Conjugated Unsaturated Systems
• Allylic Substitution and the Allyl Radical
Basic Organic Chemistry
• Stability of the Allyl Radical and Allyl Cation
• Rules for Resonance
2302202
• Stability of Conjugated Dienes
• UV‐Vis Spectroscopy
Dr Rick Attrill
Office MHMK 1405/5
• Diels‐Alder Reaction
1
Examples of Conjugated Unsaturated Systems
2
Allylic Substitution and the Allylic Radical
Propene reacts with Br2 and Cl2 by the usual electrophilic
addition reaction at room temperature in the absence of
ultraviolet radiation.
CH3CH=CH2 + X2
room temp
no h
CH3CHXCH2X
But at high temperatures, and under conditions where
[X2] is low, allylic substitution occurs:
CH3CH=CH2
All of these examples have a p orbital on an atom adjacent to a
double bond. This allows the formation of an extended or
delocalised bond, one that encompasses more than two nuclei.
These systems are called conjugated unsaturated systems and the
general phenomenon is called conjugation.
Conjugation gives these systems special properties.
+ X2
high temp
or low [X2]
XCH2CH=CH2 + HX
Allylic substitution is a general reaction that
proceeds by a free radical chain mechanism
that involves allylic radicals.
3
4
Role of Bond Dissociation Energy (Ho)
A Free Radical Chain Mechanism
The stability of the allyl radical intermediate, as revealed below
by the lower bond dissociation energy for an allylic C-H bond, is
a key feature of this reaction.
Initiation
: :
: :
: :
h or
heat
:Cl:Cl:
2 :Cl .
A Comparison of Ho Values for C-H Bonds
Propagation
steps
H
H
: :
C
H
.
H + Cl:
C
C
C
H
H
H
CH2=CHCH2-H
+ :Cl:Cl:
.
C H
C
C
H
allyl chloride
CH3
CH3-C .
CH3
CH3
CH3-C-H
CH3
-369
+ H.
-400
tert-butyl
Cl + . Cl:
H
CH3-C . + H .
CH3
H
CH3-C-H
CH3
C
H
.
CH2=CHCH2 + H .
allyl
: :
H
+ HCl
H
: :
: :
C
.
C H
allyl radical H
H
C
C
H
HH
propene
Ho (kJ/mol)
H
HH
I
N
C
R
E
A
S
I
N
G
-413
isopropyl
In this free radical chain reaction, the chain carriers are the allyl
radical and the chlorine radical. The two propagating steps recycle
thousands of times before termination reactions scavenge the chain
carriers.
CH3CH2CH2-H
5
CH 2=CH-H
.
CH3CH2CH2 + H .
-423
B
O
N
D
S
T
R
E
N
G
T
H
propyl
.
CH 2=CH + H .
-465
6
v iny l
Relative Stabilities of the Carbon Radicals
N-Bromosuccinimide (NBS), Used for Allylic Bromination
Since the DHo values are the standard heats evolved in forming the
C-H bonds, they provide a measure of the relative stabilities of the
vinyl .
several different types of carbon radicals.
N-Bromosuccinimide is a derivative of succinimide:
primary
.
allyl
.
+ H.
CH3CHCH3
tertiary
.
+ H.
CH3CCH3
CH3
+ H.
.
C OH
O
CH3CH2CH2
+ H.
N-Br
O
(kJ/mol)
NBS
CH3CH2CH3
O
N-Bromosuccinimide
+ H-Br
O
+ H
+ :Br:
N
Br
O
: :
:
400
CH3CH2CH3
N -Br
H2O
Succinimide
O
369
(CH3)CH
N-H
(NBS)
NBS reacts with HBr to produce Br2:
423
CH2=CHCH3
O
NaOBr
O
Succinic acid
465
413
DHo
Ammonium
salt
heat
=
secondary
O
=
Increasing
Stability
CH2=CHCH2
O
COH
CH2=CH
+ H.
O
N-H + Br-Br
O
Succinimide
This is the reaction that constantly provides the low concentration of bromine needed yet
never affords so much bromine that addition of bromine to the double bond occurs. It also
helps limit addition by scavenging bromide ion needed to complete the addition process.
CH2=CH2
7
8
A Mechanism for Allylic Bromination with NBS
Initiation of the free radical chain reaction may be by
any or all of the reaction sequences below:
Allylic Bromination
h or heat
RO-OR
Bromination of the allylic position is carried out in the
presence of a catalytic amount of peroxide (ROOR),
or under irradiation, to initiate radical production.
