Chapter 16 – Conjugati on, resonance, and dienes
Conjugation relies on the partial overlap of p-orbitals on adjacent double
or triple bonds. A common conjugated system involves 1,3-dienes, such
as 1,3-butadiene.
in a conjugated system, overlap of p orbitals allows electrons to be
delocalized over a larger portion of the molecule, thus lowering the
energy of the molecule and making it more stable
However, it is possible for two systems to be in “cross-conjugation” with
each other, as in the example below (the two benzene rings are crossconjugated, NOT conjugated!):
Conjugation is broken completely by the introduction of saturated (sp3)
carbon:
The allylic carbocation is another example of a conjugated system –
conjugation stabilizes an allylic carbocation (due to two resonance forms)
CH2
CH2
- the true structure is a hybrid of the two resonance forms – the π bond is
delocalized over all three C atoms and the positive charge is delocalized
over the two terminal C’s. Thus, an allylic carbocation is more stable than
a normal 1˚ carbocation; its stability is comparable to that of a 2˚
carbocation:
CH3
< RCH2 < R2CH ~~ CH2=CH-CH2
<
R3C
common examples of when resonance structures are drawn for a molecule
or reactive intermediate (besides allylic type):
1. Conjugated double bonds - acylic
:
:
H2C
CH2
H2C
CH2
1
cyclic – e.g. benzene
2. cations having a positive charge next to a lone pair
O
CH2
H3C
O
CH2
:
: :
H3C
3. double bonds having one atom more electronegative than the other
H3C
H3C
C
C
O
O
H3C
H3C
REMEMBER:
1. Resonance structures with more bonds and fewer charges are more
stable.
2. Resonance structures in which every atom has an octet are more
stable.
3. Resonance structures that place a negative charge on the more
electronegative atom are more stable.
Which of the following two resonance structures is more stable?
:
: :
HN
C
C
O
: :
HN
O:
H3C
H3C
In any system, X=Y-Z, Z is sp2-hybridized and the nonbonded electron pair
occupies a p orbital to make the system conjugated. E.g.
:
O
: :
C
O:
C
Conjugated dienes (1,3-dienes) – conformational and stereosiomers, etc.
For 1,3-dienes with alkyl groups bonded to each end carbon of the diene,
RCH=CH-CH=CHR, 3 stereoisomers are possible:
R
R
R
cis,cis isomer
or
(Z,Z)-1,3 diene
R
R
trans,trans isomer
or
(E,E)-1,3 diene
R
cis,trans isomer
or
(Z,E)-1,3 diene
2
conformational isomers are also possible because two conformations
result from rotation around the C-C bond that joins the two double bonds
s-cis conformation
s-trans conformation
e.g.
R
R
R
R
R
R
trans,trans isomer
s-trans conformation
trans,trans isomer
s-cis conformation
two conformers
cis,cis isomer
or
(Z,Z)-1,3 diene
two stereoisomers
Four features distinguish conjugated dienes from isolated dienes:
1. The C-C single bond joining the two double bonds is unusually short.
Ethylene C=C = 1.34 Å
Ethane C-C bond = 1.53 Å
1.34 Å
1.48 Å
- the shorter C-C bond in butadiene can be explained by sp2
hybridization of the C atoms or by resonance imparting partial double
bond character.
2. Conjugated dienes are more stable than similar isolated dienes – A
conjugated diene has smaller heat of hydrogenation than a similar
isolated diene
H2 (g)
Pd-C
!H = -61 kccal/mol
H2 (g)
Pd-C
!H = -54 kccal/mol
3. Conjugated dienes absorb at longer wavelengths of uv light (lower
energy).
3
4. Some reactions of conjugated dienes are different from reactions of
isolated double bonds
DO PROBLEMS 16.13 AND 16.14 IN CLASS.
Electrophilic Addition: 1,2- versus 1,4- Addition
Electrophilic addition in conjugated dienes gives a mixture of products.
