Ortho acids

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Weltzien in Annalen der Chemie und Pharmacie by Justus Liebig , 1867.

Carl Weltzien coined the terms ortho , meta and para acids around 1865. In the context of his work on silicates and the corresponding silicas , Weltzin defines ortho acids as those acids that contain as many hydrogen as oxygen atoms (only applies to Si).

Later generalized Weltzien the term to the acids of other elements and suggested the term Metasäure for the group of acids to be used, which can be traced back to a (intramolecular) dehydration of at least tribasic ortho acid. As parasacids , he describes acids that arise from di- or polybasic ortho-acids by intermolecular dehydration (for examples see original work).

He sees ortho- acids as the normal form of an acid, meta-acids as a form less watery and para-acids as their condensation products.

Stability of ortho acids

In the case of ortho acids , the difference between the elements of the second period and their higher homologues is clearly evident. Of carbon and nitrogen only the meta acids are known: H 2 CO 3 , carbon dioxide (known, but also not stable) and HNO 3 , nitric acid . The free ortho acids H 4 CO 4 and H 3 NO 4 are not known or only known in the form of derivatives ( orthocarbonic acid esters , orthocarboxylic acid esters , see also Erlenmeyer rule ).

From the third period, however, the ortho acids are often the more stable form ( silica H 4 SiO 4 , phosphoric acid H 3 PO 4 ). If the coordination number (KOZ) 4 is exceeded or if several oxygen atoms are doubly bound, the space required by the hydroxyl groups exceeds the space around the central ion. In the case of chlorine , only the meta acid HClO 3 is stable. Only in higher periods is the formation of ortho acids possible again with the increasing size of the central atom ( periodic acid H 5 IO 6 , KOZ 6).

Boric acid B (OH) 3 , which occurs as an ortho acid, is an apparent exception . Here, however, a Lewis acid with KOZ 3 is present.

Individual evidence

  1. ↑ Parasacid is an uncommon term today, the prefix di- or pyro- is used instead . For example, see pyrosulfuric acid .
  2. ↑ Heinrich Will : Annual report on the progress of chemistry , 1865, p. 212 ( Google Books ).
  3. ↑ Weltzien: "About the formation of ozone", in: New Yearbook for Mineralogy, Geology and Palaeontology 1865 , p. 418 ( Google Books ).
  4. ↑ Weltzien: "About the formation of ozone", in: Annalen der Chemie und Pharmacie by Justus Liebig 1867 , p. 110 ( Google Books ).