.
CH2=CH-CH2 + RO-H
CH2=CH-CH3 + RO .
chain carrier
or
O
O
:
CH2=CH-CH3
N-Br
+
2 RO .
homolysis
h or heat
:
N-Br
h or ROOR
O
O
N.
homolysis
CCl4
O
chain carrier
O
O
CH2=CH-CH3 +
O
.
CH2=CH-CH2 +
:
N-H
CH2=CH-CH2Br +
Allyl bromide
(3-Bromopropene)
O
+ Br .
N.
O
chain carrier
O
N-H
O
or
Br-Br
9
.
CH2=CH-CH2 + H-Br
+ Br .
chain carrier
.
(2) CH2=CH-CH2 + Br-Br
2 Br .
chain carrier
10
Extended -Systems; Conjugated Systems
Propagating Steps in Allylic Bromination
(1) CH2=CH-CH3
h
homolysis
The special properties of conjugated unsaturated systems arise
from overlap of the p-orbital on the adjacent carbon with the
-system of the alkene.
chain carrier
CH2=CH-CH2Br + Br .
chain carrier
chain carrier
C=C
C
As in all free radical chain reactions, the concentrations of the chain
carriers (radical intermediates) are extremely low, so radical-radical
recombination reactions (terminations) only occur infrequently. The
propagating steps cycle thousands of times before terminations occur.
A conjugated
unsaturated
system
The H-Br produced reacts rapidly with NBS to give succinimide and
Br2. Thus, while Br2 is not an explicit reagent, it is constantly being
produced by the reaction below. In the nonpolar solvent (CCl4), and
at the low concentrations of Br2 and HBr, little electrophilic
bromination of the alkene function occurs.
O
O
+ H-Br
: :
:
N-Br
O
+ H
+ :Br:
N
Br
O
C
C
C
An allylic cation,
radical or anion
Conjugation means a -system has been extended by
overlap with p-orbitals on adjacent carbons.
A -system of three overlapping p-orbitals is called
allylic.
O
Other important conjugated systems include those that
contain directly attached multiple bonds, e. g.
N-H + Br-Br
1,3-butadiene.
O
11
12
Resonance Theory Description of the Allyl Radical
Two equivalent resonance
structures can be drawn
H
for the allyl radical:
C
H
C
2
H
C
.
H C
2
1
. H
3C
H
C
3
1
H
H B H
A
Structures A and B are interchanged by
moving single electrons, as shown by the
use of single barbed arrows. The positions
of nuclei do not change.
THEY DO NOT
HAVE SEPARATE
EXISTENCE. The
radical is a hybrid
of the two.
The Allyl Radical
H
Hybridization of the three atomic orbitals to form three  orbitals
A single structure depiction of the allyl radical is C below. It has
the great advantage that it avoids giving the erroneous impression
that this radical is a mixture of A and B, a common misconception.
H
H
1 .
2
C
C1
H
C
1 .
2
C3
Nevertheless, representations like A and B are
often used because they employ the more
traditional depiction of covalent bonds, which
make for easier accounting of electrons.
H
H
Resonance theory predicts a symmetrical structure
for the simple allyl radical.
13
Stability of the Allyl Radical
Symmetry of the Allyl Cation
Both molecular orbital theory and resonance theory
explain the greater stability of the allyl radical relative
to simple alkyl radicals.
The hybrid structure below indicates the
symmetrical structure of the allyl cation:
H
H C C
1 2
H
H
C
3
H
H
Molecular orbital
representation
H
1 .
2
C1
C
14
1 .
2
C3
H
H
H
H
H
Resonance t heory
represent at ion
1
2
1
C
3 H
C1
2
H
A new -system of three overlapping p-orbitals with three
p-electrons is depicted with each method.
C
2
H
This structure implies (1) equivalent carbon-carbon bonds with
a bond order of one and one half and (2) an equal distribution
of the positive charge between the terminal carbons, C1 and C3.
In the extended -system, the p-electrons are delocalized over
three atoms. In simple alkyl radicals, the single nonbonding
electron is localized on one atom. This delocalization stabilizes
the allyl radical by decreasing electron density.
The allyl radical is even more stable than a tertiary radical.
15
16
In fact, the allyl cation is much more stable than a simple 1o carbocation.
The hybrid (the real molecule) is lower in energy because of stabilization
by resonance. This decrease in energy is called resonance energy.
