Chapter 20
Conjugated
Systems
20-1
Conjugated Dienes Heats of Hydrogentaion
• From heats of hydrogenation, we can compare relative
stabilities of conjugated and unconjugated dienes.
N am e
Longer
chain has
little
effect. of
Number
Steric
substituents.
S tru ctu ral
Form u l a
0
H
k J (k c al )/m o l
1-B u te n e
-127 (-30.3 )
1-Pe n te n e
-126 (-30.1 )
ci s - 2-B u te n e
-120 (-28.6 )
t r a ns - 2-B u te n e
-115 (-27.6 )
1,3 -B u ta d i e n e
-237 (-56.5 )
t r an s - 1 ,3-Pe n tad i e n e
-226 (-54.1 )
1,4-P e n tad i e n e
-254 (-60.8 )
Effects
Conjugation
stabilizes.
20-2
Conjugated Dienes Butadiene
• Conjugation of the double bonds in 1,3-butadiene
gives an extra stability of approximately 17 kJ (4.1
kcal)/mol .
If double bonds independent:
2
+ 2 H2
catal y s t
0
H = 2(-1 27 k J/ m o l )
2
= -25 4 k J/m o l )
catayl ys st t
catal
2
H
+
2
+ 2 Hdata
2
2
2
Experimental
does not
0
agree:
 H0 = -23 7 k J/m o l
H
= 2(-1 27 k J/ m o l )
Conjugation
= -25 4 k J/m o l )
is important and stabilizing.
+ 2 H2
cata l y s t
H
0
= -23 7 k J/m o l
20-3
Conjugated Dienes Butadiene
 Conjugation
of double bonds in butadiene gives
the molecule an additional stability of
approximately 17 kJ/mol.
20-4
Conjugated Systems
• Systems containing conjugated double bonds, not just
those of dienes, are more stable than those containing
unconjugated double bonds.
O
O
2-C y cl o h e xe n o n e
3-C yc l oh e xe n o n e
(m o re s tab l e )
(l e s s s tab l e )
20-5
Structure of Butadiene MOs
 Combination
of four parallel 2p atomic orbitals
gives two p-bonding MOs (this screen) and two
p-antibonding MOs (the next screen).
20-6
Structure of Butadiene MOs
two p-antibonding MOs of butadiene (higher
in energy).
 the
20-7
How do we form the orbitals of the pi system…
First count up how many p orbitals contribute to
the pi system. We will get the same number of pi
molecular orbitals.
Three overlapping p orbitals.
We will get three molecular
orbitals.
20-8
If atomic orbitals overlap with each other
they are bonding, nonbonding or
pi type anti-bond
antibonding
sigma type anti-bonding
Anti-bonding, destabilizing.
Higher Energy
non-bonded
But now a particular, simple case: distant
atomic orbitals, on atoms not directly attached
to each other. Their interaction is weak and
does not affect the energy of the system. Non
bonding
If atoms are directly
attached to each other the
interactions is strongly
bonding or antibonding.
Bonding, stabilizing the
system. Lower energy.
sigma type bonding
pi type bond
or
or
or
or
Molecular orbitals are combinations of atomic orbitals.
They may be bonding, antibonding or nonbonding molecular orbitals
depending on how the atomic orbitals in them interact.
Example: Allylic radical
Two antibonding
interactions.
Only one weak,
antibonding (nonbonding) interaction.
All bonding interactions.
Allylic Radical: Molecular Orbital vs Resonance
Molecular
Orbital. We
have three pi
electrons (two in
the pi bond and
the unpaired
electron). Put
them into the
molecular
orbitals.
Note that the
odd electron is
located on the
terminal
carbons.
Resonance Result
Again the odd,
unpaired electron is
only on the terminal
carbon atoms.
But how do we construct the molecular orbitals of the pi system? How do
we know what the molecular orbitals look like?
Key
Ideas:
For our linear pi systems
different molecular orbitals are
formed by introducing
additional antibonding
interactions. Lowest energy
orbital has no antibonding,
next higher has one, etc.
Antibonding interactions
are symmetrically placed.
2 antibonding interaction
1 weak antibonding
Interaction, “nonbonding”
0 antibonding interact
This would
be wrong.
Another example: hexa-1,3,5-triene
Three pi bonds, six pi electrons.
