EQUILIBRIUM LITHIUM ION PAIR ACIDITIES OF 2-PHENYL-1,3-DITHIANE AND 2-P-BIPHENYLYL-1,3-DITHIANE - A NEW APPROACH

摘要

A new technique has been developed to measure the equilibrium ion pair acidities of 2-phenyl-1,3-dithiane (1) and 2-p-biphenylyl-1,3-dithiane (2) with lithium as counterion. The lithium ion pair pK values (PKLi/THF) in tetrahydrofuran (THF) at 25 degrees C were determined to be 29.4 (1, contact ion pair (CIP)), 28.2 (2, CIP), and 29.3 (2, solvent separated ion pair (SSIP)), as compared to the values of 30.5 (1, CIP) and 29.1 (2, CIP) measured previously with cesium as counterion. At 25 degrees C, the lithium salt of 2-p-biphenylyl-1,3-dithiane (2L) exists as a mixture of CIP, SSIP, and free ion, with the ratio of [CIP]/[SSIP] = 11.4. The results are consistent with the stronger electrostatic interactions between lithium cation and the dithianyl anion for CIP than for SSIP, resulting in a lower pK(a) for CIP. The equilibrium constant of SSIP reversible arrow CIP for 2L was measured at 25 to -20 degrees C to give Delta H degrees = 1.43 kcal mol(-1), Delta S degrees = 9.6 eu. The SSIP of 2L was found to dissociate significantly to the free ion in dilute THF solution. The measured dissociation constants of 2L to the free ion at 25 degrees C are (1.22 +/- 0.08) x 10(-6) M and (1.39 +/- 0.09) x 10(-5) M for CIP and SSIP, respectively. The slow transmetalation reactions of 1 and 2 with different lithium indicator anions also allowed the measurements of some kinetic rate constants at 25 degrees C, from which Breinsted beta values were obtained for 1 and 2. [References: 56]