The Allyl Cation
+
1/2 +
C
3o
Substituted
allylic
Allyl
C
H
2o
1o
CH2
B
Resonance
energy
B
OR
H
C
H
+
+
> C-C+ > CH2=CH-CH2 > C-C+ > C-C+ > CH2=CH
OR
CH2-CH=CH2
A
Stability Order of Carbocations
A
+
+
CH2=CH-CH2
C=C-C-C
Unhybridized
Alternative depictions of the
hybrid allyl cation:
The allyl cation, CH2=CH-CH2+, is a very stable carbocation,
almost as stable as a tertiary carbocation.
1/2 +
CH
E
N
E
R
G
Y
CH2
Vinyl
Hybrid
cation
Benzene and other aromatic compounds provide important
additional examples of resonance stabilization.
Alternative
depictions
17
Resonance structures for benzene
The benzene hybrid
18
The  Molecular Orbitals of the Allyl Cation
Resonance Theory Description of the Allyl Cation
Resonance theory describes the allyl cation as a hybrid of the
two equivalent resonance structures A and B:
H
H
C
C
H
+ H
2
3
1
A
C
H
C
H +
C1
H
2
3
H
B
C
H
H
In the drawing of structures A and B, the interconversion is limited to
the moving of pairs of electrons. The positions of the nuclei are fixed.
The hybrid C is symmetrical in the
simple allyl cation, with the positive
charge distributed equally between
H
C1 and C3. The carbon-carbon
bonds are equivalent and have a
bond order of one and one half.
H
1
2
+
C
+
C3
C1
H
1
2
C
H
H
The stability and reactivity of allyl-type cations are explained by
resonance theory. The hybrid is lower in energy than either
contributing resonance structure of the allyl cation, with the degree of
stabilization due to resonance called resonance energy.
19
20
Rules for Writing Resonance Structures
(3) Resonance structures must be proper Lewis structures. The
valency rules must be followed.
The resonance concept gives insight into the reactivity and stability of
organic structures.
(1) Resonance structures exist only on paper. They provide a picture of the
location of valence-level electrons within a structure. When two or more
electronic representations can be written, they are called resonance
structures, and are connected by double-headed arrows (
). The
real molecule is a hybrid of all the resonance structures.
:
There are too many electrons
around the carbon atom
Not a resonance
structure
(2) In writing a set of resonance structures, only the electrons are moved.
The positions of the nuclei remain the same.
(4) All resonance structures must have the same number of
unpaired electrons.
+
+
-H +
H-C=O-H
H
: :
H
H-C-O-H
H
CH3-CH-CH=CH2
CH3-CH=CH-CH2
A
B
Resonance structures
H
C
H
C
A and B interconvert by moving an electron pair.
H
H
H
Allyl radical
+
CH2-CH2-CH=CH2
. H
C
H
three unpaired electrons
Not a resonance structure
one unpaired electron
Not a resonance structure.
It is a structural isomer
formed by shifting an H.
H
.
C
H .
C
. H
C
21
22
(7) Equivalent resonance structures make equal contributions to
the hybrid and typically afford a large resonance stabilization.
(5) All atoms that are part of a delocalized electron system
must be in a plane, or nearly in a plane.
This 1,3-butadiene acts like a
nonconjugated diene because the two
double bonds lie in different planes.
The large tert-butyl groups impose a
twist around the C2-C3 bond.
4
(CH3)3C
1
H2C
2
3
C
C
In benzene, the two resonance structures
1
1
2
6
at right are equivalent. There is a very
6
large calculated resonance stabilization of
5
3
5
about 152 kJ/mol in benzene relative to
4
4
Benz ene
the hypothetical cyclic polyene
1,3,5-cyclohexatriene (the name for each
of the resonance forms if the double bonds were fixed in position).
CH2
C(CH3)3
2,3-Di-tert-butyl-1,3-butadiene
+
Propyl cation
CH2
=
CH2-CH=CH2
Allyl cation
+
CH3CH2CH2
+
CH2
+
CH2
+
+
B
B
Hybrid
-
A
B
:O:
:CH2-C-CH3
Hybrid
:O: +
CH3-C O-H
24
Hybrid
:
:O:
+
CH3-C=O-H
:
:O:
CH3-C-O-H
: :
23
A
=
A
:O:
:CH2-C-CH3
-
:
However, because of delocalization of charge in the allyl case, it would
be an appreciably more stable ion.
