Chemistry 125: Lecture 41 January 19, 2011 Fractional-Order Kinetics Electronegativity & Bond Strength Solvation Ionophores This For copyright notice see final page of this file

[(CH 3 Li) 4 ] [CH 3 Li] 4  when Assoc = Dissoc when Term = Init [Rad] 2  [RO-OR] Generalization of Fractional-Order Kinetics (suggestion by Ayesha) Reaction of MeLi (pre-equilibrium) Number of “Machines”  [RO-OR] 1/2 1/4 [CH 3 Li]  [(CH 3 Li) 4 ] (steady state) dominant reactive (CH 3 Li) 4 4 CH 3 Li Radical-Chain Initiator RO-OR 2 RO ROH + R’ H-R’ prop X H-R’ H-X term X-X X-R’ R’-R’ Union of Ideas i.e. Progress

3. Consider the chlorination reaction : i-Pr 2 NCl + RH  i-Pr 2 NH + RCl and these approximate bond dissociation energies (kcal/mole): N-Cl (46), R-Cl (85), N-H (92), R-H (100). G.(3 min) Why should the BDEs of N-Cl and R-Cl be so different, when those for N-H and R-H are so similar? From 2009 Exam

Could e-Pair Repulsion Explain BDEs? 1.79 Å 1.73 Å 1.40 Å 1.53 Å H 3 C CH 3 HO OHH 2 N Cl H 3 C Cl 84 BDE 90 kcal/mole Total e-Density Contour (a 0 -3 ) Drawing proton(s) away from nucleus removes OMO-OMO e-density from overlap region. isoelectronic

from Wikipedia Lone pair repulsion seems a plausible explanation for weakening O-O vs. C-C or N-Cl vs. C-Cl. But might electronegativity help explain stronger C-Cl than N-Cl ? C + Cl -

Which Bond is Stronger N-Cl or C-Cl? Cl N Electron Energy separate C Compared to What? N-Cl stronger if forming Ions (N + Cl - ) C-Cl stronger if forming Radicals (Cl C) together mismatch aids Heterolysis mismatch hinders Homolysis BDE

First use in English (O.E.D.) 1837 J. D. Dana Syst. Mineral. 82 When chemistry has so far advanced, that the relative electro-negativity, (if I may so call it,) or electro-positivity, of the several elements, is fully known,..we shall probably be able to construct a natural arrangement of minerals on chemical principles. Dana House 24 Hillhouse Avenue 1849 “Electronegativity” and Bond Strength James Dwight Dana Henrietta Benjamin Silliman Silliman by kind permission of the owners Dana

“Electronegativity” and Bond Strength Linus Pauling J. Am. Chem. Soc.

H-X “normal” (average of H-H and X-X) H + X Pauling was pushing resonance. Why not use  to measure resonance stabilization? in Pauling’s theory (electron volt = kcal/mole)  Pauling thought F-F was 65 kcal/mole. Actually it is 38. Observed or A:B = (A:A * B:B) 1/2 arithmetic mean geometric mean

    1932 Pauling was pushing resonance. ~ additive! O-F polar “resonance stabilization” BDE (obs) (units of electron volts) O-OO-F F-F (actually 1.65) OF ≠ (OO+FF)/2 “Normal” [OF = (OO+FF)/2]  C-F –  C-O ≠  O-F  ≠ polarity difference  C-F ½ –  C-O ½ ) ≈  O-F ½  C-X ½  C-F  C-O

Relative to H & F 0.48 Relative to O Relative to C Relative to H Pauling was pushing resonance Mapped to a Distorted Periodic Table (screened) nuclear charge HOAO node #

1932 Pauling was pushing resonance. Is it surprising that bond strength correlates with differences in Pauling’s electronegativities,  P ? No, his  P  scale was defined by differences in bond strength. “By the 1960s, for all Pauling’s salesmanship, MO theory was generally agreed to be more con- venient [than his resonance theory] for most purposes.”. pp Beyond the Bond “More than ever before, new techniques show the bond to be a convenient fiction, albeit one that holds the field of chemistry together, finds Philip Ball.” Jan. 6, 2011 (International Year of Chemistry)

from Wikipedia Mulliken Electronegativity (1934) average of Atomic Ionization Potential and Electron Affinity Pauling Electronegativity Still we expect energy-mismatch to strengthen bonds, so crude correlation of  P with IP and EA is hardly surprising. A A + + eA A + e

Chemical & Physical Properties of Alkyl Halides Non-Bonded Interactions and Solvation (key for ionic reactions) Ionic Chemistry of  * (S N & pK a ) Free-radical halogenation introduces a functional group (  *) into alkanes. XR :Nu H SN1SN1SN2SN2pK a

PV = nRT Henri Victor Regnault ( ) For chemical affinity to act freely, substances must be dispersed, and since dispersion by mechanical pulverisation is incomplete, they must be taken into the liquid or gaseous state. Previous chemists expressed this fact by saying: Corpora non agunt, nisi soluta Premiers Éléments de Chimie ~1850 [ ]

The theory of organic chemistry became manageable because it is often possible to focus on a simple unit with strong interactions (molecules & 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.

(gravity & magnetism are for wimps; the “strong force” is for physicists)  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]   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 Solubility of NaCl vs. Me 4 NI? (e.g. J&F Table 6.7 p. 239) NaCl (mp 801°C) vs. CsI (mp 621°C)

+ - + 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 the energies fall off rapidly with increasing distance.

Halide Trends (e.g. J&F 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 from J&F

End of Lecture 41 Jan. 19, 2011 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

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