Choice of solvents

3 Choice of Solvent. - In choosing a suitable solvent, the solubility of the precursor in the solvent is only one factor that influences the interaction of metal complexes and the surface of the support. The other factors that will influence this interaction are as follows  

the solvent must not compete with the precursor for sites on the surface, 

the solvent must not react with the precursor or cause the precursor to decompose. 

While water is used as a solvent in a majority of commercial adsorbent and 

Alcohols, such as methanol and ethanol, has been used as solvents for the metal acetylacetonate systems, usually under reflux conditions. For many of these metal acetylacetonates, these alcohols are good solvents and show only a low affinity for most supports. However, as we discussed earlier, some metal acetylacetonates form p-alkoxy dimers when catalytic amounts of OH are present. The unexpected formation of these dimeric precursors may frustrate attempts to form a supported metal oxide catalysts with isolated metal cations decorating the surface. 

The choice of solvent depends on several factors. Obviously it should be liquid at the reaction temperature and, generally speaking, it is liquid at room temperature and below. Preferably it should be sufficiently volatile to be readily removed by simple distillation. Another important issue, in an industrial setting, is cost. Economic viability of the solvent will very much depend on the value of the product. 

In the context of Green Chemistry, which we are primarily concerned with in this book, there are other major issues which have an important bearing on the choice of solvent. The solvent should be relatively nontoxic and relatively nonha-zardous, e.g. not inflammable or corrosive. The word relatively was chosen with care here, as Paracelsus remarked the poison is in the dosage . 

The solvent should also be contained, that is it should not be released to the environment. In recent years these have become overriding issues in the use of solvents in chemicals manufacture and in other industries. The FDA has issued guidelines for solvent use which can be found on the web site (www.fda.gov/ cder/guidance/index.htm). Solvents are divided into four classes. 

Another class of solvents which presents environmental problems comprises the polar aprotic solvents, such as dimethylformamide and dimethyl sulfoxide, that are the solvents of choice for, e.g. many nucleophilic substitutions. They are high boiling and not easily removed by distillation. They are also water-miscible which enables their separation by washing with water. Unfortunately, this leads inevitably to contaminated aqueous effluent. 

The subject of alternative reaction media (neoteric solvents) also touches on another issue which is very relevant in the context of this book recovery and reuse of the catalyst. This is desirable from both an environmental and an economic viewpoint (many of the catalysts used in fine chemicals manufacture contain highly expensive noble metals and/or (chiral) ligands. 

PRACTICE the skill 3.31 In each of the following cases, identify whether the reagent shown is suitable to accomplish the task described. Explain why or why not. 

Bases stronger than hydroxide cannot be used when the solvent is water. To illustrate why, consider what happens if we mix the amide ion (H2N ) and water  

The amide ion is a strong enough base to deprotonate water, forming a hydroxide ion (HO ). A hydroxide ion is more stable than an amide ion, so the equiUbrium will favor formation of hydroxide. In other words, the amide ion is destroyed by the solvent and replaced with a hydroxide ion. In fact, this is true of any base stronger than HO. If a base stronger than HO is dissolved in water, the base reacts with water to produce hydroxide. This is called the leveling effect. 

Throughout the course, we will see other examples of solvents suitable for working with very strong bases. 

The physical properties of some solvents which have been employed for organomagnesium compounds are listed in Table 2.2 (see also General Ref. [A], p. 45), and notes on the solvent types follow. 

Hydrocarbons. Contrary to popular belief, some Grignard reagents are appreciably soluble in hydrocarbons, but their solubility is often low, and the composition of the solutes is variable, so purely hydrocarbon solvents 

Solutions of solvated organomagnesium compounds in largely hydrocarbon media have also been obtained by the use of hydrocarbon solvents containing small proportions of donors such as THF. 

It should be noted that an active form of magnesium chloride, formed during the reaction of magnesium with alkyl chlorides, may catalyse Friedel-Crafts alkylation of aromatic solvents [1]. 

It is important that the THF used in the still should not be too wet unless freshly opened commercial dry THF is used, it should be predried with, for example, anhydrous magnesium sulfate followed by sodium wire. The predried THF is placed in flask A, together with benzophenone. Sodium or, better, potassium chips are added. When any initial effervescence has subsided, the flask is heated, and refluxing THF is returned to flask A via the three-way tap B. When the THF is dry, the 

When utilizing microwave heating in sealed vessels, it is no longer necessary to use high-boiling solvents, as in a conventional reflux set-up, to achieve a high reaction temperature. With modern instrumentation, it is easily possible to carry out a reaction in methanol at 160 °C (see Fig. 2.8), or a transformation in dichlorometh-ane at 100 °C. 

