Role of triple ion formation in the acid–base reaction between tropolone and triethylamine in acetonitrile

Abstract

The acceleration of the proton-transfer reaction between tropolone (2-hydroxycyclohepta-2,4,6-trien-1-one) and triethylamine by the addition of various salts in acetonitrile was examined by UV–VIS spectroscopy. The addition of lithium perchlorate to the tropolone–triethylamine solution caused the formation of a cationic ‘triple ion’, C₇H₅O₂^–(Li⁺)₂(λ_max= 393 nm): C₇H₅O₂H–NEt₃+ 2 Li⁺⇄ C₇H₅O₂^–(Li⁺)₂+ Et₃NH⁺; the ‘free’ tropolonate ion (Bu₄NC₇H₅O₂) gave a band at 414 nm in acetonitrile. The effects of Na⁺ on the reaction were much smaller than those of Li⁺. Alkaline-earth metal ions (M²⁺= Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) promoted the deprotonation of tropolone by forming the species C₇H₅O₂^–(M²⁺). On the other hand, the addition of Et₄NCl to the tropolone–triethylamine mixture produced the ‘free’ tropolonate ion (λ_max= 414 nm): C₇H₅O₂H–NEt₃+ 2 Cl^–⇄ C₇H₅O₂^–+ Et₃NH⁺(Cl^–)₂. The decrease in the amount of C₇H₅O₂^– formed with Et₄NBr or Et₄Nl was attributed to a decrease in the formation constants of the anionic ‘triple ions’[Et₃NH⁺+ 2 X^–⇄ Et₃NH⁺(X^–)₂]. Thus, ‘the salt effects’ upon the deprotonation of the weak acid by the amine were explained quantitatively by the ‘coordination’ reactions and not merely by ion-pair exchange reactions.