Chemistry 125: Lecture 16October 9, 2009

Reaction Analogies and Carbonyl Reactivity

Comparing the low LUMOs that make both HF and CH3F 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, SN2

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 Cl2 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)

2pF

HOMOs

Shape of "Frontier" Orbitals

(ABN) Antibonding Node

Atomic Orbital Node (AON)

1sH

2pF

1sH

2sF

1sH

2spF

n

Alternative format froman early MO catalogueAn Organic Chemist’s

Book of Orbitals Salem & Jorgensen, 1973

Shape of "Frontier" Orbitals

F-CH3(7 valence pairs)

AON

AON

ABN

CompareEnergies

HOMOs

≈>& C-H pF

F

C H

CH

LUMO

F-CH3

Low LUMO

(7 valence pairs)HOMOs

ABN AON

AON

Why unusually low? Low LUMO

ABN

AON

Shape of "Frontier" Orbitals

CH

CH*

F

CF*

F H :OH

F H OH

"Acid-Base"

ABN

Curved ArrowsDesignate

e-Pair ShiftsStart 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

F H 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

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

"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

two localized * orbitals.

CH3 OHF

CF*CH

*

“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.

Cl

Four Functional Groups:

CarbonylAmide

Carboxylic AcidAlkyl Lithium

(then we’ll have a complete change of perspective)

Wiki onFunctional 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 importantFunctional Group

in Organic Chemistry

Strong AND Reactive

HC

CH

O

HOMO

LUMOShape of "Frontier" Orbitals

2pO C-H

AON

AON ABN

>>Which is higher?

nuclearcharge

overlap,bonding

2s

2px

2py

2pz

3s

3dxy

3dxz

Pairwise Mixing Analysis

Plum Pudding

MOs

(6 valence pairs)H

HCO

(Cf. 2pF > C-H)

±

End of Lecture 16Oct. 10, 2008

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