ROLE OF TRIPLE ION FORMATION IN THE ACID-BASE REACTION BETWEEN TROPOLONE AND TRIETHYLAMINE IN ACETONITRILE

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 so lution caused the formation of a cationic 'triple ion', C7H5O2-(Li+)2 (lambda(max) = 393 nm): CH5O2H-NEt3 + 2 Li+ reversible C7H5O2-(Li+)2EtNH+;the 'free' tropolonate ion (Bu4NC7H.02) gave a band at 414 nm in acetonitrile. The effects of Na+ on the reaction were much smaller th an 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 tropolo ne-triethylamine mixture produced the 'free' tropolonate ion (lambda(m ax) = 414 nm): C7H5O2H-NEt3 + 2 Cl- reversible C7H5O2- + EtNH+(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 'tr iple ions' [Et3NH+ + 2 X- reversible EtNH+(X-)2]. Thus, 'the salt effe cts' upon the deprotonation of the weak acid by the amine were explain ed quantitatively by the 'coordination' reactions and not merely by io n-pair exchange reactions.