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

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

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)

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.

 dielectric constant Non-Bonded “Classical” Energies  R R Charge-Charge (Coulomb’s Law) The ONLY source of true chemical potential energy. E ±Coulomb = kcal/mole / dist (Å) [long-range attraction; contrast radical bonding]   Table 6.7 p H 2 O (CH 3 ) 2 S=O CH 3 OH CH 3 CH 2 OH (CH 3 ) 2 C=O CHCl 3 (CH 3 CH 2 ) 2 O n-hexane

+ - + Non-Bonded “Classical” Energies - +  R -2 +  R  R -6  R 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.

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 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

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 compare CH 3 larger vdW radius stands off further Non-monotonic,like  ! (suggests competition)

Boiling points from Carey & Sundberg CH 4 is not polar and not very polarizable polarizability,  (Table 6.2) not just polarity

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) n-butane isobutane Cf. gas-phase ionic dissociation R-Cl  R + Cl - R + kcal/mole (CH 3 ) 3 C CH 3 CH CH

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

H H-(CH 2 ) n -X 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 ( cm 3 )

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

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

Water Dipoles

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

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

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

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 *

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

18-Crown-6 K + Cl Å Radii (Å) K O 1.4

18-Crown-6 Cs + N=C=S Å Radii (Å) Cs O 1.4

18-Crown-6 Na + N=C=S - Radii (Å) Na O Å

18-Crown-6 Li + ClO Å 2.12 Radii (Å) Li O H 2 O

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

End of Lecture 42 Jan. 22, 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