- with an isolated diene, electrophlic addition of one equivalent of HBr
gives a single product and Markovnikov’s rule is followed (H adds to the
less substituted carbon):
Br
HBr (1 equiv)
- with conjugated dienes, electrophlic addition of one equivalent of HBr
gives two products; 1,2-product from Markovnikov addition and 1,4product from conjugate addition:
Br
HBr (1 equiv)
+
Br
1,4-product
1,2- product
Discuss mechanism – section 16.10
Thus, addition of HX to a conjugated diene forms 1,2- and 1,4-products
because of the resonance stabilized allylic carbocation intermediate.
In class problem: Draw products formed when each diene is treated
with one equivalent of HCl.
(a)
(b)
(b)
H+
less reactive
double bond
ClCl
HCl (1
equiv)
more
reactive
double bond
H+
Cl
+
Cl
Cl-
+
Cl-
likely a minor product
4
Kinetic versus thermodynamic products
- the amount of 1,2- and 1,4-addition products formed in electrophilic
addition reactions of conjugated dienes depends greatly on the reaction
conditions:
At low temperatures, the major product is formed by 1,2-addition. The
kinetic product is formed faster and predominates at low temperature:
Br
HBr (1 equiv)
+
Br
-80 ˚C
1,4-product
1,2- product
20%
80%
At higher temperatures, the major product is formed by 1,4-addition. The
more slowly formed thermodynamic product is more stable and
predominates at higher temperatures
Br
HBr (1 equiv)
40 ˚C
+
Br
1,4-product
80%
1,2- product
20%
In fact, when a mixture containing mainly the 1,2-product is heated, the
1-4-addition product becomes the major product at equilibrium:
Br
!
Br
major product at low T
major product at equilibrium
Why is the 1,4-addition product the more stable (thermodynamic)
product?
Because more substituted alkenes are more stable (Remember, increasing
alkyl substitution stabilizes an alkene by an electron-donating inductive
effect)
Why is the more stable conjugate addition product formed more slowly?
Because higher activation energy is required for formation of the required
intermediate allylic carbocation – Figure 16.6 and Fig 16.7
The 1,2-addition product is formed faster because of the proximity of Brto C2; Following H+ addition to the double bond, Br- is closer to C2 than
C4 hence attack at C2 is faster.
5
Why is the product ratio temperature dependent? The activation energy
is more important at low temperature where most molecules do not have
enough kinetic energy to over the higher energy barrier. Hence they
react by overcoming the lower energy barrier and form the kinetic
product.
Preparation of Conjugated Systems
There are a number of methods for the preparation of conjugated
systems. One possibility is by allylic bromination, followed by either
conjugate (1) or normal (2) elimination:
(1)
KOBut
NBS
h! or ROOR
Br
DMSO
+
Br
(2)
Diels Alder Reaction
One of the most spectacular reactions in organic chemistry. In linking two
carbon molecules together, it forms two single bonds and one double
bond, all in one step. At its simplest, a 1,3-diene reacts with an alkene
(called a dienophile) in a concerted reaction to give a cyclohexene.
6
The Diels-Alder reaction is a thermal reaction, that is, it is initiated by
heat. No intermediates of an ionic or radical nature have been detected
for this reaction. It goes in one step (it is concerted).
The reaction depicted above requires high temperatures and pressures in
order to go. However, a Diels-Alder reaction can be performed at
moderate temperature if a number of requirements are met. Several rules
govern the course of a Diels-Alder reaction:
1. Electron withdrawing substituents in the dienophile increases the
reaction rate. An electron-poor dienophile is required because the
dienophile acts an electrophile in Diels-Alder reaction. Examples of good
dienophiles include:
- acrylonitrile, acrylic acid, acrolein, maleic anhydride, benzoquinone
Clearly, use of an electron-rich diene is helpful since the diene acts a
nucleophile in Diels-Alder reaction; e.g.
OCH3
TMSO
2. The diene MUST be able to adopt an s-cis configuration; it can only
react in this conformation
3. The stereochemistry of the dienophile is retained in the product.
A cis dienophile forms a cis-substituted cyclohexene:
7
COOH
COOH
!
COOH
or
COOH
COOH
maleic acid
COOH
an achiral meso compound
A trans dienophile forms a trans-substituted cyclohexene:
HOOC
COOH
!
