Diazomethane is an organic chemical compound with the formula CH2N2, discovered by German chemist Hans von Pechmann in 1894. It is the simplest diazo compound. In the pure form at room temperature, it is an extremely sensitive explosive yellow gas; thus, it is almost universally used as a solution in diethyl ether. The compound is a popular methylating agent in the laboratory, but it is too hazardous to be employed on an industrial scale without special precautions.<ref name=proctor>Template:Cite journal</ref> Use of diazomethane has been significantly reduced by the introduction of the safer and equivalent reagent trimethylsilyldiazomethane.<ref>Template:Cite encyclopedia</ref>
For safety and convenience diazomethane is always prepared as needed as a solution in ether and used as such. It converts carboxylic acids to methyl esters and phenols into their methylethers. The reaction is thought to proceed via proton transfer from carboxylic acid to diazomethane to give a methyldiazonium cation, which reacts with the carboxylate ion to give the methyl ester and nitrogen gas. Labeling studies indicate that the initial proton transfer is faster than the methyl transfer step.<ref>Template:Cite journal</ref> Since proton transfer is required for the reaction to proceed, this reaction is selective for the more acidic carboxylic acids (pKa ~ 5) and phenols (pKa ~ 10) over aliphatic alcohols (pKa ~ 15).<ref>Template:Cite book</ref>
A wide variety of routes have been developed for the laboratory production of diazomethane.<ref name=Liquizald>Template:Cite journal</ref> In general, the synthesis of these all involves the addition of methylamine to an electron-deficient species, before treatment with nitrite and mineral acid (nitrous acid) to form an N-methyl nitrosamide. Diazomethane is prepared by hydrolysis of an ethereal solution of these N-methyl nitrosamides with aqueous base. Examples include:
N-Nitroso-β-methylaminoisobutyl methyl ketone (Liquizald), another early precursor which remains in use in the present day.<ref name=Liquizald /><ref>Template:Cite journal</ref>
The ease with which diazomethane explodes makes it too hazardous to handle in large quantities. Despite this, it can be used on an industrial scale using on-demand flow chemistry. In these processes the rate of production is matched by the rate of consumption, such that the amount of diazomethane present at any one time is very low.<ref>Template:Cite journal</ref><ref name=proctor />
Diazomethane is both isomeric and isoelectronic with the more stable cyanamide, but they do not interconvert.
Many substituted derivatives of diazomethane have been prepared:
The very stable (CF3)2CN2 (2-diazo-1,1,1,3,3,3-hexafluoropropane; b.p. 12–13 °C),<ref>Template:Cite journal</ref>
Diazomethane is toxic by inhalation or by contact with the skin or eyes (TLV 0.2 ppm). Symptoms include chest discomfort, headache, weakness and, in severe cases, collapse.<ref name="HitCL">Muir, GD (ed.) 1971, Hazards in the Chemical Laboratory, The Royal Institute of Chemistry, London.</ref> Symptoms may be delayed. Deaths from diazomethane poisoning have been reported. In one instance a laboratory worker consumed a hamburger near a fumehood where he was generating a large quantity of diazomethane, and died four days later from fulminating pneumonia.<ref name="HitC1">LeWinn, E.B. "Diazomethane Poisoning: Report of a fatal case with autopsy", The American Journal of the Medical Sciences, 1949, 218, 556-562.</ref> Like any other alkylating agent it is expected to be carcinogenic, but such concerns are overshadowed by its serious acute toxicity.
CH2N2 may explode in contact with sharp edges, such as ground-glass joints, even scratches in glassware.<ref>Template:Cite journal</ref> Glassware should be inspected before use and preparation should take place behind a blast shield. Specialized kits to prepare diazomethane with flame-polished joints are commercially available.
The compound explodes when heated beyond 100 °C, exposed to intense light, alkali metals, or calcium sulfate. Use of a blast shield is highly recommended while using this compound.
Proof-of-concept work has been done with microfluidics, in which continuous point-of-use synthesis from N-methyl-N-nitrosourea and 0.93 M potassium hydroxide in water was followed by point-of-use conversion with benzoic acid, resulting in a 65% yield of the methyl benzoate ester within seconds at temperatures ranging from 0 to 50 °C. The yield was better than under capillary conditions; the microfluidics were credited with "suppression of hot spots, low holdup, isothermal conditions, and intensive mixing."<ref>Template:Cite book</ref>