-
:O:
CH2=C-CH3
=
+
:
CH2=CH-CH2
3
(8) Nonequivalent resonance structures make unequal contributions to the
hybrid. The more stable a structure, the more it contributes to the hybrid.
In each example below, structure A makes the larger contribution to the hybrid.
(6) The energy of the hybrid (the real molecule) is lower than the
energy of any single resonance structure.
The energy of the allyl cation
might be predicted to be similar
to the propyl cation, since both
seem to be 1o carbocations.
2
Estimating the Relative Stabilities of Resonance Structures:
Some Guidelines
Relative Stabilities of Resonance Structures, Continued
(1) The more covalent bonds in a structure, the more stable it is.
(3) Charge separation decreases stability even when the number
of covalent bonds remains the same.
Electrons are stabilized when they form covalent bonds.
Alternatively, separating charges (creating positive and negative
charges) by localizing a pair of electrons on one atom, raises the
energy of a structure.
:
CH2-CH=Cl:
Minor contribution
Major contribution
Minor contribution
1,3-Butadiene
+
:
:
CH2=CH-Cl:
CH2-CH=CH-CH2
Major contribution
-
: :
-
+
CH2=CH-CH=CH2
Vinyl chloride
(2) Structures where all the atoms have a complete valence shell of
electrons are especially stable (octet rule) and make a major
contribution to the hybrid.
+
: :
+
CH2-O-CH3
:
CH2=O-CH3
Minor contribution
(Only 6 electrons around C)
Major contribution
25
26
Alkadienes and Other Polyunsaturated Hydrocarbons
Hydrocarbons may contain more than one carbon-carbon double or triple bond.
They also may have combinations of double and triple bonds. If the multiple bonds
are separated by just one single bond, the compounds contain conjugated systems.
EXAMPLES
5
Examples
Isolated dienes have double bonds
electronically isolated from each other
(separated by two or more single bonds).
2
1
2
4
2
3
CH2=C=CH2
4
1
3
3
1
(3Z)-1,3-Pentadiene
(cis-1,3-Pentadiene)
CONJUGATED
1,3-Butadiene
1,2-Propadiene
CONJUGATED
CATEGORIES OF POLYENES
1,5-Hexadiene
Conjugated dienes have double bonds
connected by just one single bond:
1,3-Butadiene
3
2
5
2
4
3
4
1
1
Cumulenes contain two double bonds
that have a common central carbon
that is sp hybridized.
5
1-Pentene-4-yne
6
1,4-Cyclohexadiene
2
1
6
4
3
5
C=C=C
C C
H
H
C
H
C=C=C
H
Allene
(1,2-Propadiene)
8
A "cumulated" system
7
(2E,4E,6E)-2,4,6-Octatriene
(trans, trans, trans-2,4,6-Octatriene)
CONJUGATED
27
28
Conformations of 1,3-Butadiene
1,3-Butadiene: Bonding in a Conjugated Diene
Rotation around the C2-C3 bond in 1,3-butadiene leads to different
conformations, including two different planar conformations. In these
two planar conformations, overlap of the p-atomic orbitals is
maintained leading to extended -systems. The two planar
conformations (s-cis and s-trans) are more stable than the nonplanar
conformations.
The C-C bond lengths in 1,3-butadiene have been measured:
1
2
CH2=CH
o
3
4
CH=CH
2
o
o
1.34 A 1.47 A 1.34 A
For comparison
o
CH2=CH2 1.34 A
o
CH3 CH3 1.54 A
The carbon-carbon double bonds are within experimental error the
same length as the bond in ethene. But the C2-C3 bond is much shorter
than the C-C bond in ethane.
fast
s-cis
s-trans
One interpretation of this difference is based on the different
hybridizations at C, and the type of hybrid orbital projected out
from carbon as shown in the following table.
These two conformations rapidly interconvert by rotation around
the single bond. The more stable s-trans dominates in the
equilibrium at room temperature.
29
30
The  Molecular Orbitals of 1,3-Butadiene
Effect of Hybridization on C-C Bond Length
Compound
Hybridization States
of Bonding Carbons
Bond Length
o
(A)
H3C CH3
sp3--sp3
CH2=CH CH3
sp2--sp3
1.54
1.50
CH2=CH CH=CH2
sp2--sp2
1.47
1.46
HC
C CH3
sp--sp3
HC
C CH=CH2
sp--sp2
1.43
HC
C C CH
sp--sp
1.37
The varying lengths of the carbon-carbon single bonds reflect the
different lengths of the hybrid atomic orbitals projected by the
carbon atoms.
