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(λmax= 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 (λmax= 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.