chemistry 125: lecture 17 reaction analogies and carbonyl reactivity in molecular orbital terms...
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Chemistry 125: Lecture 17
Reaction Analogies and Carbonyl Reactivity
In molecular orbital terms there is a close analogy among seemingly disparate organic
chemistry reactions: acid-base, SN2 substitution, and E2 elimination. All these reactions
involve breaking existing bonds, where LUMOs have antibonding nodes, while new bonds
are being formed. The three-stage oxidation of ammonia by Cl2 is analyzed in these terms.
The analysis is extended to the reactivity of the carbonyl group and predicts the trajectory
for attack by a high HOMO. This predicted trajectory was validated experimentally by Bürgi
and Dunitz, who compared numerous crystal structures determined by X-ray diffraction.
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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)
End of Lecture 17Oct. 13, 2008
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