Diazomethane

Diazomethane is the chemical compound CH₂N₂. In the pure form at room temperature, it is a yellow gas, but it is almost universally used as a solution in diethyl ether. It is one of the more common diazo compounds. It is also toxic and potentially explosive.

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Preparation^[1]

CH₂N₂ is usually prepared as a solution in diethyl ether and used for converting carboxylic acids into their methyl esters or into their homologues (see Arndt-Eistert synthesis). In the Buchner-Curtius-Schlotterbeck reaction (1885) diazomethane reacts with an aldehyde to form ketones. Diazomethane is also frequently used as a carbene equivalent. Diazomethane is prepared in the laboratory at mmol scale from precursors such as Diazald or N-methyl-N-nitroso-p-toluenesulfonamide and MNNG or 1-methyl-3-nitro-1-nitrosoguanidine. Diazald in a solution of diglyme and diethyl ether reacts with a warm aqueous solution of sodium hydroxide and the generated CH₂N₂ is collected by distillation. Diazomethane is liberated from a solution of MNNG in diethyl ether by addition of aqueous potassium hydroxide at low temperatures.

CH₂N₂ reacts with basic solutions of²H₂O to give the deuterated derivative C²H₂N₂.^[2]

Assay

The concentration of CH₂N₂ can be determined in either of two convenient ways. It can be treated with an excess of benzoic acid in cold Et₂O. Unreacted benzoic acid is then assayed using titration with standard NaOH. Alternatively, the concentration of CH₂N₂ in Et₂O can be determined spectrophotometrically at 410 nm where its extinction coefficient, ε, is 7.2.

Other diazomethane compounds

Many substituted derivatives of diazomethane have been prepared:

• The very stable (CF₃)₂CN₂ (b.p. 12–13 °C),^[3]
• Ph₂CN₂ (m.p. 29–30 °C).^[4]
• (CH₃)₃SiCHN₂, which is commercially available as a solution and is as effective as CH₂N₂ for methylation.^[5]
• PhC(H)N₂, a red liquid b.p.< 25 °C at 0.1 mm Hg.^[6]

Safety

Diazomethane is toxic by inhalation or by contact with the skin or eyes (TLV 0.2ppm). Symptoms include chest discomfort, headache, weakness and, in severe cases, collapse.^[7] CH₂N₂ may explode when in contact with ground-glass joints or when heated to about 100.0 °C. Consequently specialized, scratch-free glassware and a blast shield should be employed for its use.

References

1. ^ J. A. Moore; D. E. Reed “Diazomethane” Organic Syntheses Collective Volume 5, page 351
2. ^ P. G. Gassman and W. J. Greenlee “Dideuterodiazomethane” Organic Syntheses Collective Volume 6, page 432.
3. ^ W. J. Middleton; D. M. Gale “Bis(Trifluoromethyl))diazomethane” Organic Syntheses Collective Volume 6, page 161
4. ^ L. I. Smith, K. L. Howard “Diphenyldiazomethane” Organic Syntheses Collective Volume 3, page 351
5. ^ T. Shioiri, T. Aoyama, S. Mori “Trimethylsilyldiazomethane” Organic Syntheses Collected Volume, Volume 8, page 612.
6. ^ X. Creary “Tosylhydrazone Salt Pyrolyses: Phenydiazomethanes” Organic Syntheses, Coll. Vol. 7, p.438 (1990); Vol. 64, p.207 (1986).
7. ^ Muir, GD (ed.) 1971, Hazards in the Chemical Laboratory, The Royal Institute of Chemistry, London.