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)

Functional Group Analysis Wiki due Thursday, Oct. 16

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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