C
C
C
sp
sp2
sp3
Increasing length
31
The four -electrons fill the two bonding molecular orbitals, 1 and 2, in the
ground electronic state. Electronic excitation from 2 to 3 (HOMO to
LUMO) occurs with absorption of a photon of wavelength 217 nm.
32
Heats of Hydrogenation Reveal Extra Stability in Conjugated Dienes
2
+ 2 H2
+ 2 H2
15 kJ/mol
Ho = -254 kJ/mol
Ultraviolet-Visible Spectroscopy
Ho
The ultraviolet portion of the electromagnetic spectrum has
wavelengths that vary from 200 to 400 nm. The visible portion includes
wavelengths from 400 to 800 nm. Absorption of radiation throughout
the ultraviolet-visible region is associated with electronic transitions
from filled to unfilled molecular orbitals.
= -239 kJ/mol
Ho
2
Possible Sources of the Extra Stability in Conjugated Dienes
(1) The shorter C2-C3 bond provides a stronger bond that stabilizes the
conjugated diene.
(2) The small amount of additional delocalization of the -electrons
stabilizes the conjugated diene.
This extra stability of conjugated dienes is also revealed in a comparison
of the electronic absorption spectra (ultraviolet-visible spectroscopy) of
hydrocarbons of these types.
33
34
Ultraviolet-Visible Spectrophotometers
The Beer-Lambert Law
Radiation from a source lamp
is split into two equal beams.
This empirical law relates the absorption of
radiation by a sample to several parameters:
IR is the initial
radiation intensity at
a wavelength .
One beam passes though a
sample (usually in solution
in a transparent cell). The
other beam serves as a
reference for determining
the amount of radiation
absorbed by the sample at
each wavelength.
IR
sample [C]
IS is the intensity of the
transmitted light at the
wavelength after passing
through a sample with a path
length l and a concentration C.
IS
l
The Law states that the transmitted light intensity at a specified
wavelength is related to the intensity of the initial light by
Where is an intrinsic constant (the molar absorptivity)
of the sample at wavelength , and the units are cm for
length and mol/L for concentration.
lC
IS = IR 10 -
Therefore, log
IS
IR
= - lC
OR log
IR
IS
= lC
AND the term adopted for this ratio of intensities is ABSORBANCE (A), so
lC
The intensities of IR and IS are compared at a series of wavelengths. This
information is displayed according to the Beer-Lambert Law.
35
36
Chromophores
Example: The UV absorption spectrum of 2,5-dimethyl-2,4-hexadiene in
The absorption bands in uv-visible spectroscopy arise from electronic
transitions from filled to unfilled molecular orbitals. Usually, these
transitions can be assigned to specific structural components within the
molecule called chromophores.
methanol at a concentration of 5.95 x 10-5 M, in a 1.00-cm cell.
Chromophores show characteristic wavelength absorption ranges and
intensities (max).
0.9
Examples of Organic Chromophores
max
Compound
Example
Solvent
max
C=C
1-hexene
C C
1-butyne
heptane
vapor
180
172
12,500
4,500
benzene
water
254
203.5
205
7,400
acetone
cyclohexane
275
190
22
1,000
Chromophore
C=O
(nm)
A plot of absorbance versus
the wavelength shows a
maximum at 242.5 nm with
Amax= 0.78.
A
B
S
O
R
B
A
N
C
E
(M-1 cm-1)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
200 210 220 230 240 250 260 270 280
nm
37
In simple alkanes, the only electronic
transitions are s s* that occur in the vacuum
UV region.
Calculation of 242.5 for
242.5 =
A242.5
Cxl
=
Alkenes show absorptions due to  *
transitions just below 200 nm (still below the
range of most UV-visible spectrophotometers).
0.78
(5.95 x 10-5 M) x (1.00 cm)
CH2=CH2
= 13,100
This experimentally determined
absorption data appears in the
literature as:
 max
methanol
2,5-Dimethyl-2,4-hexadiene
38
Ethene
1,4-Pentadiene
max 171 nm
max 178 nm
Polyenes absorb at longer and longer
wavelengths as the number of conjugated
double bonds increases.
242.5 nm ( = 13,100)
39
1,3-Butadiene
trans-1,3,5-Hexatriene
max 217 nm
max 274 nm
40
Highly Extended Polyenes
Conjugated -Systems
Polyenes with 8 or more conjugated double bonds absorb in the visible
region and are, therefore, colored. Some of these compounds occur in
nature and have biological functions. CAROTENE is an example.
