Aldehyde hydrates

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Aldehyde hydrates ( geminal diols ) are mostly unstable products of the addition of water ( hydration ) to an aldehyde .

Properties and examples

Chloral hydrate, an aldehyde hydrate
Ninhydrin, a ketone hydrate

In aqueous solution methanal (formaldehyde) is 100% aldehyde hydrate, ethanal (acetaldehyde) 58%. When the water is removed (e.g. by heating) the aldehyde hydrates decompose with elimination of water and formation of the aldehyde.

Chloral hydrate is an example of a stable aldehyde hydrate. Here the strong electron- attracting −I effect of the three chlorine atoms stabilizes the hydrate molecule.

Since two hydroxyl groups are bound to one carbon atom in aldehyde hydrates , this group of substances is an exception to the Erlenmeyer rule .

Also ketones can form hydrates, but the balance in ketones is usually far on the side of the carbonyl compound. Strongly acceptor-substituted ketones such as hexafluoroacetone or ninhydrin are, however, present as hydrate.

Educational response

As a nucleophile, the water molecule attacks the electrophilic carbonyl carbon with a free electron pair on oxygen . This creates a zwitterion , which is stabilized by deprotonation of the (OH 2 ) + group and protonation of the oxygen anion to form a hydroxyl group.

Formation of an aldehyde hydrate

The reaction is an equilibrium reaction , the equilibrium is completely on the right side only in special cases such as formaldehyde and chloral .

Web links

Individual evidence

  1. ↑ Hans-Dieter Jakubke, Ruth Karcher (Ed.): Lexicon of Chemistry , Spectrum Academic Publishing House, Heidelberg, 2001.
  2. ^ Siegfried Hauptmann : Organic Chemistry , 2nd edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig, 1985, p. 384, ISBN 3-342-00280-8 .