Microwave-mediated reactions can also be easily carried out without solvents (see Section 4.1). The requirements for these dry media reactions are different to those for reactions in solution. As no solvent is involved, the pressure built-up is rather low, and in most instances such reactions are performed under open-vessel conditions. On the other hand, these mixtures can easily be locally overheated, even though the overall bulk temperature may be comparatively low (macroscopic hotspot formation). Stirring and accurate temperature measurement can prove rather difficult within such a matrix, impeding the investigation of certain reaction conditions. Thus, degradation or decomposition of reagents can be a severe problem for these kind of reactions. 

The criteria for choosing a solvent fall into three classes (i) the solubility of A, B and AB, (ii) those specific to the experimental technique, for example IR or UV transparency when using IR or UV spectrometric methods, and (iii) the avoidance of specific solvation effects. In fact, if a solvent S is specifically associated with species A, B and AB as shown by the equilibrium 

There is some limitation in the choice of solvents which can be used with styrene-divinylbenzene copolymer packings. Widely used solvents are tetra-hydrofuran, chloroform, dichloromethane and toluene. Unsuitable solvents, which will shrink the column packing causing the formation of column voids, channelling, and loss of efficiency, include aliphatic hydrocarbons, lower alcohols and water. 

There is some difference between the various manufacturers products—for example. Polymer Laboratories claim that lOfxm PL-gel can be used with methanol and aliphatic hydrocarbons, but recommend that the column should be packed specifically for this marginal use. 

For the conventional refractive index detector, the choice of solvent depends on three main factors (i) solubility of the sample (ii) compatibility with the column packing and (iii) compatibility with the detector. Samples of low molecular mass generally are soluble in a wide range of solvents. As the molecular mass increases, there may be a decrease in solubility. Thus paraffin waxes are freely soluble in THF at room temperature, but microcrystalline waxes require elevated temperatures for complete dissolution. 

Unlike other forms of liquid chromatography, the separation is not primarily dependent on the nature of the eluent, but rather on the pore size distribution of the column packing, provided that the solvent is of reasonably high polarity. The use of lower polarity solvents may lead to loss of sample components by adsorption on the column, and care should be taken to guard against this when sample peak areas appear unexpectedly low. Tetrahydro-furan is a nearly ideal solvent since it has low viscosity, high solvent power, low refractive index and is water-miscible, and it is therefore recommended as first choice for all separations. 

Solvents should be of high purity—suitable grades can be purchased. Degassing, for example by helium purge, is desirable to remove dissolved gases which may form bubbles in the detector. Freedom from suspended particulate 

As previously discussed, the intensity of cavitational collapse is dependent on the vapour pressure of the solvent and use of involatile solvents is recommended [3], although there are a large number of examples where ethereal solvents have been used to good effect. Other liquid properties such as viscosity and surface tension may have some bearing on the threshold at which cavitation will occur, but these are of minor concern. 

Aqueous sonochemistry is dominated by reactions of OH and H radicals as a consequence of the high vapour pressure of water relative to any organic or inorganic reagents present and despite the enormous amount of attention 

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Experiment 4. Choice of Solvent and Complete Recrystallisation. Students should be supplied with distilled water and with the more common organic solvents, and also with the compounds mentioned below. Taking each compound in turn they should decide, by the methods described in (i) above, which of these six solvents is the best for recrystallisation. They should then recrystallise about 5 g. of at least two of the compounds, dry the product, and whenever possible take its melting-point. 

Choice of Solvent. In general, a higher boiling solvent is preferable to a low-boiling solvent, as evaporation is then considerably less. 

If the solid does not dissolve in the cold solvent gently heat the mixture over a micro-Bunsen burner or in a small water-ba until the liquid boils. Continue to add o-i ml. portions of solvent until the solid dissolves. [If more than about i ml. of solvent is required, the solvent is considered unsatisfactory.] If a clear solution is obtained, cool the tube and scratch it below the surface of the solution with a very fine glass rod and proceed as suggested on p. 16. In general, the products from the choice of solvent investigation are not discarded but added to the main bulk of the crude product for recrystallisation. 

The choice of solvent cannot usually be made on the basis of theoretical considerations alone (see below), but must be experimentally determined, if no information is already available. About 0 -1 g. of the powdered substance is placed in a small test-tube (75 X 11 or 110 X 12 mm.) and the solvent is added a drop at a time (best with a calibrated dropper. Fig. 11, 27, 1) with continuous shaking of the test-tube. After about 1 ml. of the solvent has been added, the mixture is heated to boiling, due precautions being taken if the solvent is inflammable. If the sample dissolves easily in 1 ml. of cold solvent or upon gentle warming, the solvent is unsuitable. If aU the solid does not dissolve, more 11,27,1. solvent is added in 0-5 ml. portions, and again heated to boiling after each addition. If 3 ml. of solvent is added and the substance... 