COOH
+
COOH
COOH
COOH
fumaric acid
enantiomers
A cyclic dienophile forms a bicyclic cis -fused product:
O
H
O
H
O
!
O
H
H
O
O
4. When exo and endo products are possible, the endo product is
preferred. e. g.
H
!
CH3
+
H
H
CH3
O
O
H CH
3
H
O
H
endo product is the
major product
exo product is the
minor product
O
O
H
O
O
-
H
!
O
O
O
+
HO
O
H
exo product is the
endo product is the
minor product
major product
in this type of reaction, we obtain a bridged bicyclic product in
which the two rings share non-adjacent carbon atoms. The endo
8
product, in which the substituent(s) on the bridging carbon(s)
end(s) up under the newly formed six-membered ring, is preferred.
- To understand this preference, see Fig 16.11 – the transition
state leading to the endo product allows more interaction between
the electron rich diene and the electron withdrawing substituent on
the dienophile; and this arrangement is energetically favorable.
In class problem: draw the product of the following Diels Alder
reaction
COOCH3
!
+
COOCH3
COOCH3
COOCH3
Regioselectivity in Diels-Alder Reactions – ortho-para rule
When an unsymmetrical diene and an unsymmetrical dienophile combine in
a Diels-Alder reaction, the simplest way to determine which product will
be formed is to draw an “ionic” stepwise mechanism for the reaction to
establish which end of the diene will react with which end of the
dienophile.
Reaction between a diene that is substituted at the terminal position with
an electron donating group and a dienophile proceeds through an
aromatic transition state that is ortho-directing and leads to 1,2substituted cycloehexene product; e.g.
O
O
O
OMe
OMe
OMe
O
OMe
Note however, that the reaction is in fact concerted. Thus the transition
state of the reaction really looks like below:
O
‡
O
O
OMe
OMe
OMe
9
Reaction between a diene that is substituted at the internal position with
an electron donating group and a dienophile proceeds through an
aromatic transition state that is para-directing and leads to 1,4substituted cycloehexene product; e.g.
O
O
O
OMe
MeO
OMe
OMe
MeO
MeO
O
OMe
Actual mechanism
MeO
O
OMe
MeO
O
‡
O
OMe
OMe
MeO
MeO
Retrosynthetic analysis of a Diels-alder product: you should be able to
look a Diels-Alder product and determine the conjugated diene and
dienophile used to make it; e.g.
H3CO
COOCH3
COOCH3
H3CO
+
COOCH3
H3COOC
Retro-Diels-Alder reaction
Note also that in some cases the Diels-Alder reaction is reversible; i.e. a
cyclohexene can revert back to a diene and a dienophile.
25˚ C
H
H
!
Occasionally, the retro Diels-Alder is more favorable than the forward
reaction. In these cases, there are some special tricks that can be used to
10
force the reaction along. One such case is the use of an orthoquinodimethane:
- aromatic character of the product drives the reaction forward
UV-Vis Spectroscopy.
UV-Vis spectroscopy is based on exciting the electronic levels in
conjugated molecules. What occurs is simply a promotion of one electron
from the molecule’s HOMO into its LUMO. The molecule generally takes on
the electronic character of the LUMO in this instance, generally having a
diradical character. The greater the degree of conjugation in the molecule
(i.e. the more levels in the M.O. picture), the easier it will be to excite an
electron into the LUMO. At sufficiently low energy level differences, the
energy required to promote an electron to the LUMO can be provided by
visible light, yielding a colored compound. Another way to say this is that
if a compund is colored, an easy route must exist for the promotion of an
electron.
Along with increasing the degree of conjugation, there are other ways to
facilitate the excitation of an electron. One method is to facilitate what is
called intramolecular charge transfer. This is usually accomplished by the
preparation of a “push-pull” system, as shown below:
The ease with which this charge-transfer reaction takes place depends on the
strength of the “push” component (in this case, the dimethylamine group) and the
strength of the “pull” component (the nitro group). Thus by varying the push and
pull moieties, and by changing the length of the conjugated bridge separating
them, we can control the color of the molecule! Some examples of colored
compounds are shown below – be sure you understand why they have different
colors!
11
12