Each atom is sp2 hybridized.
Have to form bonding and
antibonding combinations of the
atomic orbitals to get the pi
molecular orbitals.
Expect six molecular orbitals.
# molecular orbitals = # atomic
orbitals
Start with all the orbitals bonding and
create additional orbitals. The number
of antibonding interactions increases
as we generate a new higher energy
molecular orbital.
1,2- and 1,4-Addition
 Addition
of one mol of HBr to butadiene at -78°C
gives a mixture of two constitutional isomers.
CH2 = CH- CH= CH2 + HBr
-78° C
1,3-Butadiene
Br
H
C H 2 = C H - C H - CH 2
3-Bromo-1-butene
90%
(1,2-addition)
Br
+
H
C H 2 - C H = C H - CH 2
1-Bromo-2-butene
10%
(1,4-addition)
• We account for these products by the following twostep mechanism.
20-14
1,2- and 1,4-Addition
• The key intermediate is a resonance-stabilized allylic
carbocation.
C H 2 = C H - CH = CH 2 + H - Br
H
+
C H 2 = C H - CH - C H 2
H
+
C H 2 - C H = CH - C H 2
_
Br
Br
Br
H
C H 2 = C H - CH - C H 2
(1,2-Addition)
_
Br
H
C H 2 - C H = CH - C H 2
(1,4-Addition)
20-15
1,2- and 1,4-Addition
 Addition
of one mole of Br2 to butadiene at -15°C
also gives a mixture of two constitutional
isomers.
C H 2 = C H - C H = C H 2 + B r2
1,3-Butadiene
Br
Br
-15° C
C H 2 - C H - CH = CH 2
3,4-Dibromo-1-butene
(54%)
(1,2-addition)
Br
+
Br
C H 2 - C H = C H - CH 2
1,4-Dibromo-2-butene
(46%)
(1,4-addition)
• We account for the formation of these 1,2- and 1,4addition products by a similar mechanism.
20-16
Experimental Information
• For addition of HBr at -78°C and Br2 at -15°C, the 1,2addition products predominate; at higher temperatures
(40° to 60°C), the 1,4-addition products predominate.
• If the products of the low temperature addition are
warmed to the higher temperature, the product
composition becomes identical to the higher
temperature distribution. The same result can be
accomplished using a Lewis acid catalyst, such as
FeBr3 or ZnBr2.
• If either pure 1,2- or pure 1,4- addition product is
dissolved in an inert solvent at the higher temperature
and a Lewis acid catalyst added, an equilibrium
mixture of 1,2- and 1,4-product forms. The same
equilibrium mixture is obtained regardless of which
isomer is used as the starting material.
20-17
1,2- and 1,4-Addition
 We
interpret these results using the concepts of
kinetic and thermodynamic control of reactions.
 Kinetic control: The distribution of products is
determined by their relative rates of formation.
• In addition of HBr and Br2 to a conjugated diene, 1,2addition occurs faster than 1,4-addition.
+
CH 2 = CH -CH- CH 3
a 2° al l y l i c c arb oc ati on
(g re ate r co n tri b u ti on )
+
CH 2 - CH = CH- CH 3
a 1° al l yl i c carb o cati o n
(l e s s e r c on tri b u ti o n )
20-18
1,2- and 1,4-Addition
 Thermodynamic
control: The distribution of
products is determined by their relative
stabilities.
• In addition of HBr and Br2 to a butadiene, the 1,4addition product is more stable than the 1,2-addition
product.
Br
BrCH 2 CHCH= CH 2
Br CH 2
C
+
H
3,4-D i b ro m o -1-b u te n e
(l e s s s tab l e al k e n e )
H
C
CH 2 Br
(E)-1,4-D i b ro m o-2-b u te n e
(m ore s tab l e al k e n e )
20-19
1,2- and 1,4-Addition
 Kinetic
vs thermodynamic control. A plot of
Gibbs free energy versus reaction coordinate for
Step 2 of addition of HBr to butadiene.
20-20
UV-Visible Spectroscopy
Re g i o n o f
S p e ctru m
En e rg y
W a v e l e n g th
(n m )
k ca l /m o l
k J/ m o l
n e a r u l tra v i o l e t
2 0 0 -4 0 0
2 9 9 -5 98
71 .5 - 1 4 3
visib le
4 00 -7 0 0
1 7 1 -2 99
4 0 .9 - 71 .5
 Absorption
of radiation in these regions give us
information about conjugation of carbon-carbon
and carbon-oxygen double bonds and their
substitution.
