Erlenmeyer rule
The Erlenmeyer rule was formulated by Emil Erlenmeyer and states that chemical compounds that have more than one hydroxyl group (-OH) on a carbon atom are not stable, but tend to split off water.
Substances with two ( ketone or aldehyde hydrates ), with three ( orthocarboxylic acids ) or with four ( orthocarboxylic acid ) hydroxyl groups on the same carbon atom are generally unstable. The cause is the higher binding energy of a carbon-oxygen double bond compared to two CO single bonds and the mutual repulsion of the oxygen atoms that are spatially adjacent. Examples of the Erlenmeyer rule are the hydrates of carbonyl compounds , such as of aldehydes [RCH (OH) 2 ], of ketones [R 2 C (OH) 2 ] or of carboxylic acids [RC (OH) 3 ], which are normally not used as a substance are isolatable. The equilibrium of their formation from the carbonyl compound and water is usually far on the side of the starting materials. In part, this applies to carbonic acid , which, as a hydrate of carbon dioxide , breaks down again very easily.
However, derivatives of geminal diols or triols are always stable when all the hydrogen atoms of the hydroxyl groups are substituted , since they can no longer be converted into the more stable carbonyl compounds by simple protolysis . These compounds include the acetals (in the narrower sense as well as ketals ) R 2 C (OR ') 2 , the carbonic acid esters , carbamic acid esters and the orthocarboxylic acid esters [RC (OR') 3 ]. Similarly Orthokohlensäureester (Tetraalkoxymethane) known and stable.
In addition to hydroxyl groups, Erlenmeyer’s rule can also be applied to other substituents , amino groups are aminals often unstable, geminally halogenated alcohols break down easily to carbonyl compounds and the corresponding hydrogen halides .
Exceptions
The only exceptions are compounds that have strongly electron-withdrawing groups in the alpha position , such as chloral hydrate , glyoxylic acid or ninhydrin . Formaldehyde is almost completely present in aqueous solution as a hydrate , but is only stable in solution and - in contrast to chloral hydrate and ninhydrin - cannot be isolated as a substance. Half-acetals are also only exceptionally stable. These include those that are derived from aldehydes or ketones with strongly electron-withdrawing groups, but cyclic hemiacetals such as glucose are also stable. As an orthocarboxylic acid diester, tetrodotoxin does not follow the Erlenmeyer rule, but is stabilized by the formation of two rings.
Acidium ions are not geminal diols because of their mesomeric-stabilized positive charge and only formally represent a violation of Erlenmeyer's rule.
Individual evidence
- ↑ Entry on Orthoester. In: Römpp Online . Georg Thieme Verlag, accessed on June 7, 2014.
- ^ Louis Fieser and Mary Fieser: Organic Chemistry , 2nd Edition, Verlag Chemie 1982, ISBN 3-527-25075-1 .