Keto–enol tautomerism

The Erlenmeyer rule was formulated by Emil Erlenmeyer, and states that chemical compounds that contribute to a carbon atom more than one hydroxy group (-OH) are not stable, but prone to elimination of water.

Substances with two ( ketone or aldehyde hydrates ), with three ( ortho-carboxylic acids ) or four ( orthocarbonic ) hydroxyl groups on the same carbon atom are unstable in general. The reason is the higher binding energy of a carbon-oxygen double bond, with respect to two CO single bonds, and the mutual repulsion of the spatially spaced oxygen atoms. The so-called hydrates of carbonyl compounds such as aldehydes ( RCH (OH ) 2), ketones ( R2C (OH ) 2) or carboxylic acids (RC (OH ) 3) in violation of this rule and are usually not isolated as substance. This also applies to carbonic acid, which breaks down very easily into carbon dioxide and water.

But derivatives of geminal diols or triols are always stable if all the hydrogen atoms of the hydroxy groups are substituted, as they can not then be converted by simple protonolysis in the more stable carbonyl compounds. These compounds can include, for example, acetals ( ketals, as well as in a narrow sense ) R2C (OR ' ) 2, the carbonic acid ester, carbamic acid ester and the ortho carboxylate ( RC (OR' ) 3). Similarly, ortho carbonate esters ( Tetraalkoxymethane ) is known and stable.

The Erlenmeyer rule can be applied to other substituents except for hydroxy groups, eg amino groups; so aminals are often unstable and geminal halogenated alcohols readily decompose to carbonyl compounds and the corresponding hydrogen halides.

Exceptions

The only exceptions are compounds which have strong electron- withdrawing groups in the alpha position, such as 2,2,2- Trichlorethanal ( trichloroacetaldehyde, chloral ), glyoxylic acid or ninhydrin. Formaldehyde in aqueous solution is practically completely as hydrate before, the only solution is, however, resistant and - in contrast to chloral hydrate and ninhydrin - can not be isolated as substance. Also hemiacetals are only exceptionally stable. These include those derived aldehydes or ketones with strong electron- withdrawing groups, but also cyclic hemiacetals, such as glucose are stable. Also contrary tetrodotoxin as orthophosphoric acid diesters against the Erlenmeyer rule, but is stabilized by the formation of two rings.

Acidiumionen are no geminal diols and therefore no violation of the Erlenmeyer usually because of the resonance stabilized positive charge.