When multiple bonds are conjugated, their p-orbitals overlap and a new set
of -molecular orbitals form. These conjugated systems absorb at wavelengths
above 200 nm.
As shown in this diagram, the energy gap between the highest energy
occupied orbital (HOMO) and lowest energy unoccupied orbital (LUMO),
2 and 3*, respectively, is smaller than that for a simple alkene. This allows
a lower energy photon to effect the excitation, which results in a shift of the
max to longer wavelength.
Simple alkene
4*
E
N
E
R
G
Y
3*(LUMO)
171 nm
-Carotene
CH3
max 497 nm
16
 (HOMO)
1
CH3
12
10
8
Antibonding
molecular
orbitals
 x 10-4
6
UV
4
Visible
Note: As illustrated here,
absorption spectral data
sometimes are plotted as the
molar extinction coefficient ()
against .
217 nm
2 (HOMO)
CH3
Absorption at the violet end of the
visible region leads to the yelloworange color of this plant pigment.
14
CH2=CH-CH=CH2
* (LUMO)
CH3
CH3
CH3
CH3
CH3
Conjugated diene
CH2=CH2
CH3
CH3
2
Bonding
molecular
orbitals
0
41
260
300
340 380
420 460 500
540 nm
42
The Carbonyl Function
Analytical Applications of UV-Visible Spectroscopy
Electronic spectroscopy finds wide use as an analytical tool in chemistry
and other fields, especially biology. Very small amounts of materials may
be measured in samples using the Beer-Lambert Law:
A = xCxl
:
: :
The characteristic electronic absorption in the isolated carbonyl
group is the low intensity n * transition, where an electron in
one of the unshared electron pairs (a nonbonding or "n"
electron) is promoted to the carbonyl's * orbital.
n *
CH3
CH3
n *
. .
C=O
C=O
max 280 nm
CH3
CH3
max 15
Acetone
Excited state
molar absorptivity
Conjugation of the carbonyl function with an alkene double bond
shifts both the n * and  * transitions to longer wavelengths.
=
O
CH2=CH-C-CH3
But-3-ene-2-one
n *
max 324 nm
max 24
cell length in cm
absorbance
Conjugated Carbonyl Systems
concentration in mol/L
The absorbance (A) of a compound is linearly related to concentration (C)
over a range of concentrations, when the measurements are made under
identical conditions (temperature, solvent, wavelength). If not linear over
the range of interest, or to demonstrate that it is linear, a calibration
curve of the absorbance versus concentration may be prepared.
 *
max 219 nm
max 3600
43
44
Electrophilic Attack on Conjugated Dienes:
1,2- and 1,4-Addition
Example of Utility of UV-Visible Spectroscopy
Conjugated dienes display a more complicated mode of electrophilic
addition than simple alkenes. Typically, more than one product is
formed.
A synthesis product contained compound X contaminated by Y.
HCl
25 oC
CH2=CH-CH=CH2
X
Y
1,3-Butadiene
Given that X has max 242.5 nm and  13,100 M-1 cm-1 in MeOH, what
percent X is present in the mixture if a 5.00 x 10-5 M solution of it in
MeOH has an absorbance of 0.400 at 242.5 nm in a 1.00-cm cell?
H-Cl
x 100% =
Cl-
61.0%
+
+
BrCH2-CH CH CH2
-15
oC
(54%)
1,4-Addition product
1-Chloro-2-butene
+
+
CH3-CH CH CH2
HBr
If conducted at 40 oC
(20 %)
If conducted at -80 oC
(80%)
+
CH3CH=CH-CH2Br
(80%)
(20%)
Another Key Observation
Br-
While the two products are stable at -80 oC, at 40 oC the less stable 3bromo-1-butene isomerizes to the more stable 1-bromo-2-butene.
