Quenching, reaction hazard

The means by which a quench system works depends on the nature of the reactive material e.g., for water-reactive materials, a quench system will destroy the material in a last-resort situation and generally form less-hazardous products, and will at the same time absorb some of the heat of reaction. Most quench systems are designed to both cool down and dilute a material that may be reacting uncontrollably the quenching medium may also actually interfere with the chemical reaction or deactivate a catalyst. 

Three years after Narasaka and Pai s disclosure, Prasad et al. developed a modified procedure to improve syn -diastereoselecti vi ty in the reduction of certain (3-hydroxy ketones6 (Scheme 4.Id). When methoxydiethylborane, in lieu of tributylborane, reacts with p-hydroxy ketones at —70 C in anhydrous methanol, the complex 5BEt2 is formed. Subsequent treatment of the complex with sodium borohydride and quenching the reaction mixture with acetic acid affords yyn-diols in excellent levels of diastereoselectivity regardless of the structure of p-hydroxy ketones. Another practical advantage of Prasad et al. s modification may be an enhanced safety feature, as methoxydiethylborane is generally less hazardous to handle than triethylborane.6... 

Addition of N-(benzyloxycarbonyl)-(S)-proline methyl ester to phenylmagnesium chloride to give (S)-a,a-diphenyl-2-pyrrolidinemethanol (3 steps, 5 isolations, 0-50% yield from (S)-proline) ref. 5e and 5g. It should be noted that 6-10 equiv of the Grignard reagent are required to drive this reaction to completion. Quenching the excess phenylmagnesium bromide affords benzene-an environmental and health hazard. 

G. An Example of Reaction-Based Hazard Identification Reaction Quench. 241... 

Any hazard evaluation must thus include an examination of all phases of the process. As with the example above, one area which is often ignored involves the quenching, scrubbing, and disposal of reactions. It must be remembered that these reactions suffer the same consequences of scale-up as the more productive components of the process. 

Sulfonyl Azides. Alkyl- and arylmagnesium halides,305 306 as well as alkyl-307, aryl- (see Eq. 70),308-312 and heteroaryllithium313 reagents add to sulfonyl azides to give triazene salts which may be reduced to amines 305 310 312 Gr converted into azides. The latter reaction has been accomplished by an aqueous workup with the highly hindered 2,6-dimesitylphenyl azide,314 whereas quenching with aqueous potassium hydroxide (see Eq. 72)305,315 sodium bicarbonate,313 or sodium pyrophosphate305,316 (see Eqs. 67 and 74) is necessary with other arenesulfonyl azide adducts. Thermolysis of the dry triazene salts also leads to azides,307,308 but because of the hazards involved, this procedure is not recommended. 

As the kTw value for water at 250 °C is three orders of magnitude higher than under ambient temperatures, this makes HTW and SCW attractive solvents for performing acid- and base-catalysed reactions and reduces the need to add external acids or bases to the solution. Furthermore, cooling serves to neutralise the solution, which provides a relatively straightforward approach to reaction quenching. This is an attractive feature for industry, which often relies upon the addition of mineral acids or solid-state acids to catalyse reactions. The circumvention of the need for external acid catalysts therefore reduces the quantity of hazardous byproducts that require neutralisation. Examples of acid-catalysed reactions in HTW and SCW include acylations, condensations, cyclisations, eliminations and hydrolyses , which have been recently reviewed elsewhere. ... 

If we assume that the reaction rate of 35 W kg" at 90°C doubles with every 10 K rise in temperature then at the boiling point (140°C) the rate will be an uncontrollable 1120 W kg". There are many engineering solutions to this potential hazard dump tanks, quenching, independent high-capacity condensers, venting to a catch tank and so on. All of these solutions are expensive and must work reliably on the rare occasions they will be required. 

The reaction of potassium superoxide with water is the single most important chemical source of oxygen for breathing purposes in hospitals, mines, submarines (Clarke 1956), and space capsules (Bovard 1960). Even if only a small fraction of singlet oxygen survives quenching, it could prove to be a serious health hazard (Khan 1970). 

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