Chem 125 Lecture 17 10/13/08 This material is for the exclusive use of Chem 125 students at Yale and may not be copied or distributed further. It is not readily understood without reference to notes from the lecture.

F H Direction of HOMO approach for best overlap :OH FH OH ABN 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

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 CH 3 OHF "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

“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.

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

Functional Group Analysis Wiki due Thursday, Oct. 16

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

HOMO LUMO Shape of "Frontier" Orbitals Low LUMO 2p O  C-H AON Poor overlap (  ) ; Poor E-match (2p O < 2p C ) AON ABN >> Which is lower? 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

C-O  Bonding Lower of Oxygen’s “Unshared" Pairs mostly a p-rich hybrid atomic orbital of Oxygen some O-C bonding with backside of C hybrid some C-H bonding Nodes through nuclei (AON), not between atoms (ABN)

Bürgi-Dunitz Angle From what direction should a nucleophile HOMO approach the  * LUMO of the C=O group?

Bürgi-Dunitz Angle From what direction should a nucleophile HOMO approach the  * LUMO of the C=O group? furthest from nodes

Structure Superposition from many Crystals (A-O) Containing N: and C=O Bürgi-Dunitz Angle (110°) R R C C O O N N N.B. There is another R group directly behind this one. from H. B. B ü rgi, J. D. Dunitz Accts. Chem. Res. 16, 153 (1983)

End of Lecture 17 Oct. 13, 2008