Chemistry 125 Lecture 42 January 22 2010 Solvation

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

Puzzle Answer(s) B H O R CH 2 H free-radical chain O (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 -. R O CH elimination H-O Cl H-O+ R CH 2 N H i-Pr Cl R CH H Cl HOMO-LUMO n. O *N-Cl H + i-Pr N i-Pr Cl R O H + i-Pr N i-Pr H elimination R CH H B

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

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 interactions give organic materials many of their most valuable properties.

Non-Bonded “Classical” Energies R + - dielectric constant R-1 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] H 2 O 78 Table 6. 7 p. 239 (CH 3)2 S=O 49 CH 3 OH 33 CH 3 CH 2 OH 25 (CH 3)2 C=O 21 CHCl 3 5 (CH 3 CH 2)2 O 4 n-hexane 2

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

Halide Trends (text sec. 6. 2) van der Waals Radius of X (Å) Bond Distance of X-CH 3 (Å) 2 Dipole Moment of X-CH 3 i. e. non-bonded distances are Debye units = 4. 8 (electrons) about twice bonded distances. charge separation (Å) 1 Non-monotonic conflicting nonlinear trends The dipole moment ( ) is the product of two properties, with opposing trends. Both are monotonic, but one is nonlinear. 0 H F Cl atom Br I “Charge” of X , CH 3 (e) = Debye / (4. 8 dist) (monotonic)

Halide Trends (text sec. 6. 2) van der Waals Radius of X (Å) Bond Distance of X-CH 3 (Å) compare CH 3 2 ! 1 0 Non-monotonic, like (suggests competition) “A-Value” of X Eaxial-Eequatorial larger vd. W radius (kcal/mol) stands off further another measure H F Cl atom Br I of substituent “size”

Boiling points (Table 6. 2) 0 1. 85 1. 87 1. 81 1. 62 CH 4 is not polar and not very polarizable - + + + -+ polarizability, not just polarity from Carey & Sundberg -

gas-phase ionic dissociation Boiling. Cf. points R-Cl R+ Cl R+ kcal/mole CH 3+ 229 CH 3 CH 2+ 193 (CH 3)3 C+ 176 n-butane Hf (gas) Intra- vs. Intermolecular “Solvation” isobutane -35. 1 n-Pentane 36°C -36. 9 iso-Pentane 28°C Polarizability does its job well only when the atoms can get really near one another. -40. 3 neo-Pentane 10°C Atoms near surface count!

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

200 I Br Cl H-(CH 2)n-X H Boiling Point (°C) F Dipole Moment (D) Polarizability (10 -24 cm 3) CH 3 -H CH 3 -F CH 3 -Cl 0 1. 8 1. 9 3 3 5 CH 3 -Br CH 3 -I 1. 8 1. 6 6 8 100 0 -100 2 4 n 6 8 10

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

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

Water Dipoles

Calculated Water Dimer Lengthened by only ~0. 5% (not much * occupancy) Klopper, et al. , PCCP, 2000, 2, 2227 -2234

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

Calculated Water Dimer Dissociation energy = 3. 3 kcal/mole Klopper, et al. , PCCP, 2000, 2, 2227 -2234 Cf. Goldman, et al. , J. Chem. Phys. , 116, 10148 (2002)

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

Text Section 6. 10 Crown Ethers and Tailored Ionophores Nobel Prize in Chemistry 1987 “ion carriers” 18 -c-6

2. 82 2. 83 Å 2. 78 Radii (Å) K+ 1. 33 18 -Crown-6 • + K Cl O 1. 4

3. 10 3. 27 3. 16 3. 04 Å 3. 27 Radii (Å) Cs+ 1. 67 O 18 -Crown-6 • + Cs N=C=S 1. 4

2. 62 2. 47 Å 2. 58 2. 55 Radii (Å) Na+ 0. 98 O 18 -Crown-6 • + Na N=C=S 2. 32 2. 45 1. 4

1. 92 1. 91 2. 07 2. 12 2. 71 3. 11 3. 52 3. 79 Å Radii (Å) Li+ 0. 68 O 18 -Crown-6 • + Li Cl. O 1. 4 - • 2 H O 4 2

Relative binding constants for 18 -crown-6 with various alkali metal ions

End of Lecture 42 Jan. 22, 2010 Copyright © J. M. Mc. Bride 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-Non. Commercial-Share. Alike 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. Mc. Bride, Chem 125. License: Creative Commons BY-NC-SA 3. 0