Chemistry 125: Lecture 16 October 9, 2009 Reaction Analogies and Carbonyl Reactivity Comparing the low LUMOs that make both HF and CH 3 F acidic underlines the distinction between atomic orbital nodes (AON) and antibonding nodes (ABN). In molecular orbital terms there is a close analogy among seemingly disparate reactions: acid-base, S N 2 substitution, and E2 elimination. All these reactions involve breaking existing bonds, at sites where LUMOs have antibonding nodes, as new bonds are being formed. The three-stage oxidation of ammonia by Cl 2 is analyzed in these terms. The analysis extends to the carbonyl group. For copyright notice see final page of this file

 * LUMO Low LUMO H-F (4 valence e-pairs; 5 valence AOs) 2p F HOMOs Shape of "Frontier" Orbitals (ABN) Antibonding Node Atomic Orbital Node (AON) 1s H 2p F 1s H 2s F 1s H 2sp F n Alternative format from an early MO catalogue An Organic Chemist’s Book of Orbitals Salem & Jorgensen, 1973

Shape of "Frontier" Orbitals F-CH 3 (7 valence pairs) AON ABN Compare Energies HOMOs ≈ > &  C-H pFpF F CH  CH

LUMO F-CH 3 Low LUMO (7 valence pairs) HOMOs ABN AON Why unusually low? Low LUMO ABN AON Shape of "Frontier" Orbitals C H  CH * F  CF *

F H :OH FH OH "Acid-Base" ABN Curved Arrows Designate e-Pair Shifts Start arrow at e-pair location in starting material. End arrow at e-pair location in product. LUMO Reaction Analogies Direction of HOMO approach for best overlap ? **

F H Direction of HOMO approach for best overlap :OH FH OH Besides creating a new bond, mixing HOMO with LUMO can break a bond where the LUMO has an AntiBonding Node. LUMO Reaction Analogies ** "Acid-Base" Make & Break ABN

F H:OH FH OH "Acid-Base" F CH 3 :OH CH 3 OHF LUMO Reaction Analogies  "S N 2 Substitution" ABN (Could have been called S N 2 Substitution at H) Make & Break Same

Make & Break  F CH 3 :OH "S N 2 Substitution" F CH 2 CH 2 H F H:OH FH OH "Acid-Base" :OH F H OH CH 2 "E2 Elimination" ABN AON  Bonding Between Carbons This LUMO is the favorable mixture of  * C-H  * F-C ABN Make Two Break Two (Could have been called S N 2 Substitution at H) LORE! (LUMO calculation biased by stretching C-H, C-F) ? LUMO Reaction Analogies two localized  * orbitals. CH 3 OHF  CF *  CH *

“Oxidation” of Ammonia by Chlorine 3 NH 3 + Cl 2 H 2 N-NH 3 Cl + NH 4 Cl High HOMO? Low LUMO? n NH 3 :NH 3  * Cl 2 Cl-Cl High HOMO? Low LUMO? n NH 3 :NH 3  * N-H Cl-NH 2 -H + Cl + Cl-NH 3 +_ Low LUMO?  * Cl-N Cl-NH 2 High HOMO? n NH 3 :NH 3 Cl + NH 2 -NH 3 +_ Cl NH 4 Cl-NH 3 + Cl-NH 2 +NH 4 + H 2 N-NH 3 Cl Three Cycles of Make & BreakNH 3 s attack Cl, then H, then N. Cl

Four Functional Groups: Carbonyl Amide Carboxylic Acid Alkyl Lithium (then we’ll have a complete change of perspective)

Wiki on Functional Group Analysis due 11:59 pm Wednesday, Oct. 14 (Claim your functional group soon, while the good ones last.)

The Carbonyl Group C=O Probably the most important Functional Group in Organic Chemistry Strong AND Reactive

H C  CH O HOMO LUMO Shape of "Frontier" Orbitals 2p O  C-H AON ABN >> Which is higher? nuclear charge overlap, bonding 2s 2p x 2p y 2p z 3s 3d xy 3d xz Pairwise Mixing Analysis Plum Pudding MOs (6 valence pairs) H H C O (Cf. 2p F >  C-H ) ±

End of Lecture 16 Oct. 10, 2008 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