Self-Flammable Metal-Organic Compounds

Compounds of metals and alkyl or aryl groups are frequently spontaneously ignitible in air, especially the lower members of homologous scries, such as the methyl and ethyl compounds. The metals in the most active compounds are the alkali metals and aluminum, zinc, and arsenic. The nonmetals boron and phosphorus also furnish active members, which conventionally are classed with the metal-organic (sometimes called organo-metallic) compounds. It is noteworthy that the alkyls of silicon do not flame in air. 

The first discovered members of this class were dimethyl zinc and diethyl zinc (Frankland, 1849). They are relatively easy and safe to handle—for example, blanketed by carbon dioxide gas with which they do not react, whereas, remarkably, beryllium and magnesium alkyls can react under glow with carbon dioxide gas. Many of these compounds will react explosively with water and also with carbon tetrachloride. However, spontaneous ignitibility in air and stability toward various reagents may well occur with the same compound. 

Borderline cases of spontaneous ignition are frequent and it should be kept in mind that probably no substance can unequivocally be called spontaneously ignitible in air. Three parameters must be taken into account temperature, air pressure, and dryness of the air. Even if spontaneous combustion occurs at extremely low ambient temperature, the reaction may be slow or uncertain enough to make it useless or impractical in a device such as a flame thrower. Experimental addition of the more active alkyls to liquid fuels and pairing the manageable ones with oxidizers as high-energy rocket fuels seem to be the only practical applications found for these metal alkyls. 

By strict definition, alkyls of boron and phosphorus are not metal- 

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