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 Generalization of Fractional-Order Kinetics (suggestion by Ayesha) Reaction of MeLi (pre-equilibrium) (CH3Li)4 dominant 4 CH3Li when Assoc = Dissoc [CH3Li]4 [(CH3Li)4] 1/4 [CH3Li] [(CH3Li)4] reactive Radical-Chain Initiator (steady state) Union of Ideas i.e. Progress RO-OR 2 RO• H-R’ ROH + •R’ when Term = Init [Rad•]2 [RO-OR] H-X prop •X H-R’ term Number of “Machines” [RO-OR]1/2 X-X X-R’ R’-R’ From 2009 Exam 3. Consider the chlorination reaction : i-Pr2NCl + RH i-Pr2NH + 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? Could e-Pair Repulsion Explain BDEs? HO OH Total e-Density Contour (a0-3) 0.002 H2N Cl 1.40 Å 0.01 1.73 Å 51 0.05 46 isoelectronic isoelectronic 0.25 H3C CH3 1.53 Å Drawing proton(s) away from nucleus removes BDE 90 kcal/mole OMO-OMO e-density from overlap region. H3C Cl 1.79 Å 84 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- from Wikipedia Which Bond is Stronger N-Cl or C-Cl? Electron Energy Compared to What? N-Cl stronger if forming Ions (N+ Cl-) mismatch aids Heterolysis • Cl • ••• •• •• C N BDE C-Cl stronger if forming Radicals (Cl • •C) mismatch hinders Homolysis separate together separate by kind permission of the owners 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. by kind permission of the owners “Electronegativity” and Bond Strength Dana House James Dwight Dana Benjamin Silliman Henrietta Dana Silliman 24 Hillhouse Avenue 1813-1895 1779-1864 1823-1907 1849 “Electronegativity” and Bond Strength Linus Pauling 1901-1994 http://nobelprize.org/ J. Am. Chem. Soc. 1932 Pauling was pushing resonance. X- H-X “normal” (average of H-H and X-X) Why not use to measure resonance stabilization? (electron volt = 23.06 kcal/mole) in H+ Pauling’s theory arithmetic mean Observed Pauling thought F-F was 65 kcal/mole. Actually it is 38. geometric mean or A:B = (A:A * B:B)1/2 1932 Pauling was pushing resonance. C-F – C-O ≠ O-F½ C-X ≠ polarity difference O-O O-F OF ≠ (OO+FF)/2 BDE (obs) “Normal” [OF = (OO+FF)/2] O-F polar “resonance stabilization” ~ additive! (C-F½ – C-O½) ≈ O-F½ F-F (units of electron volts) (actually 1.65) 1932 Pauling was Relative to H & F Relative to O pushing Relative to C resonance. HOAO node # Relative to H 1.00 0.58 0.48 1.480.34 (screened) nuclear charge Mapped to a Distorted Periodic Table 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. pp. 26-28 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.” “By the 1960s, for all Pauling’s salesmanship, MO theory was generally agreed to be more convenient [than his resonance theory] for most purposes.” . Jan. 6, 2011 (International Year of Chemistry) Pauling Electronegativity Still we expect energy-mismatch to strengthen bonds, so crude correlation of P with IP and EA is hardly surprising. from Wikipedia Mulliken Electronegativity (1934) average of Atomic Ionization Potential and Electron Affinity A A+ + e A A+e Free-radical halogenation introduces a functional group (*) into alkanes. Chemical & Physical Properties of Alkyl Halides Ionic Chemistry of * (SN & pKa) X H R :Nu S S pK 1 2 N N a Non-Bonded Interactions and Solvation (key for ionic reactions) Premiers Éléments de Chimie ~1850 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: PV = nRT Corpora non agunt, nisi soluta Henri Victor Regnault (1810-1878) 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 interactions give organic materials many of their most valuable properties. Non-Bonded “Classical” Energies + R - Charge-Charge R-1 (Coulomb’s Law) The ONLY source of true chemical potential energy. E±Coulomb = -332.2 kcal/mole / dist (Å) dielectric constant Solubility of NaCl vs. Me4NI? NaCl (mp 801°C) vs. CsI (mp 621°C) H 2O 78 (CH3)2S=O 49 CH3OH 33 CH3CH2OH 25 (CH3)2C=O 21 CHCl3 5 (CH3CH2)2O 4 n-hexane 2 (e.g. J&F Table 6.7 p. 239) (gravity & magnetism are for wimps; [long-range attraction; contrast radical bonding] the “strong force” is for physicists) Non-Bonded “Classical” Energies + - + + - R R-1 -2 R-4 T R-3 What if the dipole orientation is not fixed? - ++ + Charge-Charge (Coulomb’s Law) Charge-Dipole (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 the energies fall off rapidly with increasing distance. Halide Trends (e.g. J&F sec. 6.2) van der Waals Radius of X (Å) Bond Distance of X-CH3 (Å) 2 Dipole Moment of X-CH3 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 , CH3 (e) = Debye / (4.8 dist) (monotonic) Halide Trends (text sec. 6.2) van der Waals Radius of X (Å) Bond Distance of X-CH3 (Å) compare CH3 2 ! 1 0 Non-monotonic,like (suggests competition) “A-Value” of X Eaxial-Eequatorial larger vdW radius (kcal/mol) stands off further another measure H F Cl atom Br I of substituent “size” from J&F Boiling points (Table 6.2) 0 1.85 1.87 1.81 1.62 CH4 is not polar and not very polarizable - + + + - + + -+ polarizability, not just polarity from Carey & Sundberg - End of Lecture 41 Jan. 19, 2011 Copyright © J. M. McBride 2011. 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). 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) B H O R CH2 H free-radical chain Note: the base that removes H+ could be a very weak one, like ROH or HSO4-. R O O CH (might fail with 30% H2SO4) elimination Cl H-O+ i-Pr R H-O CH2 N H Cl R CH H i-Pr C H Cl HOMO-LUMO nO *N-Cl H + i-Pr N i-Pr Cl R elimination R O CH H + i-Pr N i-Pr H H B