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

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

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 molar  H -T  S kcal/mole >1/2 complexed at 1  M [18-c-6]

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

Cryptands

Nonactin a bacterium-generated antibiotic

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

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

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.

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.

Factors that Influence Acidity

Learning from pK a Values HOH pK a * -4 H 2 OH HSH 7.0 FH 3.2 H 3 NH (BDE 119) (BDE 91) (BDE 136)

Brønsted Acidity Chapter 3 BDE Overlap!

Learning from pK a Values pK a * CH 3 -COH O 2.9 ClCH 2 -COH O HOH 15.7 H 2 OH HSH 7.0 FH 3.2 H 3 NH CH 3 -C-CH-C-CH 3 O H O H 3 NCH-COH OCH 3 +

Titration of Alanine

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 HA 2 equilibria for bases stronger that HO - cannot be measured in water. One must “bootstrap” by comparing acid-base pairs in other solvents pK a * : : (allylic)

1 st of 6 pages from

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.

End of Lecture 43 Jan. 25, 2010 Copyright © J. M. McBride 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