In general, the increase in electron-withdrawing effect of the fluorinated groups on the BU benzyl substituent causes an increase in the Ka of BU to anions. Figure A lower binding strength was found for BUs 1 and 9 (both having a similar Ka to chloride), followed by 7. BUs 6, 8, 11 and 12 were found to be weaker in chloride binding than 7 (Table ). We note that the estimated Ka for BU 6 is lower than for 7, which corresponds well to the result obtained for BUs 1 and 2, where the derivative with -SCF3 groups (2) has stronger binding than the one with -CF3 (1) groups.