chem 125 lecture 17 10/13/08 this material is for the exclusive use of chem 125 students at yale and...
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Chem 125 Lecture 1710/13/08
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
F H 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
F H OH
"Acid-Base"
F CH3 :OH
CH3 OHF
LUMO Reaction Analogies
"SN2 Substitution"
ABN
(Could have been calledSN2 Substitution at H)
Make & BreakSame
SameMake & Break
F CH3 :OH
CH3 OHF
"SN2 Substitution"
F CH2
CH2 H
F H :OH
F H OH
"Acid-Base"
:OHF
H OH
CH2
CH2"E2 Elimination"
ABNABN
AON
BondingBetween Carbons
This LUMO is the favorable
mixture of
*C-H
*F-C
ABN
Make TwoBreak Two
(Could have been calledSN2 Substitution at H)
LORE!(LUMO calculation biased by stretching C-H, C-F)
?
LUMO Reaction Analogies
“Oxidation” of Ammonia by Chlorine
3 NH3 + Cl2 H2N-NH3Cl + NH4Cl
High HOMO?
Low LUMO?
nNH3 :NH3*Cl2
Cl-Cl
High HOMO?
Low LUMO?
nNH3 :NH3*N-H
Cl-NH2-H+
Cl + Cl-NH3
+_
Low LUMO? *Cl-N
Cl-NH2High HOMO? nNH3 :NH3 Cl + NH2-NH3
+_
ClNH4
Cl-NH3
+Cl-NH2+NH4
+
H2N-NH3Cl
Three Cycles of Make & BreakNH3s attack Cl, then H, then N.
Four Functional Groups:
CarbonylAmide
Carboxylic AcidAlkyl Lithium
(then we’ll have a complete change of perspective)
The Carbonyl Group
C=O
Probably the most importantFunctional Group
in Organic Chemistry
Strong AND Reactive
HOMO
LUMOShape of "Frontier" Orbitals
Low LUMO
2pO C-HAON
Poor overlap () ; Poor E-match (2pO < 2pC)
AON
AON ABNABN
>>Which is lower?
nuclearcharge
overlap,bonding
2s
2px
2py
2pz
3s
3dxy
3dxz
Pairwise Mixing Analysis
Plum Pudding
MOs
(6 valence pairs)H
HCO
C-O Bonding
Lower of Oxygen’s “Unshared" Pairs
mostly a p-rich hybrid atomic orbitalof Oxygen
some O-C bondingwith backside of C hybrid
some C-H bonding
Nodes through nuclei (AON),not between atoms (ABN)
Bürgi-DunitzAngle
From what direction should a nucleophile
HOMO approachthe * LUMO
of the C=O group?
Bürgi-DunitzAngle
From what direction should a nucleophile
HOMO approachthe * LUMO
of the C=O group?
furthestfrom nodes
Structure Superposition from many Crystals (A-O)
Containing N: and C=O
Bürgi-DunitzAngle(110°)
R
R
C
C
O
O
N
N
N.B. There is another R group directly behind this one.
from H. B. Bürgi, J. D. DunitzAccts. Chem. Res. 16, 153 (1983)