The pK(a) values for calixarenes in MeCN have been determined by selective titration with bases using a spectroscopic method. These values are as follows: calix[4]arene pK(a)(1) = 19.06 +/- 0.22, pK(a)(2) > 33; calix[6]arene pK(a)(1) = 15.59 +/- 0.06, pK(a)(2) = 23.85 +/- 0.35, pK(a)(3) > 33; calix[8]arene pK(a)(1) = 17.20 +/- 0.20, pK(a)(2) = 20.32 +/- 0.31, pK(a)(3) > 33. The trends in acidity are rationalized using structures generated by a DFT model. For mono-deprotonation, the degree and nature of hydrogen bonding in the anion is the dominant factor; for di-deprotonation, spatial separation of the anionic charges becomes important.
The pK(a) values for calixarenes in MeCN have been determined by selective titration with bases using a spectroscopic method. These values are as follows: calix[4]arene pK(a)(1) = 19.06 +/- 0.22, pK(a)(2) > 33; calix[6]arene pK(a)(1) = 15.59 +/- 0.06, pK(a)(2) = 23.85 +/- 0.35, pK(a)(3) > 33; calix[8]arene pK(a)(1) = 17.20 +/- 0.20, pK(a)(2) = 20.32 +/- 0.31, pK(a)(3) > 33. The trends in acidity are rationalized using structures generated by a DFT model. For mono-deprotonation, the degree and nature of hydrogen bonding in the anion is the dominant factor; for di-deprotonation, spatial separation of the anionic charges becomes important.