1,4
+
1,4
1,2
Br
CH3CH-CH=CH2
(46%)
1,2
46
Br-
+
+
CH3-CH CH CH2
(20 %)
1,2-Addition product
1,4
Br
BrCH2CHCH=CH2 + BrCH2CH=CHCH2Br
Br
CH3CH-CH=CH2
CH3-CH=CH-CH2Cl
CH2=CH-CH=CH2
1,2
HBr
40 oC
CH3-CH-CH=CH2
Cl
The ratio of products obtained from the reaction of butadiene and
HBr depends on the temperature:
Br-
CH2=CH-CH=CH2
Cl-
Kinetic Control versus Thermodynamic Control
Other Examples of Competing 1,2- and 1,4-Additions
Br2
More stable allylic-type cation
3-Chloro-1-butene
45
CH2=CH-CH=CH2
Less stable primary carbocation
MAJOR
+
+
H-CH2-CH CH CH2
0.400
M-1 cm-1)(1.00 cm)
= 3.05 x 10-5 M
3.05 x 10-5 M
5.00 x 10-5 M
(22%)
H
NOT
+
CH2-CH-CH=CH2 Evidenced
H-Cl
CH2 CH-CH CH2
0.400 = M-1 cm-1 x C M x 1.00 cm
And the percent X in the mixture =
3-Chloro-1-butene
(78%)
Formation of the above two products, and absence of ClCH2CH2CH=CH2,
could have been predicted by consideration of the carbocation intermediates.
SOLUTION
[Recall that Y would not absorb light above 200 nm.] So, using A =  x C x l
And then C (the concentration of X in the solution) =
CH3-CH-CH=CH2 + CH3-CH=CH-CH2Cl
Cl
1-Chloro-2-butene
CH3CH=CH-CH2Br
(80%)
Thus 1-bromo-2-butene, CH3CH=CH-CH2Br, is favored
But if this reaction is carried out at -80 oC, the ratio of products is exactly
reversed. And, on warming this cold reaction product up to 40 oC, it
equilibrates to the 20:80 ratio that would have formed at 40 oC.
47
Br
thermodynamically, but 3-bromo-1-butene, CH3CH-CH=CH2 ,
is favored kinetically.
48
Ki
Kinetic Control of Product Mixture
h
Thermodynamic Control of Product Mixture
At the higher temperature, the products can revert back to the
carbocation intermediate, providing a pathway for interconversion of
the two products. Under this condition, the two products equilibrate,
and the product mixture reflects the relative stabilities of the two
products.
+Br-
faster
CH2=CH-CH=CH2
+
H+
+Br-
slower
+
CH3CH CH CH2
-Br-
+Br-
at 40 oC
+Br-
Br
CH3CH-CH=CH2
(20 %)
+
CH3CH CH CH2
Equilibration of 3-bromo-1-butene and 1-bromo-2-butene.
+
Br
CH3CH-CH=CH2
(80%)
at -80 oC
CH3CH=CH-CH2Br
(20%)
IRREVERSIBLE at -80 oC
-Br-
The faster rate of attack by Br- at C3 is due to the greater
CH3CH=CH-CH2Br
(80%)
charge density at that position relative to C1:
+
Product mixtures produced under such equilibrating
conditions are formed under thermodynamic control.
Br-
+
CH3CH-CH=CH2
CH3CH=CH-CH2
major contributor
(charge on 2o C)
minor contributor
(charge on 1o C)
49
50
Energetics of the Product-Forming Steps
The Diels-Alder Reaction
A 1,4-Cycloaddition Reaction of Dienes
slower
faster
-Br-
-BrFE
RN
E
ER
EG
Y
A very useful cycloaddition addition reaction of 1,3-dienes and alkenes to
produce cyclohexenes was discovered in 1928 by Otto Diels and Kurt
Alder. They received the Nobel Prize in Chemistry in 1950 for this
discovery.
O
O
G1,4
G1,2
+
+
CH3CH CH CH2
Go
formation
Br
CH3CH-CH=CH2
less stable
+
A diene in
s-cis conformation
CH3CH=CH-CH2Br
more stable
O
O
An alkene
(maleic anhydride)
O
O
A cyclohexene
"adduct"
This is an example of a 4 + 2 -electron cycloaddition.
It involves a conjugated diene (a 4 -electron system) and an alkene (a 2 electron system). The latter is called a dienophile.
Reaction Coordinate
Under conditions of thermodynamic control, the product mixture is
determined by the difference in the standard free energies of formation
of the products: Go

formation
Under conditions of kinetic control, the product mixture is determined by
the relative rates of product formation. The more rapid formation of the
1,2-addition product is because the energy of activation required there
( G1,2 ) is less than that required for 1,4-addition ( G ).
1,4

51

+ 


During the reaction, the two -bonds in
the diene and the -bond of the dienophile
are transformed into two new -bonds and
one new -bond in the adduct.
52
Factors Favoring the Diels-Alder Reaction
Stereochemistry of the Diels-Alder Reaction
This cycloaddition reaction is promoted by electron-withdrawing groups
in the dienophile and electron-releasing groups in the diene.
