Chem 125 Lecture 674/13/08

Projected material

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.

Bringing the ends of a conjugated chain together to form a ring gives a

lowest MO with an additional bonding interaction.

In a conjugated rings peripheral nodes must come in even numbers. e.g. cyclopropenyl

0 nodes 2 nodes

Lowest MO will have energy = -N/N = -1

2 nodesE = -1 E = +1/2 E = +1/2

Energy Shifts on “Ring Formation”

+1

-1

0

.

.

.

.

::M

O E

nerg

y (u

nits

of

2)

::

End to End Interaction

favorable

favorable

unfavorable

unfavorable

Shifts Alternate (because of increasing number of nodes).

Hückel’s Rule: 4n+2 electrons is unusually favorable in a conjugated ring.

On bringing the ends of a chain together, odd-numbered MOs (1, 3, 5, etc.) decrease in energy

(favorable terminal overlap for 0,2,4… nodes), while even-numbered MOs (2, 4, 6, etc.) increase in energy

(unfavorable terminal overlap for 1,3,5… nodes).

Thus having an odd number of occupied orbitals (more odd-numbered than even-numbered)

insures overall stabilization of ring (compared to chain).

[though there may be strain in the bonds]

an odd number of e-pairs

(where n in an integer)

:

:

:

:

..

There is always an MO at -1.

Circle Mnemonic for MO Energy in Conjugated Rings.

Inscribe regular polygon with point down.

+1

-1

0

MO

Ene

rgy

(uni

ts o

f 2

)

Same radius as for open chain

Read MO energies on vertical scale.

:

4 cyclobutadiene6 benzene

3 cyclopropenyl

..

.

.

.

.

.

. .

..

reactive SOMOs !

Cation strongly stabilized(vs. allyl+)

::

:4n “Antiaromatic”!

slightly destabilized(vs. butadiene)

Stabilized(vs. hexatriene):

::

Radical less stabilized (vs. allyl•)

..

Anion destabilized •-

open

-cha

in

ene

rgie

s fr

om s

emic

ircl

e m

nem

onic

Generalization of Aromaticity:

4n+2 Stability

NMR Spectroscopy

Transition State “Aromaticity”

:

H

X-

Y

:

H

.

H

:

N

Pyridine

HH

H H

O

Furan

H

HH

H H

N

PyrroleHH

H

N

NImidazole

Heteroaromatic Compounds(pp. 725, 1221-1225)

::

::

.

Y-

HX

:

.

Relay for long-rangeproton transfer by enzymesN.B. Single . denotes contribution of 1 e to

system (redundant with double bond).

(occurs in amino acid histidine)

Furan

0 anti-bonding nodes

2 ABNs

4 ABNs

N

SHMo2 (Simple Hückel Molecular Orbital Program)

N

Benzene Pyridine

N

lower energy

node on N

identical shapeenergy

larger on N

lower energyhigh N density

Crude calculation shows heterocycle analogy.

Generalized Aromaticity

pKa 15vs. 16 for H2O

H H

HH

H H

pp. 725-6

cyclo-C7H8 cyclo-C7H7- pKa 39 (despite more resonance structures)

6 electrons (4n+2)

8 electrons (4n, antiaromatic)

R H

R R

+ Ph3C+

2 electrons (4n+2)

H

HH

H H

OH-

unusually stable cation (triply benzylic)

+ Ph3CH

R

R R

+

even more stable

Aromaticity: PMR Chemical Shift Criterion

H Cl

H H

+ SbCl5 SbCl6-

H

H H

+

10.4Downfield!

(diamagnetic anisotropy and loss of e-density)

Aromaticity: PMR Chemical Shift Criterion

HCCl3

TMS

-4.23

14 electrons(43 + 2)

DIAMAGNETIC ANISOTROPY!

?

DIAMAGNETIC ANISOTROPY

Pericyclic Reactions(in which transition states are “aromatic”)

Cycloadditions: Diels-Alder (Ch. 15)

Electrocyclic Reactions (Ch. 27)

Cycloadditions: Diels-Alder (Sec. 15.3)

4 + 2 electrons

Ring

4 + 2 electrons

ene

diene

enediene

Cycloadditions: Diels-Alder (Sec. 15.3)

Stereochemistry (ene)Ene just “sits down” on Diene

Cycloadditions: Diels-Alder (Sec. 15.3)

Stereochemistry (diene)

Diene just “sits down” on Ene

Cycloadditions: Diels-Alder (Sec. 15.3)

LUMO

HOMO

Diels-Alder Reactioncyclic electrontransition state

HOMO

LUMO

TransitionState

Motion

front view side view

TransitionState

HOMO-1

TransitionState

HOMO

p. 1351

Diels-Alder Reaction

cyclicelectrontransition state

TransitionState

Motion

front view side view

p. 1254

?

HOMO () orthogonal to LUMO (*)

h

Shift electron from HOMO to LUMO

pp. 1351-2

End of Lecture 67April 13, 2009