text: The study by Nesmeyanov of the products of the addition of mercuric chloride to ethylene, acetylene and their derivatives  led to the concept of the “dual reactivity” of a substance and the “transfer of the reaction center” along the chain π, π-, σ, π-, σ, σ- and p,π-conjugations in a molecule. Further research showed that these phenomena are fundamentally different from tautomerism. With the participation of his colleague A.E. Borisov, Nesmeyanov formulated the rule according to which electrophilic and homolytic substitution at the olefin carbon atom occurs with the preservation of the geometric configuration (Nesmeyanov – Borisov rule). Thanks to his research, which showed the enolate structure of ketone derivatives with alkali metals and magnesium, that is, the existence of O-derivatives of ketones, Nesmeyanov refuted Knorr 's concept of "pseudomerism ".