O
O
C
CH3
CH3
H
H
+
=
=
CH3
CH3
Propenal
2,3-Dimethyl1,3-butadiene
(1) The Diels-Alder reaction is a syn addition and the configuration of
the dienophile is retained in the product.
H
COOCH3
H
COOCH3
H
COOCH3
+
(100%)
Dimethyl maleate (cis)
(68%)
H
COOCH3
Dimethyl 4-cyclohexenecis-1,2-dicarboxylate
The Diels-Alder reaction is enhanced by high temperature and pressure.
200 oC
+
pressure
=
=
The Diels-Alder reaction is enhanced by the presence of Lewis acids.
O
O
CH3
CH3
OH
CH3 O
NOTE: The pair of dienophiles above are among the very few
geometric isomers that have different names (maleate vs. fumarate)
and do not require use of cis/trans or Z/E designations.
OH
(80%)
53
(2) In the Diels-Alder reaction, the diene must react in an s-cis
conformation. In 1,3-butadiene, there is an equilibrium between the scis and s-trans conformations. Although the latter is more stable by 9.6
kJ/mol, this difference does not prevent reaction.
fast
Exo and endo describe the stereochemistry of groups attached to bridged
bicyclic ring systems. The longest bridge (excluding the one carrying the
exo or endo group) is the reference point. Groups that project away from
this bridge are in exo positions. Groups that project toward this bridge
are in endo positions.
dienophile
Diels-Alder adduct
s-cis
This requirement is responsible for the reactivity order of the following
1,3-dienes in the Diels-Alder reaction:
H
CH3
(rigid s-trans)
No Reaction
CH3
CH3
H
cis, cis-2,4Hexadiene
<<
H
54
(3) Cyclic dienes, such as cyclopentadiene, give endo products in the
Diels-Alder reaction.
slow
s-trans
(more stable)
(95%)
Dimethyl 4-cyclohexene-
COOCH3 trans-1,2-dicarboxylate
H
OH
=
=
O
OH
H
D imethyl fumarate (t rans)
ethyl ether, 25 oC
O
OCH3
CH 3OOC
AlCl3
+
H
COOCH3
COOCH 3
H
+
Cyclohexene
Ethene
1,3-Butadiene
Example: a substituted bicyclo[2.2.1]heptane
H
H
H
H
<<
H
R
(rigid s-cis)
cyclopentadiene
H
CH3
trans, trans-2,4Most Reactive
Hexadiene
Steric hindrance makes s-cis
conformation very difficult to achieve.
Reference bridge
55
H
exo Position
R
endo Position
56
Stereoselectivity in the Diels-Alder Reaction
It is suggested that the diene and dienophile approach each other in parallel
planes. The stereoselectivity arises because of the preference for the endo
approach, which provides the more extensive  orbital overlap. This results
in a lower G for the creation of the endo adduct than for the exo.
The Endo Rule
The Diels-Alder reaction is stereospecific in regard to the configuration in
the dienophile (syn addition), and it is stereoselective in regard to the
orientation of unsaturated groups of the dienophile relative to a cyclic diene.
H
CO2CH3
H
CO2CH3
The endo adduct is
formed more rapidly
and is the major product.
H
+
H CO2CH3
CO2CH3
Dimethyl maleate
Stereospecificity: CO2CH3 groups are cis
(In both reactant and product; if they had
been trans in the reactant, they would have
been trans in the product.)
Stereoselectivity: endo product is formed
(When, as in this case, the reaction is
under kinetic control.)
57
58
Molecular Orbital Picture of the Diels-Alder Reaction
Additional Diels-Alder Examples
Coplanar, so orbital
overlap favors
approach that leads
to the kinetically
favored endo adduct.
The dimerization (and the laboratory preparation)
of cyclopentadiene:
room temperature
(slow)
+
Cyclopentadiene
=
O
H-C
+
H
CH 3
+
CH 3
H
H
high temperature
("cracking")
endo-Dicyclopentadiene
H
H
C
C
H
H
COOCH 3
C
C
COOCH 3
C =O
H
endo Adduct
H
CH 3
COOCH 3
COOCH 3
Electron-deficient
alkynes also are
good dienophiles.
CH 3
59
Interaction of the orbitals that lead to the new C-C  bonds of the
adduct are shown in purple, and interaction of the orbitals that
guide the reactants into the endo mode are shown in green.
60