bottle containing an aqueous solution of a reducing agent, such as ferrous sulfate or sodium bisulfite. The material can then be handled as a waste chemical; however, it must not be mixed with other chemicals for disposal. Spilled peroxides should be absorbed on vermiculite or other absorbent as quickly as possible. The vermiculite-peroxide mixture can be burned directly or may be stirred with a suitable solvent to form a slurry that can be treated according to institutional procedures. Organic peroxides should never be flushed down the drain.

Large quantities (more than 25 g) of peroxides require special handling. Each case should be considered separately, and handling, storage, and disposal procedures should be determined by the physical and chemical properties of the particular peroxide (see also Hamstead, 1964).

Peroxides can be formed during storage of some materials in air, and a peroxide present as a contaminant in a reagent or solvent (e.g., 1,4-dioxane) can be very hazardous and change the course of a planned reaction. Especially dangerous are ether bottles that have evaporated to dryness. Excluding oxygen by storing potential peroxide-formers under an inert atmosphere (N2 or argon) or under vacuum greatly increases their safe storage lifetime. In many instances, it is possible to purchase the chemical stored under nitrogen in septum-capped bottles. In some cases, stabilizers or inhibitors (free-radical scavengers that terminate the chain reaction) are added to the liquid to extend its storage lifetime. Because distillation of the stabilized liquid removes the stabilizer, the distillate must be stored with care and monitored for peroxide formation. Furthermore, HPLC-grade solvents generally contain no stabilizer, and the same considerations apply to their handling.

(Also see Chapter 3, section 3.D.3.2; for disposal information, see Chapter 7, section 7.D.2.5.)

5.G.4 Explosive Gases and Liquefied Gases

A substance is more concentrated in the form of a liquefied gas than in the vapor phase and may evaporate extremely rapidly. Contact with liquid oxygen, in particular, may introduce extreme risk. Liquefied air is almost as dangerous as liquid oxygen because the nitrogen boils away, and as it does, it leaves an increasing concentration of oxygen. Other cryogenic liquids, such as nitrogen and helium, if they have been open to air, may have absorbed and condensed enough atmospheric oxygen to be very hazardous. When a liquefied gas is used in a closed system, pressure may build up, so that adequate venting is required. Relief devices are required to prevent this dangerous buildup of pressure. If the liquid is flammable (e.g., hydrogen), explosive concentrations in air may develop. Because flammability, toxicity, and pressure buildup may become serious when gases are exposed to heat, gases should be stored only in specifically designed and designated areas (see Chapter 8, section 8.E).

5.G.5 Hydrogenation Reactions

Hydrogenation reactions are often carried out under pressure with a reactive catalyst and so require special attention. Along with observation of the precautions for the handling of gas cylinders and flammable gases, additional attention must be given to carrying out hydrogenation reactions at pressures above 1 atm. The following precautions are applicable:

  • Make sure that the autoclave, pressure bottle, or other apparatus is appropriate for the experiment. Most preparative hydrogenations of substances such as alkenes can be carried out safely in a commercial hydrogenation apparatus using a heterogeneous catalyst (e.g., Pt and Pd) under moderate (<80 psi H2) pressure.

  • Review the operating procedures for the apparatus, and inspect the container before each experiment. Glass reaction vessels are subject to scratches or chips that render them unsuitable for use under pressure. Never fill the vessel to capacity with the solution; filling it about half full (or less) is much safer.

  • One of the most important precautions to be taken with any reaction involving hydrogen is to remove as much oxygen from the solution as possible before adding hydrogen. Failure to do this could result in an explosive oxygen-hydrogen (O2/H2) mixture. Normally, the oxygen in the vessel is removed by pressurizing the vessel with inert gas (N2 or argon), followed by venting the gas. If available, vacuum can be applied to the solution. Repeat this procedure of filling with inert gas and venting several times before the hydrogen or other high-pressure gas is introduced.

  • Do not approach the rated safe pressure limit of the bottle or autoclave, with due regard to increased pressure upon heating. A limit of 75% of the rating in a high-pressure autoclave is advisable, but if this limit is exceeded accidentally, replace the rupture disk upon completion of the experiment.

  • Monitor the pressure of the high-pressure device periodically as the heating proceeds to avoid too high a pressure in case of unintentional overheating.

  • Purge the system of hydrogen by repeated "rinsing" with inert gas at the end of the experiment to avoid producing hydrogen-oxygen mixtures in the presence of the catalyst during work-up. Handle cata-

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