the name used in the Soviet literature for a rule stating that enols (unsaturated aliphatic hydrocarbons containing a hydroxyl group at a carbon-carbon double bond) are unstable and, at the time of formation, convert into isomeric carbonyl compounds—aldehydes and ketones. For example, upon hydrolysis, isopropenyl acetate yields (in addition to acetic acid) acetone rather than 2-propen-2-ol:
The El’tekov rule was formulated by A. P. El’tekov in 1877 and, independently, by E. Erlenmeyer in 1880. It was later shown that the rule holds only for the simplest enols. In the case of many compounds, such as acetoacetic ester, isomerization is not carried out to the end, and a dynamic equilibrium is established between the enol and carbonyl forms (the keto-enol tautomerism). The enol forms of certain fluorine-containing ketones, such as CF2 ═ C(CF3)OH, are stable.