Selection of solvents. The choice of solvent will naturally depend in the first place upon the solubility relations of the substance. If this is already in solution, for example, as an extract, it is usually evaporated to dryness under reduced pressure and then dissolved in a suitable medium the solution must be dilute since crystallisation in the column must be avoided. The solvents generally employed possess boiling points between 40° and 85°. The most widely used medium is light petroleum (b.p. not above 80°) others are cycZohexane, carbon disulphide, benzene, chloroform, carbon tetrachloride, methylene chloride, ethyl acetate, ethyl alcohol, acetone, ether and acetic acid. 

Reference has already been made to the choice of solvent for introducing the mixture to the column. Generally speaking, adsorption takes place most readily from non-polar solvents, such as petroleum ether or benzene, and least from highly polar solvents such as alcohols, esters and pyridine. Frequently the solvent for introducing the mixture to the column and the developer are so chosen that the same solvent serves the dual purpose. 

Choice of solvent for recrystallisation. Obtain small samples (about 0 5 g.) of the following compounds from the storeroom (i) salicylic acid, (Li) acetanilide, (iii) m-dinitrobenzene, (iv) naphthalene, and (v) p-toluene-sulphonamide. Use the following solvents distilled water, methylated spirit, rectified spirit, acetone, benzene and glacial acetic acid. 

The substance is examined in a dilute solution in a solvent. A wide choice of solvents, transparent to ultraviolet radiation, is available. The paraffin hydrocarbons are all suitable, as are the ahphatic alcohols and the chlorinated hydrocarbons, such as chloroform and carbon tetrachloride. The most useful solvents are re-hexane, cycZohexane, chloro-... 

Activity of Mn02 depends on method of preparation and choice of solvent... 

To isolate polymer chains from one another, we consider a solution which is sufficiently dilute that the domains of the individual polymer molecules are well separated from each other. For the present, we assume the solvent has no influence on the polymer but merely supports the molecule. In fact, this is not generally the case, although it can be achieved by proper choice of solvent or temperature. 

The factor 1 - p/p2 cannot be too close to zero, nor can the refractive index of the polymer and the solvent be too similar. These additional considerations limit the choice of solvents for a synthetic polymer, while their values are optimal for aqueous protein solutions. 

Extraction, a unit operation, is a complex and rapidly developing subject area (1,2). The chemistry of extraction and extractants has been comprehensively described (3,4). The main advantage of solvent extraction as an industrial process Hes in its versatiHty because of the enormous potential choice of solvents and extractants. The industrial appHcation of solvent extraction, including equipment design and operation, is a subject in itself (5). The fundamentals and technology of metal extraction processes have been described (6,7), as has the role of solvent extraction in relation to the overall development and feasibiHty of processes (8). The control of extraction columns has also been discussed (9). 

Emulsives are solutions of toxicant in water-immiscible organic solvents, commonly at 15 ndash 50%, with a few percent of surface-active agent to promote emulsification, wetting, and spreading. The choice of solvent is predicated upon solvency, safety to plants and animals, volatility, flammabiUty, compatibihty, odor, and cost. The most commonly used solvents are kerosene, xylenes and related petroleum fractions, methyl isobutyl ketone, and amyl acetate. Water emulsion sprays from such emulsive concentrates are widely used in plant protection and for household insect control. 

In general, polycarbonate resins have fair chemical resistance to aqueous solutions of acids or bases, as well as to fats and oils. Chemical attack by amines or ammonium hydroxide occurs, however, and aUphatic and aromatic hydrocarbons promote crazing of stressed molded samples. Eor these reasons, care must be exercised in the choice of solvents for painting and coating operations. Eor sheet appHcations, polycarbonate is commonly coated with a sihcone—sihcate hardcoat which provides abrasion resistance as well as increased solvent resistance. Coated films are also available. 

Studies have shown that, in marked contrast to carbanionic polymerisation, the reactivity of the free oxonium ion is of the same order of magnitude as that of its ion pair with the counterion (6). On the other hand, in the case of those counterions that can undergo an equiUbrium with the corresponding covalent ester species, the reactivity of the ionic species is so much greater than that of the ester that chain growth by external attack of monomer on covalent ester makes a negligible contribution to the polymerisation process. The relative concentration of the two species depends on the dielectric constant of the polymerisation medium, ie, on the choice of solvent. 

