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Chemistry 125: Lecture 54
February 22, 2010
Linear and Cyclic Conjugation
Allylic Intermediates
(4n+2) Aromaticity
This
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Is There a Limit to 1 Energy for Long Chains?
Chain
length
Normalized
AO size
Overlap
per 
bond
Number
Total
of 
overlap
bonds stabilization
(AO product)
N.B. Here we are using our own
“overlap stabilization” units, which are
twice as large as conventional “” units.
2
1/2
1/2
1
1/2
4
1/4
1/4
3
3/4
8
1/8
1/8
7
7/8
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..
MO Energy (units of 2)
+1
0
Semicircle Mnemonic for  MO
Energy in Conjugated Chains.
.
.
.
p
.
: :: :
:
.
.
.
.
.
Place points denoting length of chain
evenly along circumference between
upper and lower limit (+1 and -1).
.. .
N=2 an
N=3
N=1
ethylene
allyl
isolated 2p AO
N=4
1,3-butadiene
etc.
:
All odd chains
have
a nonAs
the
conjugated
chain
lengthens,
(difference
is resonance
stabilization
bonding
MO
with
nodes
on


more
and
more
levels
are
crowded
of butadiene
vs. 2 isolated
ethylenes)
alternant
carbons.
It is the
between -1 and +1, and the HOMOlocus
of the
“odd” electron
p andgap
)
allylic stabilization (vs. isolated
LUMO
decreases.
in the radical, and of + (-)
same 2 electron stabilization
Color
shiftintoward
red. (anion).
charge
the cation
.
-1
Radius of circle = 2  stabilization of H2C=CH2
[ limit of ±(N-1)/N ]
.
for cation, radical, anion
Allylic Intermediates:
Allylic Free-Radical Bromination
Sec. 11.8 pp. 497-500, Sec. 12.11c p. 543
NBS
Allylic Intermediates:
Addition of HX to Butadiene
Sec. 12.9-12.10 pp. 534-541
Butadiene
H+
Propenyl Cation
-21.4
best
potential best
overlap
LUMO+1
HOMO-1
HOMO
LUMO
+17.6
best overlap
hyperconjugated C-H
HOMO-4
LUMO+1
HOMO
LUMO
best product
Propenyl Cation
+132
best
+152
potential
+144 +99
best 
potential
Surface Potential
D
Cl
p. 1288
Cl-
symmetrical
(but for D)
3.1 : 1
-78°
25°
1.6 : 1
rapid ion-pair collapse
competes with motion
In a Very Viscous Solvent Can Short-Range Motion
Constitute a Rate- (and Product-) Determining Step?
CH3
H3C
•
CH3
(2) Shift D atom
CH3
N
N
CD3
•
H3C
CD3
exothermic/easy/fast
D
CD2
CH3
CD3
CD3
If Step 1 (motion) is rate-limiting,
H- and D-transfer products should
form in equal amounts.
CH3
(2) Shift H atom
exothermic/easy/faster
(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”)
CD3
H3C
CD3
CH2 H
CD3
CD3
Kinetic vs. Thermodynamic Control
Sec. 12.10 pp. 537-540
Allylic Intermediates, Transition States:
SN1 and SN2
Sec. 12.11a,b pp. 541-543
Allylic Intermediates:
RH Acidity
Sec. 12.11d pp. 543-544
Cf. Benzylic Intermediates (sec. 13.12)
e.g. Ph-CH2-H pKa = 41
AROMATICITY
Ch. 13-14
predicted
Conjugation worth ~30 kcal !
observed !
Cf. 13.5a pp. 580-581
Bringing the ends of a conjugated
chain together to form a ring gives a
lowest  MO with one additional
bonding interaction.
Lowest MO will have energy = -N/N = -1
In a conjugated ring peripheral nodes must come in
even numbers. e.g. cyclopropenyl
E = -1
E = +1/2
0 nodes
E = +1/2
2 nodes
2 nodes
Energy Shifts on “Ring Formation”
Shifts Alternate (because of node parity).
.
unfavorable
.
favorable
:
MO Energy (units of 2)
+1
0
:
-1
End to End
Interaction
:
.
:
.
unfavorable
favorable
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
Hückel’s Rule:
4n+2  electrons is unusually
favorable in a conjugated ring.
(where n in an integer)
.
0
.
.
Same radius as for open chain
.
..
.
Inscribe regular polygon
with point down.
Read MO energies
on vertical scale.
.
.
reactive SOMOs !
3 cyclopropenyl
4 cyclobutadiene
6 benzene
4n “Antiaromatic”!
Stabilized
slightly
destabilized
Cation (vs.
strongly
stabilized
hexatriene)
:
: : ::
-1
:
.
.
::
MO Energy (units of 2)
+1
open-chain  energies from semicircle mnemonic
Circle Mnemonic for  MO
Energy in Conjugated Rings.
.
(vs. butadiene)
+
(vs. allyl )
•
There
is always
an MO at
Anion
destabilized
Radical
less
stabilized
(vs.-1.
allyl•)
Generalization of
Aromaticity:
4n+2 Stability
Sec. 13.6 pp. 582-595
Transition State “Aromaticity”
Heteroaromatic Compounds
(Sec. 13.9 pp. 598-601)
N
.
O
H
Pyridine
H
Furan
H
YY
H
H
:
N
H
H
Pyrrole
Imidazole
H
H
H
N.B. Single . denotes contribution of 1 e to 
system (redundant with double bond).
(occurs in
amino acid
histidine)
N
.
H
H
.
N
H
HH
Relay for long-range
proton transfer by enzymes
X
X-
End of Lecture 54
Feb. 22, 2010
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J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0