20-22
UV-Visible Spectroscopy
• Typically, UV-visible spectra consist of one or a small
number of broad absorptions.
20-23
UV-Visible Spectroscopy
 Beer-Lambert
law: The relationship between
absorbance, concentration, and length of the
sample cell (cuvette):
B e e r-L am b e rt L aw : A = e c l
• A = absorbance (unitless): A measure of the extent to
which a compound absorbs radiation of a particular
Io
wavelength.
A b so rb an c e (A ) = lo g
I
• e = molar absorptivity (M-1cm-1): A characteristic
property of a compound; values range from zero to 106
M-1cm-1.
• I = length of the sample tube (cm)
20-24
UV-Visible Spectroscopy
• The visible spectrum of b-carotene (the orange
pigment in carrots) dissolved in hexane shows intense
absorption maxima at 463 nm and 494 nm, both in the
blue-green region.
b-c a ro te n e
 m a x 46 3 (lo g e 5 .1 0); 49 4 (lo g e 4. 77 )
20-25
UV-Visible Spectroscopy
• A p to p* transition in excitation of ethylene.
20-26
UV-Visible Spectroscopy
• A p to p* transition in excitation of 1,3-butadiene
20-27
UV-Visible Spectroscopy
• Wavelengths and energies required for p to p*
transitions of ethylene and three conjugated polyenes
N am e
S tru ctu ral Form u l a
En e rg y
 m ax
(n m ) [k J (k c al )/m ol ]
Eth y l e n e
165
724 (173)
1,3-B u tad i e n e
217
552 (132)
(3E)-1,3,5-H e xatri e n e
268
448 (107)
(3E,5E)-1,3,5,7-O ctate trae n e
290
385 (92)
20-28
UV-Visible Spectroscopy
 Absorption
of UV-Vis radiation results in
promotion of electrons from a lower-energy,
occupied MO to a higher-energy,unoccupied MO.
• The energy of this radiation is sufficient to promote
electrons in a pi- bonding (p) MO to a pi-antibonding
(p*) MO.
• Electrons in sigma bonding MOs are lower in energy
and the UV radiation energy is no longer sufficient to
promote the electrons to the empty anti-bonding MOs.
• Following are three examples of conjugated systems.
O
O
1,3-B u tad i e n e
3-B u te n -2-on e
H
B e n z al d e h y d e
20-29
UV-Visible Spectroscopy
 UV-Visible
spectroscopy of carbonyls.
• Simple aldehydes and ketones show only weak
absorption in the UV due to an n to p* electronic
transition of the carbonyl group.
• If the carbonyl group is conjugated with one or more
carbon-carbon double bonds, intense absorption
occurs due to a p to p* transition.
O
2-Pe n tan o n e
 m ax 180 n m (e 900)
O
3-P e n te n -2-o n e
 m ax 224 n m (e 12,590)
O
A c e top h e n o n e
 m a x 246 n m (e 9,800)
20-30
Diels Alder Reaction/Symmetry Controlled Reactions
Quick Review of formation of
chemical bond.
Elect
ron
dono
r
Electro
n
accept
or
Note the overlap of the hybrid (donor) and the s orbital which
allows bond formation.
For this arrangement there is no overlap. No
donation of electrons; no bond formation.
20-31
Diels Alder
Reaction of
butadiene and
ethylene to yield
cyclohexene.
We will analyze in terms of the pi electrons of the two
systems interacting. The pi electrons from the highest
occupied pi orbital of one molecule will B
donate
an lowest
HOMOinto
donates
into
energy pi empty of the other. Works in both
directions: A
A LUMO
Note
donates into B, B donates into A. LUMO
the
accept
overlap
LUMO
or
leading
accept
to bond
A HOMO donates
into
orHOM
HOMB LUMO
formati
Note
O
O
on
the
B
dono
dono
overlap
r
A
r
20-32
leading
Try it in another reaction: ethylene + ethylene 
cyclobutane
LU
MO
HOM
O
LUM
O
HOMO
Equal bonding and
antibonding
interaction, no overlap,
no bond formation, no
reaction
20-33