the very simple acyclic diazo compound CH₂N₂; a highly toxic and explosive yellowish gas with an unpleasant odor. Melting point, -145°C; boiling point, -23° C. The structure of diazomethane can be represented by the following formulas: .

Diazomethane is usually produced by treating nitrosomethylurea (1) or nitrosomethylurethane (2) with an alkali:

Diazomethane may react with or without the liberation of nitrogen. The most important reactions of diazomethane involve the liberation of N₂: the generation of carbene upon pyrolysis or photolysis of diazomethane or under the catalytic action of powdered copper:

(3) CH₂N₂→:CH₂+N₂

reactions with acids, phenols, and alcohols, with the formation of corresponding methyl ethers and esters, such as

(4) CH₃COOH + CH₂N₂→CH₃COOCH₃ + N₂

and interaction with halogens and halides of other elements:

Diazomethane reacts without the liberation of nitrogen with such compounds as the acid halides of carboxylic acids, resulting in the production of diazoketones. When the diazoketones decompose in the presence of water, alcohols, or amines and a catalyst (Ag₂O), regrouping takes place, resulting in the formation of an acid (its ester or amide) with one more carbon atom than in the initial acid chloride (the Arndt-Eistert reaction):

The synthesis of carboxylic acids is one of the most important applications of diazomethane; it is also used extensively as a methylizing agent.