Chemistry 125: Lecture 48 February 8, 2010 Addition to Alkenes a Physical-Organic MO Perspective This For copyright notice see final page of this file

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Butenes Combustion of 4 C graphite + 8 H H-H gives kcal/mole

Use NIST Webbook to study factors influencing relative stability of butenes For high precision convert to a common product, butane, using catalytic hydrogenation, which is fast and clean, and much less exothermic than combustion.

Isomeric Hexenes  H f Problem: Are these data consistent with those for the butenes?

C 6 H 12 Alkene Isomer Stability  H f (kcal/mol) Strain Energy (MM2) sp 2 C-H  sp 2 C-C ~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

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H  * LUMO  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  * LUMO  HOMO CCCC  * LUMO  HOMO HOMO/LUMO : Stepwise “Electrophilic” (“works” with Pt/C Catalyst! Sec 10.2, 410ff ) (Section , Regioselective) Br-H CC CC H Br CC H + (for any H-Hal Ch. 3, Ch. 9) (Markovnikov) HOMO-HOMO repulsive empty C C Br H C C H

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-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. Sec HX addn via R + Energetics Regiochemistr y (Markovnikov) Hydration R + + H 2 O

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

Loudon 4.7C Cation Stability (source?) 1° to 2°  = 21 kcal/mole vs. 2.5 kcal/mole for n-alkenes Table 10.2 p. 445 Compared to what?

Doubleor ? CC H CC H CC H MinimumSingle HOMO-1 LUMO HOMO LUMO+1 “Hyperconjugation” (  HOMO-LUMO mixing) CC H CC H CC H CC H

CCCCCC H 2 CH Doubleor ?MinimumSingle LUMO+1 HOMO H H H

CH 3 CCCCCC H 2 CH Doubleor ?MinimumSingle “ ” LUMO+1 HOMO-7

Cation Rearrangement do Problem 9.11 (ring expansion)

End of Lecture 48 Feb. 8, 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