Post on 21-Dec-2015
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