Chemistry 125: Lecture 48 February 7, 2011 Alkenes: Stability and Addition Mechanisms Electrophilic Addition This For copyright notice see final page of this file

C 6 H 12 Alkene Isomer Stability  H f (kcal/mol) Correct for strain by subtracting Molecular Mechanics energy to leave “intrinsic” bonding energy sp 2 C-H  sp 2 C-C (+hyperconjugation?) ~2.5 kcal/mol cis  trans ~1.5 kcal/mol  H f from Pedley (1994) 3214 # of C - C= bonds Cf. Table 10.1, p. 413

H-ClH 2 C=CH 2 H-CH 2 -CH 2 -Cl + Alkene / HCl Thermochemistry single + doublethree singles = 249= 263 favorable by 14 kcal/mol

 H f (NIST) Problem : Use the pK a values of HCl and H 2 O to estimate this shift in water. H-ClH 2 C=CH 2 H-CH 2 -CH 2 -Cl  H gas OH - + Alkene / HCl Thermochemistry HOH + Cl Addition in Acid Elimination in Base single + doublethree singles difference in proton affinities (NIST) Addition Mechanism?

H  * LUMO H  HOMO orthogonal Addition to Alkenes SOMO : Radical Chain H-Br Br CC CC CC H H Br HOMO/LUMO : Concerted CC H CC H H CCCC  * LUMO  HOMO HOMO/LUMO : Stepwise “Electrophilic” Stepwise works with Pt/C Catalyst! (e.g. J&F 10.2, 410ff ) (e.g. J&F , ) (Regioselective) Br-H CC CC H Br CC H + for any H-Hal (e.g. J&F Ch. 3, Ch. 9) “Markovnikov” HOMO-HOMO repulsive empty C C Br H C C H “anti-Markovnikov” Pt

-22 kcal/mole +17 kcal/mole Surface Potential (energy of proton on van der Waals surface) Electrostatics is important in positioning fragments, but new bonding requires orbital mixing. Electrophilic Addition to Alkenes. (e.g. J&F ) HX addn via R + Regiochemistr y (Markovnikov) Hydration R + + H 2 O

34:08-42:50

Section Addition of Hydrogen Halides Regioselectivity Intermediate Cations ; Cation Stability Cation Rearrangement

Cation Stability e.g. Loudon 4.7C pp Compared to What? (Evidence) : Bond Energy: sp 2 C-H vs. sp 2 C-C Hyperconjugation: C H vs. C C H + + C H + Starting Alkene C C or Halide C X (ease of formation) + H Product or Transition State C : Nu (reactivity) + Rationale (Theory) :

e.g Loudon 4.7C Cation Stability e.g. J&F Table 10.2 p. 445 Compared to What? different numbers of atoms in their standard states not relative to each other, nor to their respective starting materials or products. IRRELEVANT (source?) 1° to 2°  = 21 kcal/mole vs. 2.5 kcal/mole for n-alkenes Bond Energy: sp 2 C-H vs. sp 2 C-C Hyperconjugation: C H vs. C C H + + Intramolecular “Solvation” (Polarizability) (In EtOH/H 2 O S N 1 of t-Bu-Br is only ~5 kcal/mol easier than i-Pr-Br)

Cl CH 3 CCH CH 3 83% of product CH 3 H-Cl 2° Cation + CH 3 CCH CH 3 3° Cation (more stable) CH 3 CCH CH 3 + CCH CH 2 CH 3 CCH CH 3 Cl 17% of product Cl H Cl - CH 3 CCH CH 3 OH HO CH 3 CCH CH 3 H 2 O -H + H-Catalyzed Hydration Subject to Rearrangement + Rearrangement Competes with Reaction

Markovnikov Regiochemistry + HCl CH 3 CCH CH 3 CCH 2 Cl - CH 3 CCH 2 Cl unstable vinyl cation times slower than alkene HCl (excess) CH 3 C Cl 56% 44% HBr CH 3 CC CCH2CH2 Br CH 3 60% Markovnikov Regiochemistry CH 3 C Cl + Is halogen favorable or unfavorable for cation? Second Step Slow BUT Markovnikov? Yes! Hard to make cation because of  electron withdrawal by halogen. But if you’re going to make it, make it where you can get  help. Influence of Halogens

End of Lecture 48 February 7, 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