1. Chemistry 125: Lecture 42 January 22, 2010 Solvation, Ionophores and Brønsted Acidity preliminary This For copyright notice see final page of this file

2. Puzzle Answer(s) H-O CH 2 R i-Pr N + H Cl B free-radical chain (might fail with 30% H 2 SO 4 ) Note: the base that removes H + could be a very weak one, like ROH or HSO 4 -. C R O H elimination B HOMO-LUMO i-Pr N H H-O CH R H Cl + i-Pr N + H H O CH R H Cl elimination n O  * N-Cl O CH 2 R H O CH R Cl H

3. Chapter 6: R-X X = Halogen, OH(R), NH(R) 2, SH(R) Non-Bonded Interactions and Solvation (key for ionic reactions) Ionic Chemistry of  * (pK a and Ch. 7) (electrostatic - gravity & magnetism are for wimps, and the “strong force” is for physicists)

4. The theory of organic chemistry became manageable because it is often possible to focus on a simple unit with strong interactions (bonds with well defined geometry and energy), neglecting the much weaker (and more numerous and complex) intermolecular interactions. But the weak intermolecular inter- actions give organic materials many of their most valuable properties.

5.  dielectric constant Non-Bonded “Classical” Energies  R -1 + - R Charge-Charge (Coulomb’s Law) The ONLY source of true chemical potential energy. E ±Coulomb = -332.2 kcal/mole / dist (Å) [long-range attraction; contrast radical bonding]   Table 6.7 p. 239

6. + - + Non-Bonded “Classical” Energies - +  R -2 +  R -3 - + - + - + - +  R -6  R -1 + - R Charge-Charge (Coulomb’s Law) + Charge-Dipole (Dipole Moment) Charge-Induced Dipole (Polarizability) Dipole-Dipole (Dipole Moments) Induced-Induced - + - + - + - + - + (Cf. Correlation Energy) What if the dipole orientation is not fixed?  R -4 T Nonpolar The latter interactions are weak because dipoles moments and polarizabilities are small - and because of the energies fall off rapidly with increasing distance.

7. Halide Trends (text sec. 6.2) Bond Distance of X-CH 3 (Å) van der Waals Radius of X (Å) Dipole Moment of X-CH 3 “Charge” of X, CH 3 (e) HFClBrI atom 0 1 2 Debye units = 4.8  charge (electrons)  separation (Å) = Debye / (4.8  dist) i.e. non-bonded distances are about twice bonded distances. Non-monotonic (monotonic) The dipole moment (  ) is the product of two properties, with opposing trends. Both are monotonic, but one is nonlinear.  conflicting nonlinear trends

8. Halide Trends (text sec. 6.2) Bond Distance of X-CH 3 (Å) van der Waals Radius of X (Å) “A-Value” of X E axial - E equatorial (kcal/mol) another measure of substituent “size” HFClBrI atom 0 1 2 compare CH 3 larger vdW radius stands off further Non-monotonic,like  ! (suggests competition)

9. Boiling points from Carey & Sundberg CH 4 is not polar and not very polarizable polarizability,  (Table 6.2) 0 1.85 1.87 1.81 1.62 not just polarity - + - + - + - + - + - +

10. Boiling points n-Pentane 36°C iso-Pentane 28°C neo-Pentane 10°C Polarizability does its job well only when the atoms can get really near one another. Atoms near surface count! Intra- vs. Intermolecular “Solvation”  H f (gas) -35.1 -36.9 -40.3 n-butane isobutane Cf. gas-phase ionic dissociation R-Cl  R + Cl - R + kcal/mole (CH 3 ) 3 C + 176 CH 3 CH 2 + 193 CH 3 + 229

11. What does molecular weight have to do with b.p.? Could be plotted more informatively

12. H H-(CH 2 ) n -X 100 -100 0 200 246 8 10 n Boiling Point (°C) I Br Cl F CH 3 -Cl1.95 CH 3 -Br1.86 CH 3 - I 1.68 CH 3 -H CH 3 -F Dipole Moment (D) Polarizability (10 -24 cm 3 ) 0 1.83 3

13. Like Dissolves Like “Solvophobic” Forces Hg does not “wet” glass

14. Like Dissolves Like “Solvophobic” Forces Hg does not “wet” hydrocarbon Alkanes and water (or Hg and glass) do not repel one another. but Hg has good reason to be near Hg, and water near water. nor does H 2 O Hg attracts H 2 O

15. Water Dipoles

16. Calculated Water Dimer Lengthened by only ~0.5% (not much  * occupancy) Klopper, et al., PCCP, 2000, 2, 2227-2234

17. Water Multipoles Surface potential -45 to +50 Surface potential +35 to +50Surface potential -45 to -35 6-311+G**

18. Calculated Water Dimer Klopper, et al., PCCP, 2000, 2, 2227-2234 Cf. Goldman, et al., J. Chem. Phys., 116, 10148 (2002) Dissociation energy = 3.3 kcal/mole

19. The small size of H allows the unusually close approach that makes O-HO-H worth R R. calling a “hydrogen bond”. * Typically ~ 5% as strong as a covalent bond *

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

21. 18-Crown-6 K + Cl - 2.82 2.78 2.83Å Radii (Å) K + 1.33 O 1.4

22. 18-Crown-6 Cs + N=C=S - 3.10 3.04 Å 3.16 3.27 Radii (Å) Cs + 1.67 O 1.4

23. 18-Crown-6 Na + N=C=S - Radii (Å) Na + 0.98 O 1.4 2.62 2.55 Å 2.58 2.47 2.62 2.32 2.45

24. 18-Crown-6 Li + ClO 4 - 3.52 3.11 2.71 3.79 2.07 Å 2.12 Radii (Å) Li + 0.68 O 1.4 1.91 1.92 2 H 2 O

25. Relative binding constants for 18-crown-6 with various alkali metal ions K = [M + Ligand] [M + ]  [Ligand] (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 By making cation large 18-c-6 allows KMnO 4 to dissolve in hydrocarbons

26. Cryptands

27. Nonactin a bacterial antibiotic

28. Nonactin K eq (MeOH) Na + 512 K + 31,000

29. 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  10 -(3/4  386)  10 -290 BDE HO-H 120 e transfer similar

30. 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.

31. 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

32. 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) (Acidity of 1-Alkynes Sec. 14.7) (Problems 14.15-17, 14.18acd, 14.21-22)

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

35. Factors that Influence Acidity

36. End of Lecture 42 Jan. 22, 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