1. Chemistry 125: Lecture 43 January 25, 2010 Solvation, Ionophores and Brønsted Acidity This For copyright notice see final page of this file

2. Text Section 6.10 Crown Ethers and Tailored Ionophores Nobel Prize in Chemistry 1987 “ion carriers” 18-c-6

3. Relative binding constants for 18-crown-6 with various alkali metal ions K = [M + 18-c-6] [M + ]  [18-c-6] (mol -1 ) 23,000 1,150,000 in MeOH at 25°C 29  10 6 stronger than MeOH ! 0.79 g/mlmol.wt. 32 25 molar  H -T  S -13.4 5.2 kcal/mole -8.4 2.5 >1/2 complexed at 1  M [18-c-6]

4. By making cation large 18-c-6 “destabilizes” solid or aqueous KMnO 4 allowing the salt to dissolve in hydrocarbons. (“purple benzene”) Phase-Transfer Catalysis H2OH2O organic solvent KMnO 4 organic substance to oxidize Similar effect from adding other salts with large organic cations, e.g. R 4 N + Cl - R 4 P + Cl - Avoids need for expensive, dangerous solvents like (CH 3 ) 2 SO that dissolve both reagents

5. Cryptands

6. Nonactin a bacterium-generated antibiotic

7. Nonactin K eq (MeOH) Na + 512 K + 31,000 moves K + selectively through a membrane

8. H 2 O (aq) kcal/mol 400 300 200 100 0 H 2 O (g) 6.3 H 3 O + (aq) OH - (aq) H + + OH - (g) 392 H 3 O + (g) 164 ! 106 100 Sum = 370 H + (aq) + OH - (aq) pK a = 15.8 The Importance of Solvent for Ionic Reactions 21.5 E ±Coulomb = -332.2 / dist (Å) [long-range attraction; contrast radical bonding] H + :OH 2 bonding plus close proximity of + to eight electrons (polarizability shifts e-cloud) + - + - + - 28 18 etc, etc From small difference of large numbers! K (G)  10 -(3/4  386)  10 -290 BDE HO-H 120 e transfer similar

9. Fortunately solvation energies of analogous compounds are similar enough that we can often make reasonably accurate predictions (or confident rationalizations) of relative acidities in terms of molecular structure.

10. When pK a = pH Why should organic chemists bother about pH and pK a, which seem like topics for general chemistry? a) Because whether a molecule is ionized or not is important for predicting reactivity (HOMO/LUMO availability), conformation, color, proximity to other species, mobility (particularly in an electric field), etc. b) Because the ease with which a species reacts with a proton might predict how readily it reacts with other LUMOs (e.g.  * C-X or  * C=O ). KaKa = [H + ] [B - ] [HB] [B - ] [HB] pK a = pH - log = pH, when HB is half ionized Single indicators work best over ~2.5 pH units (95:5 - 5:95). Bootstrap with overlapping indicators for wide coverage.

11. Factors that Influence Acidity

12. Learning from pK a Values HOH 15.7 16 12 8 4 0 pK a * -4 H 2 OH -1.7 + HSH 7.0 FH 3.2 H 3 NH 9.2 + (BDE 119) (BDE 91) (BDE 136)

13. Brønsted Acidity Chapter 3 BDE 105 108 119 136 91 103 88 71 Overlap!

14. Learning from pK a Values 16 12 8 4 0 pK a * -4 4.8 CH 3 -COH O 2.9 ClCH 2 -COH O HOH 15.7 H 2 OH -1.7 + HSH 7.0 FH 3.2 H 3 NH 9.2 + 9 CH 3 -C-CH-C-CH 3 O H O H 3 NCH-COH OCH 3 +

15. Titration of Alanine http://themedicalbiochemistrypage.org/images/titrate.jpg

16. Approximate “pK a ” Values CH 3 -CH 2 CH 2 CH 2 H ~ 52 CH 3 -CH 2 CH=CHH ~ 44 CH 3 -CH 2 C CH ~ 25 ~ 34 H 2 NH = 16 HOH CH 3 -CH=C=CHH CH 3 -C C-CH 2 H ~ 38 sp 3 C _ sp 2 C _ (no  overlap) sp C _ (no  overlap) C _ HOMO -  overlap (better E-match N-H ) (bad E-match O-H ) (best E-match C-H ) * Values are approximate because HA 1 + A 2 - = A 1 - + HA 2 equilibria for bases stronger that HO - cannot be measured in water. One must “bootstrap” by comparing acid-base pairs in other solvents. 50 40 30 20 10 pK a * : : (allylic)

17. 1 st of 6 pages from http://evans.harvard.edu/pdf/evans_pKa_table.pdf

18. 1)List factors that help determine pK a for an acid. 2)Choose a set of several acids from the Ripin-Evans Tables or from the text (inside back cover) and explain what they teach about the relative importance of these factors. 3)Explain your conclusions to at least one other class member and decide together how unambiguous your lesson is. Problems for Wednesday: Feel free to consult a text book and its problems or the references at the end of the Tables. Hint: this could provide a good question.

19. End of Lecture 43 Jan. 25, 2010 Copyright © J. M. McBride 2010. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0) Use of this content constitutes your acceptance of the noted license and the terms and conditions of use. Materials from Wikimedia Commons are denoted by the symbol. Third party materials may be subject to additional intellectual property notices, information, or restrictions. The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0