The term metalation denotes the replacement of hydrogen by metal to yield a true organometallic compound. The replacement of hydrogen by metal in a compound such as acetoacetic acid is not generally classed as a metalation reaction in this sense because the resulting salt is not a typical organometallic compound. This distinction is matter of degree. The ability to yield ortho products, usually unmixed with para or other isomers, distinguishes the metalation reaction from the more familiar types of substitutions and makes possible the preparation of many products not readily available by other routes. The derivative most commonly prepared from a metalation product has been the carboxylic acid, made by the reaction of the lithium compound with carbon dioxide. This reaction is useful in the study of metalation products because the usual metalating agents, the alkyllithium compounds, yield volatile acids which are easily separated from the acids of higher molecular weight obtained from the metalated products themselves.