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Chemistry 125: Lecture 54February 22, 2010
Linear and Cyclic Conjugation Allylic Intermediates(4n+2) Aromaticity
This
For copyright notice see final page of this file
Is There a Limit to 1 Energy for Long Chains?
8 1/8 1/8 7 7/8
4 1/4 1/4 3 3/4
Chain length
2
Normalized AO size
1/2
Overlapper bond
(AO product)
1/2
Number of
bonds
1
Total overlap
stabilization
1/2
N 1/N 1/N N-1 (N-1)/N
Yes, the limit is 1, i.e. twice the stabilization of the H2C=CH2 bond.
Similarly, the LUMO destabilization limit is twice that of the H2C=CH2 MO..
N.B. Here we are using our own “overlap stabilization” units, which are twice as large as conventional “” units.
N=2 ethyleneN=3 allylN=4 1,3-butadiene
:
.p
:
N=1 an isolated 2p AO
:
.
+1
-1
0
Semicircle Mnemonic for MO Energy in Conjugated Chains.
Radius of circle = 2 stabilization of H2C=CH2
Place points denoting length of chain evenly along circumference between upper and lower limit (+1 and -1).
.
.
..
.
.
.
.
.
:::
. etc.. . .All odd chains have a non-bonding MO with nodes on alternant carbons. It is the locus of the “odd” electron in the radical, and of + (-)
charge in the cation (anion).
As the conjugated chain lengthens, more and more levels are crowded
between -1 and +1, and the HOMO-LUMO gap decreases.
Color shift toward red.
MO
Ene
rgy
(uni
ts o
f 2
)
allylic stabilizationsame 2 electron stabilization
for cation, radical, anion
(vs. isolated p and )
(difference is resonance stabilization of butadiene vs. 2 isolated ethylenes)
[ limit of ±(N-1)/N ]
+17.6
-21.4
HOMO-4HOMOLUMO+1LUMOHOMO-1LUMO+1LUMOHOMO
Butadiene Propenyl CationH+
hyperconjugated C-H
bestoverlap
bestpotential
best productbest overlap
symmetrical(but for D)
p. 1288
3.1 : 1-78°
1.6 : 125°
Cl-
DCl
rapid ion-pair collapse competes with motion
D CD2 CD3
CD3
HCH2
If Step 1 (motion) is rate-limiting, H- and D-transfer products should
form in equal amounts. (because their motions should be equally fast)
If Step 2 (atom shift) is rate-limiting, more H-transfer product should form.
kH/kD > 1 (kinetic “isotope effect”)
In a Very Viscous Solvent Can Short-Range Motion Constitute a Rate- (and Product-) Determining Step?
CH3
CH3
H3C CD3
CD3
CD3
•
•
CH3
CH3
H3C
(2) Shift D atomexothermic/easy/fast
N
N
(2) Shift H atomexothermic/easy/faster
CD3
CD3
CD3
CH3
H3C
NN
NN
Allylic Intermediates:RH Acidity
Sec. 12.11d pp. 543-544
Cf. Benzylic Intermediates (sec. 13.12)
e.g. Ph-CH2-H pKa = 41
Bringing the ends of a conjugated chain together to form a ring gives a
lowest MO with one additional bonding interaction.
In a conjugated ring 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 node parity).
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 de stabilized •-
open
-cha
in
ene
rgie
s fr
om s
emic
ircl
e m
nem
onic
:
H
X-
Y
:
H
.
H
:
N
Pyridine
HH
H H
O
Furan
H
HH
H H
N
PyrroleHH
H
N
NImidazole
Heteroaromatic Compounds(Sec. 13.9 pp. 598-601)
::
::
.
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)
End of Lecture 54Feb. 22, 2010
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The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0