using a higher alcohol or other solvating solvent. This removes phosphoric acid and some hydrochloric acid from the system driving the equiHbrium of equation 42 to the right. The same principle can be appHed in other salt—acid reactions of the form... 

Extraction of hemiceUulose is a complex process that alters or degrades hemiceUulose in some manner (11,138). Alkaline reagents that break hydrogen bonds are the most effective solvents but they de-estetify and initiate -elimination reactions. Polar solvents such as DMSO and dimethylformamide are more specific and are used to extract partiaUy acetylated polymers from milled wood or holoceUulose (11,139). Solvent mixtures of increasing solvent power are employed in a sequential manner (138) and advantage is taken of the different behavior of various alkaUes and alkaline complexes under different experimental conditions of extraction, concentration, and temperature (4,140). Some sequences for these elaborate extraction schemes have been summarized (138,139) and an experimenter should optimize them for the material involved and the desired end product (102). 

By-products include ozonides (17). Other peroxidic products including polymeric peroxides and polymeric ozonides can form, depending on reaction conditions, solvent, and olefin used. A variety of cycHc diperoxides (4) have been obtained by ozonolysis of olefins. Both cis- and... 

The compound R X is a chain-transfer agent, with X usually H or Cl. The net effect of chain transfer is to kill a growing chain and start a new one in its place, thus shortening the chains. Mercaptan chain-transfer agents ate often used to limit molecular weight, but under appropriate conditions, almost anything in the reaction mass (solvent, dead polymer, initiator) can act as a chain-transfer agent to a certain extent. 

Reaction and Heat-Transfer Solvents. Many industrial production processes use solvents as reaction media. Ethylene and propylene are polymerized in hydrocarbon solvents, which dissolves the gaseous reactant and also removes the heat of reaction. Because the polymer is not soluble in the hydrocarbon solvent, polymer recovery is a simple physical operation. Ethylene oxide production is exothermic and the catalyst-filled reaction tubes are surrounded by hydrocarbon heat-transfer duid. 

The benzazetes produced by this method are of insufficient stability for purification from the other components in the pyrolysate. However, they can be washed from the cold receiver using solvents cooled to below -40 °C and their reactions studied. 

Toluene is commonly used. It can be dried by molecular sieves or direct distillation from calcium hydride into the reaction flask. Solvent stored over calcium hydride for several days is usually sufficiently dry to decant directly into the reaction flask, but distillation gives more consistent results. Any solvent with a boiling point sufficiently high to melt sodium is satisfactory. The submitters have also used methyl-cyclohexane and xylene in acyloin condensations. After the sodium is dispersed, the high-boiling solvent can be removed and replaced with anhydrous ether (as noted by the submitters) or can be retained and used in combination with ether (checkers). 

The present method offers several advantages over earlier methods. The use of carbon tetrachloride instead of diethyl ether as solvent avoids the intrusion of certain radical-chain reactions with solvent which are observed with bromine and to a lesser degree with chlorine. In addition, the potassium bromide has a reduced solubility in carbon tetrachloride compared to diethyl ether, thus providing additional driving force for the reaction and ease of purification of product. The selection of bro-... 

Inherent An atmospheric pressure reaction using nonvolatile solvents which is incapable of generating any pressure in the event of a runaway reaction. There is no potential for overpressure of the reactor because of the chemistry and physical properties of the materials. 

An extension ot this reaction provides a number of other perfluorovinylic halides [54] The type of reaction products from the thermal decomposition reaction and the type of hydrocarbon Grignard reagent used in the exchange reaction are solvent-dependent When an excess ot phenylmagnesium bromide is used, a variety of phenylated products are formed depending on the excess amount used [4S (equation 23)... 

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