Safer Solvents

Keywords Green chemistry alternative feedstocks benign reagents catalytic synthetic pathways natural processes biocatalysis alternative solvents safer chemicals alternative reaction conditions minimization of energy consumption... 

Often a safer solvent can be substituted for a more hazardous solvent. Safer can be either, or both, of the goals of reducing flammability or toxicity. Two publications have described green solvents. Supercritical carbon dioxide has proved to be an excellent substitute for some organic solvents, particularly if newly designed polymers are added, which can increase solute solubility. ... 

Carbon disulphide should never be used if any alternative solvent is available, as it has a dangerously low flash-point, and its vapours form exceedingly explosive mixtures with air. Ether as a solvent for recrystallisation is much safer than carbon disulphide, but again should be avoided whenever possible, partly on account of the danger of fires, and partly because the filtered solution tends to creep up the walls of the containing vessel and there deposit solid matter by complete evaporation instead of preferential crystallisation. 

High purity acetaldehyde is desirable for oxidation. The aldehyde is diluted with solvent to moderate oxidation and to permit safer operation. In the hquid take-off process, acetaldehyde is maintained at 30—40 wt % and when a vapor product is taken, no more than 6 wt % aldehyde is in the reactor solvent. A considerable recycle stream is returned to the oxidation reactor to increase selectivity. Recycle air, chiefly nitrogen, is added to the air introducted to the reactor at 4000—4500 times the reactor volume per hour. The customary catalyst is a mixture of three parts copper acetate to one part cobalt acetate by weight. Either salt alone is less effective than the mixture. Copper acetate may be as high as 2 wt % in the reaction solvent, but cobalt acetate ought not rise above 0.5 wt %. The reaction is carried out at 45—60°C under 100—300 kPa (15—44 psi). The reaction solvent is far above the boiling point of acetaldehyde, but the reaction is so fast that Httle escapes unoxidized. This temperature helps oxygen absorption, reduces acetaldehyde losses, and inhibits anhydride hydrolysis. 

Bitumen Soluble in Carbon Disuffide (ASTMD4). Asphalt is defined as a mixture of hydrocarbons that are completely soluble in carbon disulfide. Trichloroethylene or 1,1,1-trichloroethane have been used in recent years as safer solvents for this purpose. The procedure for these and other solvents for asphalt with Htde or no mineral matter are described in ASTM D2042. 

A refined grade of MTBE is used ia the solvents and pharmaceutical iadustries. The main advantage over other ethers is its uniquely stable stmctural framework that contains no secondary or tertiary hydrogen atoms, which makes it very resistive to oxidation and peroxide formation. In addition, its higher autoignition temperature and narrower flammabihty range also make it relatively safer to use compared to other ethers (see Table 3). 

Substitution If intensification is not possible, then an alternative is to consider using a safer material in place of a hazardous one. Thus it may be possible to replace flammaole solvents, refrigerants, and heat-transfer media by nonflammable or less flammable (high-boiling) ones, hazardous products by safer ones, and processes which use hazardous raw materials or intermediates by processes which do not. As an example of the latter, the product manufactured at Bhopal (carbatyl) was made from three raw materials. Methyl isocyanate is formed as an intermediate. It is possible to react the same raw materials in a different order so that a different and less hazardous intermediate is formed. 

Hazardous chemicals or mixtures may be replaceable by safer materials. These may be less toxic per se, or less easily dispersed (e.g. less volatile or dusty). Substitution is also applicable to synthesis routes to avoid the use of toxic reactants/solvents or the production, either intentionally or accidentally, of toxic intermediates, by-products or wastes. 

In the laboratory, a range of slush baths may be used for speciality work. These are prepared by cooling organic liquids to their melting points by the addition of liquid nitrogen. Common examples are given in Table 8.2. Unless strict handling precautions are instituted, it is advisable to replace the more toxic and flammable solvents by safer alternatives. 

Benzene is a flammable liquid and its vapors are toxic and explosive. Low concentrations are dangerous on continued inhalation because benzene affects the blood forming function of the bone marrow and it is a cancirogen. Dermatitis may result from repeated skin contact. Alkyl derivatives such as toluene and xylenes are far less toxic and are, therefore, much safer than benzene for use in solvents. Some of the symptoms of benzene poisoning are dizziness, constriction of the chest, and tightening of the leg muscles. 

In many cases, the inherent safety advantages of one process are clear when compared with alternatives. One or more hazards may be significantly reduced, while others are unaffected or only marginally increased. For example, aqueous latex paints are clearly inherently safer than solvent based paints, although there are applications where the increased performance of solvent based paints justifies their use, with the appropriate layers of protection. 

The best way to control exposure is to replace dangerous agents with safer ones. Today, highly toxic solvents, such as benzene, bromobenzenc, carbon tetrachloride, and chloroform are no longer extensively used. Benzene remains, however, an important chemical in the petrochemical industry, but the processes where it is used are clc)sed. 

Morpholinosulfur trifluoride is more thermally stable and therefore safer to handle than DAST and gives a slightly higher yield of the fluoride in the fluorination of cyclohexanol [95, ISO] Both solvent and conformational effects are pronounced in the fluorination of cyclohexanols [550] The chiral (S)-2-(methoxymethyl)pyr-rolidin-l-ylsulfur tnfluonde is an effective enantioselective fluorodehydroxylating agent [ISI]... 

In recent years the realisation of the danger to health from the presence of unreacted formaldehyde monomer in the working environment has led to the development of resins having very low free formaldehyde content, less than 0.5% instead of the usual 2-3%. This produces resins that are safer and less unpleasant to work with, though the solvent blend itself, xylene and butanol, has a very pronounced odour. 

Use safer solvents. Minimal use should be made of solvents, separation agents, and other auxiliary substances in a reaction. 

To an 8-L beaker a solution of 3.78 g (0.630 mol) of ethylene diamine and 0.126 mol of potassium hydroxide in 4.5 L of distilled water are added and stirred with a high-speed mixer. To this, 12.79 g (0.634 mol) of terephthaloyl chloride dissolved in 1 L of methylene chloride (a safer solvent is THF) is added and stirred for 10 min. The suspension is filtered and washed twice with methanol. After drying, the polymer has an of 1.0 as measured in 96% sulfuric acid (0.5% solution at 30°C). The melting temperature of the polymer is 455° C. 

A commercial solution of f-BuLi in pentane may be substituted for t-BuLi in heptane. However, the lower volatility and higher flash-point of the heptane solvent makes t-BuLi in heptane much easier and safer to handle than this alternative. 

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