Tautomerism (from the Greek. Ταὐτός - the same and μέρος - part) - the phenomenon of reversible isomerism , in which two or more isomers easily pass into each other. [1] In this case, tautomeric equilibrium is established , and the substance simultaneously contains molecules of all isomers (tautomers) in a certain ratio.
Most often, during tautomerization, hydrogen atoms move from one atom in a molecule to another and vice versa in the same compound. A classic example is acetoacetic ester , which is an equilibrium mixture of ethyl acetoacetic ester (I) and oxycrotonic acid (II).
Tautomerism is strongly manifested for a whole range of substances derived from hydrogen cyanide . So, hydrocyanic acid itself already exists in two tautomeric forms:
At room temperature, the equilibrium of the conversion of hydrogen cyanide to hydrogen isocyanine is shifted to the left. It has been shown that less stable isocyanine hydrogen is more toxic.
A similar transformation is known for cyanic acid, which is known in three isomeric forms, however, the tautomeric equilibrium binds only two of them: cyanic and isocyanic acids:
Esters are known for both tautomeric forms, that is, products of substitution of hydrogen for hydrocarbon radicals in cyanic acid. Unlike the indicated tautomers, the third isomer, explosive (fulminic) acid, is not capable of spontaneous conversion to other forms.
Many chemical processes are associated with the phenomenon of tautomerism, especially in the field of the synthesis of drugs and dyes (production of vitamin C - ascorbic acid, etc.). The role of tautomerism in processes occurring in living organisms is very important.
Amide-iminol tautomerism of lactams is called lactam-lactim tautomerism. It plays a large role in the chemistry of heterocyclic compounds. The equilibrium in most cases is shifted towards the lactam form.
Notes
- ↑ * Antonov L. Tautomerism: Methods and Theories. - 1st. - Weinheim: Wiley-VCH , 2013 .-- ISBN 978-3-527-33294-6 .
Literature
- Tautomerism // Tatars - Topopik. - M .: Soviet Encyclopedia, 1956. - P. 10. - ( Great Soviet Encyclopedia : [in 51 vols.] / Ch. Ed. B. A. Vvedensky ; 1949-1958, vol. 42).