Silanes react with alkyllithium compounds, forming various alkylsilanes. Complete substitution is generally favored however, less substituted products can be isolated by proper choice of solvent. AH four methylsHanes, vinylsHane [7291-09-1and divinylsilane [18142-56-8] have been isolated from the reaction of SiH and the appropriate alkyllithium compound with propyl ether as the solvent (35). MethylsHane and ethyldisHane [7528-37-2] have been obtained in a similar reaction (36). 

Solution polymerization can use various solvents, primarily aUphatic and aromatic hydrocarbons. The choice of solvent is usually dictated by cost, avaHabihty, solvency, toxicity, flammabiUty, and polymer stmcture. SSBR polymerization depends on recovery and reuse of the solvent for economical operation as well as operation under the air-quaUty perrnitting of the local, state, and federal mandates involved. 

This reaction can be extended to unsaturated nitriles, eg, acrylonitrile, which can give trihalostannyl-functional carboxyHc acids, esters, and amides by the proper choice of solvents and reaction conditions (156). 

Extraction and Extractive Distillation. The choice of an extraction or extractive distillation solvent depends upon its boiling point, polarity, thermal stabiUty, selectivity, aromatics capacity, and upon the feed aromatic content (see Extraction). Capacity, defined as the quantity of material that is extracted from the feed by a given quantity of solvent, must be balanced against selectivity, defined as the degree to which the solvent extracts the aromatics in the feed in preference to paraffins and other materials. Most high capacity solvents have low selectivity. The ultimate choice of solvent is deterrnined by economics. The most important extraction processes use either sulfolane or glycols as the polar extraction solvent. 

Industrial examples of phase-transfer catalysis are numerous and growing rapidly they include polymerisa tion, substitution, condensation, and oxidation reactions. The processing advantages, besides the acceleration of the reaction, include mild reaction conditions, relatively simple process flow diagrams, and flexibiHty in the choice of solvents. 

Polyarylether Ketones. The aromatic polyether ketones are tme thermoplastics. Although several are commercially available, two resins in particular, poly ether ether ketone [31694-16-3] (PEEK) from ICI and poly ether ketone ketone (PEKK) from Du Pont, have received most of the attention. PEEK was first synthesized in 1981 (20) and has been well studied it is the subject of numerous papers because of its potential use in high performance aircraft. Tough, semicrystalline PEEK is prepared by the condensation of bis(4-fiuorophenyl) ketone with the potassium salt of bis(4-hydroxyphenyl) ketone in a diaryl sulfone solvent, such as diphenyl sulfone. The choice of solvent is critical other solvents, such as Hquid HE, promote the reaction but lead to premature low molecular-weight crystals, which do not exhibit sufficient toughness (21). 

Organic Halides. Alkyl halides and aiyl halides, activated by election withdrawing groups (such as NO2) in the ortho or para positions, react with alkyleneamines to form mono- or disubstituted derivatives. Product distribution is controlled by reactant ratio, metal complexation or choice of solvent (16,17). Mixing methylene chloride [75-09-2J and EDA reportedly causes a mnaway reaction (18). 

Additional work showed that the dimerization of arylazirines to 1,3-diazabicy-clo[3.1.0]hex-3-enes is a general reaction which is independent of the nature of the substituent groups attached to the C atom of the azirine ring. Care is required in the choice of solvent, photolysis time and substituents since the 1,3-diazabicyclohexenes are themselves photochemically labile (72JA7788). 

Choice of Solvent The solvent selected will offer the best balance of a number of desirable characteristics high saturation hmit and selec tivity for the solute to be extrac ted, capability to produce... 

Choice of solvents. The best solvents for recrystallisation have the following properties ... 

In general care should be taken in the choice of solvent to ensure that neither the chromatographically separated substances nor their reaction products are soluble in the solvent of the dipping reagent. 

Many researchers choose to buy expensive GPC/SEC columns from one of the major producers because that producer s columns had been used in the past or because of a successful marketing campaign by one particular producer. It should be noted that repacked columns can be obtained for a fraction of the cost of new columns. American Polymer Standards repacked columns are guaranteed to perform just as well as new columns from any company. When a column is repacked the only parts reused are the stainless-steel tube and end caps. This hardware is then repacked using new frits and new ST-DVB gel. Each column is individually tested in a quality control laboratory and shipped in the customer s choice of solvent. American Polymer Standards offers a column repacking service because it is a practical, inexpensive way for customers to acquire state of the art GPC/SEC columns. 

Eor amine-containing polymers, DMF is often a good choice of solvent. DMF can also be a good choice for polymers of higher carboxylic acid content. However, DMF does present some experimental difficulties. It must be run at an elevated temperature, typically 60°C, because of its viscosity. Also, because most polymers have a much lower refractive index response in DMF, the signal-to-noise ratio for a polymer in this solvent is diminished versus the same ratio for common acrylates in THF. 

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